Battery Management systems (BMS) for LiFePO4 batteries in real situations

karrakkarrak Solar Expert Posts: 326 ✭✭✭✭
There seems to be some interest to start a dedicated discussion on this so I thought might do just that.

I am responsible for two off grid installations which use LiFePO4 batteries. My own is described in my signature, the other is a 48 volt system using off the shelf components with the same amount of battery storage.

My friend who owns the 48 volt system does not have much electrical knowledge but is very practical. His system is about 300km from where I live. With these two factors in mind I decided that his system would have to be self reliant and deal with any faults or problems preferably without operator intervention. I couldn't find an off the shelf BMS that would balance the cells at 2.45 V/cell and provide low and high level alarms and disconnects so ended up cobbling together something using Cellog8s, relays and balancing devices that were in use by the RC community. This turned into a bit of a wiring nightmare as I needed four Cellog8s to monitor 16 cells and provide both high and low level alarms. I certainly wouldn't do it like this again!

The battery monitoring section of the BMS has probably saved his batteries from potential damage on a few occasions , the first time caused by initial balance problems, another couple of times because my friend forget about monitoring their consumption and nearly ran the battery flat and once because of a fault in one of the cell balancers that lead to one cell being discharged more than the others. I have since disconnected the cell balancing devices and have done a thorough manual balance, which is quite a fiddly procedure with 16 cells to balance. My system battery monitoring has saved my batteries on a few occasions, see this thread for details http://forum.solar-electric.com/showthread.php?19472-Precautions-to-take-with-LiFePO4-batteries I manually balance my batteries and am happy with this. I am not quite so sure if this is the best long term approach for a system that is 300km away.

Unfortunately I think it is still difficult if not impossible to purchase an off the shelf BMS with automatic battery balancing to suit off-grid applications, although I must say I haven't researched this recently. I hope I am wrong. I have designed and made my own monitoring system which is detailed here http://forum.solar-electric.com/showthread.php?24630-Open-source-battery-monitoring-system-%28BMS%29-with-a-difference which can now control outputs so could be used as a BMS, but this does require some electrical and computer knowledge and is hardly an off the shelf turn key approach.

Simon
Off-Grid with LFP (LiFePO4) battery, battery Installed April 2013
32x90Ah Winston cells 2p16s (48V), MPP Solar PIP5048MS 5kW Inverter/80A MPPT controller/60A charger, 1900W of Solar Panels
modified BMS based on TI bq769x0 cell monitors.
Homemade overall system monitoring and power management  https://github.com/simat/BatteryMonitor
 

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Comments

  • BB.BB. Super Moderators, Administrators Posts: 29,980 admin
    Re: Battery Management systems (BMS) for LiFePO4 batteries in real situations

    At least for monitoring, if not balancing, I would like to see wireless--WIFI or some decient way of managing a bunch of Bluetooth (one cell, one blue tooth device) to a central monitoring station.

    A rat's nest/spider web of wires and connections--Just too likely to be damaged/miss-wired/etc. during the life/servicing of the battery bank.

    My concern--Even if the LiFePO4 cells turn out to be "perfect" and perform to specifications--The chances of killing a battery bank before they die a natural death is too high.

    And when you go from 12 volt to 48 volt banks--A single cell problem/failure is that much more difficult to detect from the +/- battery bus (2 volt cell failure out of 12 volts--easy to "see". 2 or 2.45 volt cell failure on a 48-59 volt or so battery bank--very difficult).

    If there is a good/cost effective wireless battery monitoring system--I would think there would even be a market for large cell Lead Acid systems--Especially on 48 volt battery banks.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • zonebluezoneblue Solar Expert Posts: 1,218 ✭✭✭✭
    Re: Battery Management systems (BMS) for LiFePO4 batteries in real situations

    It is an interesting idea, using radio to provide electrical isolation. You could use 433meg , zigbee, 2.4 nrf etc class of device, one per cell or group of 4 cells. Rossw from midnite designed and build a cheap little pic board intially for his lead bank, now LFP, which does 3 cells, and daisy chains using optocouplers to the next unit. Hence scalable. The one on the end of the chain connects via serial to his monitoring system. He also had problems with balance boards , ending up unbalancing his bank, and so removed them. Seems like a theme there.
    1.8kWp CSUN, 10kWh AGM, Midnite Classic 150, Outback VFX3024E,
    http://zoneblue.org/cms/page.php?view=off-grid-solar


  • Blackcherry04Blackcherry04 Solar Expert Posts: 2,490 ✭✭✭
    Re: Battery Management systems (BMS) for LiFePO4 batteries in real situations

    I fail to see why anyone would build a system that uses so many small cells to start with. I'v yet to see anyone taking advantage with the 400, 700, 1,000 or 1,200 amp hr cells. Why would you build a system plagued with so many parallel connections and so many points of failure if you believed in the concept to start with unless you wanted a toy / hobby system ? How could anyone makes any decisions on some with a couple batteries or a cobbled up system and a bunch of talking points.
  • BB.BB. Super Moderators, Administrators Posts: 29,980 admin
    Re: Battery Management systems (BMS) for LiFePO4 batteries in real situations

    I think even a 48 volt/24 cell lead acid battery bank could use per cell monitoring.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Blackcherry04Blackcherry04 Solar Expert Posts: 2,490 ✭✭✭
    Re: Battery Management systems (BMS) for LiFePO4 batteries in real situations
    BB. wrote: »
    I think even a 48 volt/24 cell lead acid battery bank could use per cell monitoring.

    -Bill
    It could, if they would come up with a reliable Specific Gravity probe with a remote readout, it'd be about all you need. It'd make me a happy guy.
  • BB.BB. Super Moderators, Administrators Posts: 29,980 admin
    Re: Battery Management systems (BMS) for LiFePO4 batteries in real situations

    That too, but from what I have read, not as likely to happen vs cheap rf battery probe.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • ReedReed Solar Expert Posts: 55 ✭✭
    Re: Battery Management systems (BMS) for LiFePO4 batteries in real situations

    Thought I had read that AM Solar (very highly regarded outfit and fairly conservative in design/fabrication: if you have a solar fabrication yard with several months waiting list, you do not build systems that might not perform) might have another look at LFP. Copied this from their web-site

    "...The tried and true Lifeline AGM battery has proven itself in the field and is what we sell the most of. However, there is a new Lithium battery available that we are testing right now in our RV and will be adding it to our offerings if it proves to be reliable and lives up to the claims of its manufacturer...", http://www.amsolar.com/home/amr/multilist_15/batteries.html

    Our son has been in contact with one fabricator vendor about developing a BMS that is more compatible with solar discharge/charge rates. They contacted him recently that they are doing this. If they do, this will be incorporated into our system. It may require nothing more than re-programming what they already have for the EV crowd.

    We do have four batteries of four CALB (Chinese Aviation Lithium Batteries) cells in series (12 V nominal). The four batteries are in series for a 48V nominal battery bank. I should not like to be trying to manually balance all 16 cells.

    The question arose with Blackcherry as to why batteries are not fabricated with large amperage cells such as he noted: 400, 700, 1,000 or 1,200 amp-hr. Perhaps someone has an answer. I may note that for RV use, such batteries would be excessively heavy.
    Reed and Elaine
  • PNjunctionPNjunction Solar Expert Posts: 762 ✭✭✭
    Re: Battery Management systems (BMS) for LiFePO4 batteries in real situations
    How could anyone makes any decisions on some with a couple batteries or a cobbled up system and a bunch of talking points.

    How about a user video marine 12v / 400ah bank based on Winstons. Use the housepower bms, which incorporates lvd, hvd, and *optional* balancing:

    https://www.youtube.com/watch?v=oJ8AfJsOhsM

    While I am personally only running 12v, if I was running 48v I'd have NO problem doing it the way I'm doing by being very conservative with no balancers at all. Do I have an lvd or hvd - yes! My charger is the hvd. My steps for a 12v application:

    1) Charge each cell to 3.65v, or just until you observe it starting to go into absorb. You could bulk and then enlist the aid of a single-cell charger or equivalent. 0.05C to no more than 1C, or better yet no more than 0.5C.
    2) Discharge as usual - no more than 80% DOD. Typically 12.7v or maybe no deeper than 3.17v per cell.
    3) Recharge conservatively to no more than 14v (3.5v per cell). Allows one a little headroom if things aren't perfect. For 48v overall, maybe I'd drop that to 3.475v. BABYSIT that first recharge.
    4) Since we are in a "Sub-C" application, the state of balance tends to stay the same, unless you do something out of the ordinary to change that.

    This means I am relying on the manufacturer to be supplying closely matched batteries in both capacity and internal resistance, and so far that has proven the case with both of my lifepo4 prismatics. There is *some* minor differences, but these are easily handled by not running them wall-to-wall in the charge-discharge curve, AND again, being "sub-c".

    With no balancing boards to worry about failing or presenting a constant parasitic load, I'm pretty happy. But I understand those like Reed, who do favor this type of operation. It is just a difference in trust.

    Single cell monitoring may be ideal, although from a practical standpoint, perhaps monitoring 12v "modules" for a 48v battery might be in order, and manual checks once in awhile for individual cell voltage could be performed for sanity. It all depends on your trust level. Note that your cell-monitoring or coulomb-counting gear should NOT be powered by the cells under test, but from another power source.
  • BB.BB. Super Moderators, Administrators Posts: 29,980 admin
    Re: Battery Management systems (BMS) for LiFePO4 batteries in real situations
    PNjunction wrote: »
    Single cell monitoring may be ideal, although from a practical standpoint, perhaps monitoring 12v "modules" for a 48v battery might be in order, and manual checks once in awhile for individual cell voltage could be performed for sanity. It all depends on your trust level. Note that your cell-monitoring or coulomb-counting gear should NOT be powered by the cells under test, but from another power source.

    If the voltage prob+RF transceiver is small enough load (a very small button cell can run a Bluetooth headset for days in standby)--Then I would not worry--Plus it could always "auto off" after transmitting a low voltage cell fault.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • karrakkarrak Solar Expert Posts: 326 ✭✭✭✭
    Re: Battery Management systems (BMS) for LiFePO4 batteries in real situations
    BB. wrote: »
    At least for monitoring, if not balancing, I would like to see wireless--WIFI or some decient way of managing a bunch of Bluetooth (one cell, one blue tooth device) to a central monitoring station.

    I would second this, Zigbee as suggested by Zoneblue with its low power consumption is probably the way to go. With careful design it might be possible to have the monitoring/balancing board consume less power than the battery leakage for reasonably size batteries.
    My concern--Even if the LiFePO4 cells turn out to be "perfect" and perform to specifications--The chances of killing a battery bank before they die a natural death is too high.
    As long as you don't take then out of their proper voltage range I don't think there is much likelihood of this happening. I think you have to be much more careful with lead acid batteries, having to keep them fully charged and trying not to discharge them too much and recharging them with the correct current to avoid the dreaded sulphation.
    If there is a good/cost effective wireless battery monitoring system--I would think there would even be a market for large cell Lead Acid systems--Especially on 48 volt battery banks.

    I agree, I wonder how many times a complete battery bank has been replaced because a single cell has died prematurely or because a single or multiple cells within a bank have been damaged because of poor battery equilisation. All these sort of faults would be picked up with individual cell monitoring. In the past the cost of this monitoring could have made it uneconomic, but if made in quantity these monitoring devices should cost next to nothing.
    Reed wrote: »
    I should not like to be trying to manually balance all 16 cells.
    Yes it did take several hours over a couple of days!
    The question arose with Blackcherry as to why batteries are not fabricated with large amperage cells such as he noted: 400, 700, 1,000 or 1,200 amp-hr. Perhaps someone has an answer. I may note that for RV use, such batteries would be excessively heavy.
    Reed and Elaine

    I think that for their primary use as power storage for vehicles they need to fit into nooks and cranies within the car and you need to distribute the weight, also the car propulsion systems usually work at high voltage to keep the current down so more cells not large Ah capacity is what is needed. Winston actually do have a 1000Ah cell, that sells at a very good price. This is much too large for our system.

    Simon
    Off-Grid with LFP (LiFePO4) battery, battery Installed April 2013
    32x90Ah Winston cells 2p16s (48V), MPP Solar PIP5048MS 5kW Inverter/80A MPPT controller/60A charger, 1900W of Solar Panels
    modified BMS based on TI bq769x0 cell monitors.
    Homemade overall system monitoring and power management  https://github.com/simat/BatteryMonitor
     

  • Blackcherry04Blackcherry04 Solar Expert Posts: 2,490 ✭✭✭
    Re: Battery Management systems (BMS) for LiFePO4 batteries in real situations
    Winston actually do have a 1000Ah cell, that sells at a very good price. This is much too large for our system.
    I guess if your building systems for a RV or something small. Even the 12v bank on my Boat is 6, 2v, 1,300 amp hr batteries. Trying to balance 32 batteries when you could have 4 for 12v or 8 for 24v makes no sense with all the connections. Thats the one thing I find good about the availability about the large cell size selection, it's much easier to custom size the bank without a bunch of small cells.
  • karrakkarrak Solar Expert Posts: 326 ✭✭✭✭
    Re: Battery Management systems (BMS) for LiFePO4 batteries in real situations
    I fail to see why anyone would build a system that uses so many small cells to start with. I'v yet to see anyone taking advantage with the 400, 700, 1,000 or 1,200 amp hr cells. Why would you build a system plagued with so many parallel connections and so many points of failure if you believed in the concept to start with unless you wanted a toy / hobby system ? How could anyone makes any decisions on some with a couple batteries or a cobbled up system and a bunch of talking points.

    The two "toy/hobby" systems you talk about have been reliably providing all the power with no generator backup for two households for nearly 18 months. The only failures in these systems have been the cell balancer I mentioned in my first post and an expensive well known brand, Australian made Inverter that was replaced under warranty.

    Using LiFePO4 batteries in off-grid applications is still in its infancy and in my humble opinion there is still little commercial equipment that integrates well with these batteries in this application so one is forced to make "cobbled up" systems. Hopefully this situation will change. I am quite happy to make "talking points" if it furthers the development of this CO2 reducing technology.
    I guess if your building systems for a RV or something small. Even the 12v bank on my Boat is 6, 2v, 1,300 amp hr batteries. Trying to balance 32 batteries when you could have 4 for 12v or 8 for 24v makes no sense with all the connections. Thats the one thing I find good about the availability about the large cell size selection, it's much easier to custom size the bank without a bunch of small cells.

    Our daily peak current draw from our battery can be around [email protected] volts. I would not want to go to a 12 volt system where the current draw would be 400A.

    We live in a very small market where our choices are limited. Our local supplier was only importing Winston cells up to 160Ah and the large 1000Ah cells. I calculated my requirements to be around 300-400Ah so ordered the 160Ah cells. After waiting months for them to arrive, it was discovered that there were insufficient 160Ah cells in the batch, so I was forced to get the 90Ah cells due to time constraints, so my configuration is 4x90 Ah cells in parallel and my friends is 2x90 Ah in parallel. For my configuration with 4p8s cells I only have to monitor 8 blocks of cells and for my friend's 2p16s system 16 block of cells. I agree that the extra interconnects could be source of failure but have been very careful to make sure that they will not be.

    Simon
    Off-Grid with LFP (LiFePO4) battery, battery Installed April 2013
    32x90Ah Winston cells 2p16s (48V), MPP Solar PIP5048MS 5kW Inverter/80A MPPT controller/60A charger, 1900W of Solar Panels
    modified BMS based on TI bq769x0 cell monitors.
    Homemade overall system monitoring and power management  https://github.com/simat/BatteryMonitor
     

  • BB.BB. Super Moderators, Administrators Posts: 29,980 admin
    Re: Battery Management systems (BMS) for LiFePO4 batteries in real situations

    Just to be clear--I don't mean that LiFePO4 cells are easy to screw up vs Lead Acid--I intended to say that it is easy to mess up any battery bank with over discharging, over charging, failed/dirty interconnects, etc..

    A "simple" cell level battery monitor would help identify any battery defects, as well as when a battery bank is pressed near its limits through either "driver error" or hardware issues.

    BMS is probably difficult to justify on a per cell basis for automotive packs that are tons of small cells in series/parallel arrangements. But for large AH cells like used in off grid/backup power systems for homes/offices/etc., I think they could be very cost effective in the long term. Especially if they could be made inexpensively (easy to replace if damaged, using 24 of them per 48 volt battery bank, etc.).

    I believe that many issues are driver error (over discharging, deficit charging, not recognizing a failed component or cell, poor maintenance for cables, etc.). And even if you end up with a cell problem (mfg. related), catching/replacing the failed cell before it takes down the entire bank is helpful too).

    I do believe that a well designed/properly manufactured LiFePO4 battery appears to be much less easy to damage by an end user vs lead acid (no sulfation/watering issues, balancing/equalization not needed as often--if ever, etc.).

    Am more worried about the typical failures of the owner/guests/hardware/etc... Using power until inverter won't turn on, no monitoring of cells, BMS system with active equalization "goes bad", etc. taking out the expensive battery bank (or even theft), failed/crashed charge controller, etc.

    Should a cell monitoring system be connected to a "kill switch" for the battery bank? Or would that create even more problems (a DC kill switch is not an insignificant task, what happens when battery is removed from off grid power system, etc.).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • karrakkarrak Solar Expert Posts: 326 ✭✭✭✭
    Re: Battery Management systems (BMS) for LiFePO4 batteries in real situations
    PNjunction wrote: »
    How about a user video marine 12v / 400ah bank based on Winstons. Use the housepower bms, which incorporates lvd, hvd, and *optional* balancing:

    Thanks for the info, I did look at this but the balance voltage is fixed at 3.6 volts. It would be really nice to have a programmable balance voltage.
    Do I have an lvd or hvd - yes! My charger is the hvd.
    I think it is a good idea to try to design systems where any single fault will definitely not create a hazardous situation and will hopefully not damage any other parts of the system, especially if these parts are expensive. What happens if a fault in your charger shorts your solar panels to your battery or the voltage monitoring circuitry in your charger develops a fault where it under-reads the battery voltage, or any number of other faults or software glitches, or unforeseen circumstances.
    Note that your cell-monitoring or coulomb-counting gear should NOT be powered by the cells under test, but from another power source.

    Could you give the reasons for this
    Thanks
    Simon
    Off-Grid with LFP (LiFePO4) battery, battery Installed April 2013
    32x90Ah Winston cells 2p16s (48V), MPP Solar PIP5048MS 5kW Inverter/80A MPPT controller/60A charger, 1900W of Solar Panels
    modified BMS based on TI bq769x0 cell monitors.
    Homemade overall system monitoring and power management  https://github.com/simat/BatteryMonitor
     

  • karrakkarrak Solar Expert Posts: 326 ✭✭✭✭
    Re: Battery Management systems (BMS) for LiFePO4 batteries in real situations
    BB. wrote: »
    Am more worried about the typical failures of the owner/guests/hardware/etc... Using power until inverter won't turn on, no monitoring of cells, BMS system with active equalization "goes bad", etc. taking out the expensive battery bank (or even theft), failed/crashed charge controller, etc.

    Should a cell monitoring system be connected to a "kill switch" for the battery bank? Or would that create even more problems (a DC kill switch is not an insignificant task, what happens when battery is removed from off grid power system, etc.).

    Bill, you have such a good way of summing things up and getting to the heart of the matter, you are not an engineer by any off-chance:-)

    I agree that the big DC kill relay or whatever is problematic, to get around this I have implemented the following:

    The 'any cell low voltage detected' generates an audible alarm which has been sufficient in the two systems I have detailed. I had thought about turning off the AC side of things by generating an 'earth fault' on the house power circuit and tripping an RCD breaker, but am a little hesitant to do this. I would have to look at our wiring regs for this as well. The other preferable option would be to shut down the Inverter, would be good if the inverter manufacturers gave you a facility to do just this.

    In my friends system the 'any cell high voltage detected' generates an alarm and disconnects the solar panels from the solar controller via a latching relay. In my system my MPPT controller has a relay on the panel side with separate contacts for each panel that can be overridden.

    One of the main reason for using the Beaglebone in my monitoring system is that it can be very easily connected to the Internet or a 3G dongle and so you could send an email or text message when a fault is detected and then log into the Beaglebone remotely to see what is going on.

    Simon
    Off-Grid with LFP (LiFePO4) battery, battery Installed April 2013
    32x90Ah Winston cells 2p16s (48V), MPP Solar PIP5048MS 5kW Inverter/80A MPPT controller/60A charger, 1900W of Solar Panels
    modified BMS based on TI bq769x0 cell monitors.
    Homemade overall system monitoring and power management  https://github.com/simat/BatteryMonitor
     

  • Blackcherry04Blackcherry04 Solar Expert Posts: 2,490 ✭✭✭
    Re: Battery Management systems (BMS) for LiFePO4 batteries in real situations
    Our daily peak current draw from our battery can be around [email protected] volts. I would not want to go to a 12 volt system where the current draw would be 400A.
    I don't think I suggested anyone pull 400 amps on a 12v system, if fact it'd be crazy to pull 200 amps on a 24v system especially when there is not enough PV to put it back. My rule is if it's over 100 amps for a minute ( Microwave ), I start the Generator.

    I am not belittling your system, just pointing out that the systems that have been talked about on these threads are all made with many parallel connections and small cells that create problems that are not necessary to create a quality system that still costs 3x a FLA system. If you built a FLA system this way you'd have the same problems and worse. If your going to build a 24v or 48v system the best practice is to use the proper cell size to give you the amp hour required, then a 24v system would have 8 cells and not 32 to worry about. Since you could not get the proper cells, you built what you built, I just wouldn't spend $4,000 that way.

    Talking Point : LifePO4 have no maintenance.

    Yeah, instead of a hydrometer you'll be spending your time with a volt meter obsessing about balancing cells. If you have a BMS then you'll always have to be second guessing it.

    Premium quality ( Surrettes ) have a 2 1/2 electrolyte reserve over the plates and water is about ounce a year unless your over absorbing. If your obsessive , put a blinky on one of the cells.
  • PNjunctionPNjunction Solar Expert Posts: 762 ✭✭✭
    Re: Battery Management systems (BMS) for LiFePO4 batteries in real situations

    The reason for not powering your monitoring equipment on the cells under test/monitoring is that the equipment is a parasitic load, which if left on long enough, can cause an imbalance condition. This is usually a bigger problem for EV'ers who "tap off" a 12v module in a larger system.

    Sure, the parasitic load can be small, BUT if it fails, or degrades and draws much more idle current than spec, you have an even bigger problem. In essence, all of these tacked-on monitoring systems may be nifty when they work, but you are also taking the chance that they will actually take out your system if they fail. Or, if they simply degrade, it becomes a veritable circus of balancing activity, each one trying to cover up the evidence of it's failed neighbor circuit. :)

    I think it is key to separate all the "what if's" and protection circuitry that ANY chemistry should use, like lvd, hvd, and so forth from the actual operation of the battery itself, and then merge those concerns together, and evaluate the risks involved.

    The concerns are valid, but if we don't keep in mind our relatively MILD application for these cells, we go overboard like we are flying RC-modeling quad-copters in the park, or trying to emulate EV'ers who take these cells to heaven and back. We don't when sized appropriately. This allows us the headroom to not try and achieve perfection, but live in a relative comfort zone without masses of complexity. We can afford to be a bit unbalanced since you don't have to drive them to 100% SOC. Since we still plan on about 50% DOD, we have plenty of room down below for a small imbalance there and in my own testing achieve 80% DOD without any balancing drama. Thus, all we have to maintain is a *reasonable* balance without external balancing circuitry risks, and that maintenance is eased by the fact that under our "sub-c" condition, the cells stay in the state of balance we left them in. Tighten it up, or run a little loose - just stay within spec, ie not driving higher than 3.6v per cell.

    Individual cell monitoring is fun. Then again, if you buy quality, and run with simple 4S (12v), 8S(24v) and so on, and not parallel a huge bunch of dinky cells together like you would for *some* EV's, then the statistics for failure decreases - and again, especially because we treat these things to maybe what - 0.1 to 0.2C max charge / discharge? The cells are actually overbuilt for our application, and will lead a very pampered life with us - even more so if we run conservatively.

    That's one reason I run around naked and conservative. Well maybe not the way it sounds. :) But much like any serious FLA installation, I emulate the sanity checks one would make with a hydrometer now and then with my Flukes. I don't treat it as a set-n-forget battery system. I keep the individual cell monitoring obsession under control.

    Part of the reason for my own testing of the small 20 and 40ah GBS batteries was to see if I could operate them as simply as I could with lead. If it turns out I have to treat them with kid-gloves, with anything more than I would in the way of protection that I would with a lead-based battery, then despite lifepo4's advantages, I'll have no part of it, and stick to lead. Fortunately that has not proven to be the case with a dash of conservatism.
  • PNjunctionPNjunction Solar Expert Posts: 762 ✭✭✭
    Re: Battery Management systems (BMS) for LiFePO4 batteries in real situations
    Reed wrote: »
    The question arose with Blackcherry as to why batteries are not fabricated with large amperage cells such as he noted: 400, 700, 1,000 or 1,200 amp-hr. Perhaps someone has an answer. I may note that for RV use, such batteries would be excessively heavy.

    Actually, they do (example only - not an endorsement as I have no experience with them)
    http://www.ev-power.eu/Winston-300Ah-1000Ah/

    The reason for the generally smaller sizes, although obviously a LOT larger than a huge bank of dinky cylindricals, is that these are primarily EV batteries! You can't stuff them into door-panels, but they may fit in your trunk. And like Balqon does, will fit inside larger trucks and other industrial EV vehicles. Keeping the cell count down means that you may be able to get cells off the line that are more alike in both capacity and internal resistance. It also cuts down on cell interlinks, which also need to be clean and torqued to the proper specs. Loose connections, or those that have been man-handled and twisted internally are a problem. So a lower cell count just leads to lesser problems.

    Of course like all products you may run into an initially failing product, but if you deal with reputable battery dealers like you should with ALL batteries, this should be covered. Part of the reports of failure on the net turn out to be from newcomers who first abuse the batteries with improper charge procedures, and THEN do the right thing, but by then the damage has been done.

    One of the biggest mistakes was trying to take these cells to 4.2v for an "initial" charge, which is NOT NEEDED, and came from a very old ancient translated document which should be burned and forgotten.

    I'm pretty sure that this system at Bourns College in U.C. Riverside uses the larger format.
    http://www.engr.ucr.edu/news/2014/2014-03-14.html

    My own little lifepo4 test-bed pales in comparison! :)
  • PNjunctionPNjunction Solar Expert Posts: 762 ✭✭✭
    Re: Battery Management systems (BMS) for LiFePO4 batteries in real situations
    karrak wrote: »
    Using LiFePO4 batteries in off-grid applications is still in its infancy and in my humble opinion there is still little commercial equipment that integrates well with these batteries in this application so one is forced to make "cobbled up" systems. Hopefully this situation will change. I am quite happy to make "talking points" if it furthers the development of this CO2 reducing technology.

    By running conservatively, you can run without a cobbled up system, and use standard componentry. Just look for adjustments that mimic a lower voltage requirement, like those used with GEL.

    For example, the Samlex series of AC chargers I reviewed here can be set to 14v, and does a very nice job on my GBS lifepo4 batteries. One dip-switch to 14v and I was done. There are other programmable ac chargers that have 14-14.1v limits. Good enough for our purposes. Float (unnecessary unless there is a parasitic load), is at 13.6v, which produces no current after a full charge so could be considered benign. I just pull it since it is not needed, but don't freak out about it.

    My lvd / hvd? Powerwerx ITS-12. While designed for lead, the 12.7 / 12.8v timed cutoff (user selectable timer delay) does a great job. In addition, it has an 11.2v hard-dropout, and a 16v hard dropout at the high end. While these "hard" limits are not ideal for lifepo4, they DO prevent going catastrophic, but yes, 16v is definitely damaging. But it prevents garage catastrophe even though the cells may be spent. In use, the timer is what is activated well prior to the hard lvd, and my charger / controller at 14v is the soft hvd. Works well, 35A continous, and costs me 150ma in operation - 5ma or so when timed out.

    Tecmate-Optimate lifepo4 charger TM-291. Does a GREAT job.

    Single-cell 3.7v chargers - batteryspace has them. 0.8 to 40A. Great for those that don't need constant balancing, but want to charge each individually at least at first:
    http://www.batteryspace.com/smartchargerfor32v-96v1-3cellslifepo4batterypackscells.aspx

    Need a bench-supply / charger instead? Use the "EX" series recommended:
    http://www.mastechpowersupply.com/dc-power-supply/switching-power-supply/volteq-power-supply-hy3010ex-30v-10a-over-voltage-over-current-protection/prod_70.html

    From big to small, you just need to look around and by being conservative, can easily charge and maintain lifepo4 without a mess. For larger stuff, then BlueSea and other big name / big system stuff can be used. Just match the voltage to your application.

    Sure, other than the Optimate, these things wouldn't be handed to my next door neighbor. However, that is probably not our lurker-demographic. :)
  • PNjunctionPNjunction Solar Expert Posts: 762 ✭✭✭
    Re: Battery Management systems (BMS) for LiFePO4 batteries in real situations

    How about an example of simple conservatism with no external balancing:

    Battery: 40ah GBS 12v. NO external circuitry.
    Pre-Charge: Initially charged each cell to 3.65v with single-cell charger and stopped.
    Normal cycle charge: Discharged 10a, and recharged with Samlex 12v charger set to 14v. Balance was slightly out among each cell but none over 3.6v.
    Load: 2A continuous for 16 hours. Approximately 80% DOD
    Protection circuit: Powerwerx ITS-12, with 4 minute timeout at 12.7v

    Result after 16 hours (32ah withdrawn)
    Still under load here just before disconnect:

    Cell1: 3.178v
    Cell2: 3.177v
    Cell3: 3.181v
    Cell4: 3.172v

    If this were an EV under accelaration pulling hundreds of amps, I'd be worried and do a bottom-balance instead. But at 0.05C, this kind of imbalance is not an issue.

    3.15v is what I would consider about 90% DOD. I easily achieved 80% without any major imbalance from nothing more than an initial charge to 3.65v, and later using a standard lead acid charger limited to 14v for my normal operations. (or solar CC limited to 14v)

    Note: taking cells beyond 80% DOD should be recharged with limited current until about 3.2v per cell is reached. The Optimate noticed this, went into a current limited "safe-mode" recharge. Once out of the limited current recharge mode, I pulled it, and let the Samlex finish the job. Those using larger systems will want to do much the same with whatever gear they are using if going beyond 80%. Moral: just don't go beyond 80% and you'll keep things simple.

    I'm satisfied.
  • vtmapsvtmaps Solar Expert Posts: 3,738 ✭✭✭✭
    Re: Battery Management systems (BMS) for LiFePO4 batteries in real situations
    PNjunction wrote: »
    How about an example of simple conservatism with no external balancing:
    <snip>
    If this were an EV under accelaration pulling hundreds of amps, I'd be worried and do a bottom-balance instead. But at 0.05C, this kind of imbalance is not an issue.

    Where do you draw the line? I mean, how large can the discharges be before you recommend bottom balancing?

    I ask because one of the attractions of LiFePO4 (and AGM LA batteries) is the ability to draw large currents with minimal voltage sag. I find that my present FLA bank (380 ah @ 24 volts) stores adequate kwh for my needs, but that when I'm at 80% SOC (especially with cool batteries) I can't draw 500 watts for 30 minutes without tripping my LVD (set to 24 volts).

    With LiFePO4 (or AGM) batteries, I could probably be happy with 300 ah or even less @ 24 volts... but of course the lower my ah capacity is, the higher the discharge rate is for a given wattage draw. In a household RE system, what sort of peak discharges would require a BMS? Would the temperature of the batteries affect your recommendation for or against a BMS?

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • karrakkarrak Solar Expert Posts: 326 ✭✭✭✭
    Re: Battery Management systems (BMS) for LiFePO4 batteries in real situations
    vtmaps wrote: »
    Where do you draw the line? I mean, how large can the discharges be before you recommend bottom balancing?

    I can't see any reason for bottom balancing in off-grid applications.
    I ask because one of the attractions of LiFePO4 (and AGM LA batteries) is the ability to draw large currents with minimal voltage sag. I find that my present FLA bank (380 ah @ 24 volts) stores adequate kwh for my needs, but that when I'm at 80% SOC (especially with cool batteries) I can't draw 500 watts for 30 minutes without tripping my LVD (set to 24 volts).

    With LiFePO4 (or AGM) batteries, I could probably be happy with 300 ah or even less @ 24 volts... but of course the lower my ah capacity is, the higher the discharge rate is for a given wattage draw.
    Here is some data hot off the press from my battery monitoring system which should give you some idea of how an 18 month old set of LiFePO4 works under load.

    [alltime]
    timestamp = '20141001164506 '
    maxvoltages = [3.395, 3.405, 3.395, 3.41, 3.398, 3.398, 3.418, 3.404, 27.24]
    minnoload = [3.254, 3.255, 3.253, 3.256, 3.256, 3.254, 3.261, 3.256, 26.07]
    minvoltages = [3.144, 3.15, 3.13, 3.14, 3.12, 3.139, 3.153, 3.13, 25.13]
    deltav = [0.005, 0.081, 0.081]
    amps = [-34.2, 192.3]
    ah = [2.7, 111.14, 151.62, 4016]

    The 'alltime' means just for the last few days as I haven't had this version of the data logging going for very long.
    'maxvoltages' is the maximum voltage measured over the time period of each cell and the pack as a whole
    'minnoload' is the minimum voltage measured over the time period of each cell and the pack as a whole with a load of less than 10A
    'minvoltages' is the minimum voltage measured over the time period for each cell and the pack as a whole
    'deltav' is the difference in voltage between the highest and lowest cell in mV measured over the time period, the first reading is the lowest difference, the second in the maximum difference with a load less than 10A, the third is the maximum difference
    'amps' is the maximum charge and maximum discharge current measured over the time period, negative is charge
    'ah' is the DOD of the battery in Ah measured over the time period, my pack has a nominal rating of 360Ah, the first figure is the lowest DOD, with 0 being 'full', the second figure if the average DOD in this case over the last few days, the third figure if the maximum DOD 360 should be empty, the forth figure is the number of samples the average is taken over, there is a minute between samples.

    In this instance 'minvoltages' would be the voltages under the load of 192A. I would estimate that the battery would have been at about 70% SOC when this occurred, the resting battery voltage would have been around 26.2 volts and the battery temperature about 18 degrees C. The load would have been on for around 3-5 minutes.
    In a household RE system, what sort of peak discharges would require a BMS? Would the temperature of the batteries affect your recommendation for or against a BMS?

    I probably disagree with PNJunction on this, but I can't see how you can guarantee that a set of cells is going to stay in balance and or there won't be some fault or problem in a system that is being charged and discharged on a daily basis for ten years that will not drive one or more cells out of the proper operating voltage range. I suppose it is a personal choice as to how much risk you want to take and therefore what sort of BMS you want to install. I think even a simple Battery Cell Monitor and alarm is very cheap insurance.
    Off-Grid with LFP (LiFePO4) battery, battery Installed April 2013
    32x90Ah Winston cells 2p16s (48V), MPP Solar PIP5048MS 5kW Inverter/80A MPPT controller/60A charger, 1900W of Solar Panels
    modified BMS based on TI bq769x0 cell monitors.
    Homemade overall system monitoring and power management  https://github.com/simat/BatteryMonitor
     

  • Blackcherry04Blackcherry04 Solar Expert Posts: 2,490 ✭✭✭
    Re: Battery Management systems (BMS) for LiFePO4 batteries in real situations
    vtmaps wrote: »
    I find that my present FLA bank (380 ah @ 24 volts) stores adequate kwh for my needs, but that when I'm at 80% SOC (especially with cool batteries) I can't draw 500 watts for 30 minutes without tripping my LVD (set to 24 volts). --vtMaps
    24 V is a very conservative LVD setting for a FLA deep cycle set of batteries under load. It is also a clue to your battery health and capacity. The question is what voltage do they recover to after the load is dropped, to me that's the real dod. There is always a drop in the cabling and the voltage the Inverter sees, even that is not necessary a exact 24v. LiFePO4 can be scarey as they drop without any warning.

    It's funny, I can have a set of GC-2's and a set of L-16's and the L-16 will have a larger voltage sag than the GC-2's with the same load. it's just the nature of the beast and their ability to release current, the question is ? what does the sag hurt ? Does the Inverter have the ability to compensate for the drop, doesn't seem to be a problem. In some applications it could be, I don't see it in ours.
  • vtmapsvtmaps Solar Expert Posts: 3,738 ✭✭✭✭
    Re: Battery Management systems (BMS) for LiFePO4 batteries in real situations
    It's funny, I can have a set of GC-2's and a set of L-16's and the L-16 will have a larger voltage sag than the GC-2's with the same load.

    That's my point... I have L-16 and the voltage sag is bothersome at times.
    24 V is a very conservative LVD setting for a FLA deep cycle set of batteries under load.

    I agree, but Cariboocoot recommends the LVD be set at battery nominal.
    I wouldn't want to change the LVD without moderator permission :p

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • Blackcherry04Blackcherry04 Solar Expert Posts: 2,490 ✭✭✭
    Re: Battery Management systems (BMS) for LiFePO4 batteries in real situations
    vtmaps wrote: »
    That's my point... I have L-16 and the voltage sag is bothersome at times.
    --vtMaps
    I don't know exactly how LiFeP04's react to a the load and again it's the cabling and connections that that can add a little here and there that add up. Thats one reason that all these small cells with parallel connections would worry me that there could be a lot of sag. One clue is that the lugs they use are for #4 cable. A .30 - .50 v sage is about what a 500w load does to mine depending on the SOC and then there can be some recovery as the load stabilizes.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Battery Management systems (BMS) for LiFePO4 batteries in real situations
    vtmaps wrote: »
    That's my point... I have L-16 and the voltage sag is bothersome at times.



    I agree, but Cariboocoot recommends the LVD be set at battery nominal.
    I wouldn't want to change the LVD without moderator permission :p

    --vtMaps

    Don't get smart.

    LVD at system nominal works in most cases because it prevents the batteries going below 50% DOD for certain. Some systems have no other method of control for this.

    As always, settings need to be tailored to the individual application. An Outback inverter needs to go below LVD for a certain (programmable) period of time before it will shut off. If you have a load that regularly dips below this point for an extended time you can test the sag and length and adjust accordingly.

    Although you should also recognize that means you are regularly running the batteries down near 50% SOC and that probably means you have insufficient capacity for the loads. Keep in mind that recommended average daily DOD is 25%, so nearing 50% on a regular basis indicates you need more battery.

    And yes the small GC2's are better at supplying instantaneous current than the L16 size.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Battery Management systems (BMS) for LiFePO4 batteries in real situations
    I don't know exactly how LiFeP04's react to a the load and again it the cabling and connections that that can add a little here and there that add up. Thats one reason that all these small cells with parallel connections would worry me that there could be a lot of sag. One clue is that the lugs they use are for #4 cable.


    Regardless of battery chemistry, fewer larger capacity cells is better than more small capacity ones for exactly the reasons BC04 mentions; you just do not want all that complexity of wiring (and monitoring if used).

    The more things there are, the more things there are to go wrong.
  • vtmapsvtmaps Solar Expert Posts: 3,738 ✭✭✭✭
    Re: Battery Management systems (BMS) for LiFePO4 batteries in real situations
    LVD at system nominal works in most cases because it prevents the batteries going below 50% DOD for certain. Some systems have no other method of control for this.
    <snip>
    Although you should also recognize that means you are regularly running the batteries down near 50% SOC and that probably means you have insufficient capacity for the loads.

    My present battery bank has never been as low as 50% SOC. Three times in 4 years it has been to 60% SOC. Your advice to set the LVD to battery nominal works as a fail-safe to make sure I never do get to 50%. A very good failsafe system if the voltage sag isn't too great... my L-16's have a bit more sag than I expected when I commissioned them (they are my first L-16).

    I think I recall reading that some inverters with integrated battery monitors can protect the battery by considering both SOC and voltage.

    I could set the LVD lower, but I'd just as soon start up the honda if I need 500 watts for 30 minutes when at 80% SOC, especially in the winter when the cool batteries have more sag.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • Blackcherry04Blackcherry04 Solar Expert Posts: 2,490 ✭✭✭
    Re: Battery Management systems (BMS) for LiFePO4 batteries in real situations

    I usually have voltage problems on day 2 from 5:00 - 8:00 with large loads. ( A/C and Coffee pot ). I will start the Honda after the wife wakes up. The A/C is 500w and the Coffee Pot 1100W , I'll just cut the A/C off while I make coffee. I can get a big sag on either the 24v or 12v in this situation ( .30 to .50 v ), I would expect that with any battery technology. The rest of the 22 hrs it's less than 100w max. It's hard to size a system to run everything, you learn to adapt.

    I look at sag a couple different ways. Sag from cabling, connections and Inverter conversion disappear quickly as the load is dropped. Current sag is a little slower to recover the Voltage, batteries in series are fairly fast, but batteries with a lot of paralleling is much slower. The first battery on the positive side will drop much lower than the others and takes longer for all the voltages to adjust as they flow from the higher voltage batteries.. At least thats my experience.
  • inetdoginetdog Solar Expert Posts: 3,123 ✭✭✭✭
    Re: Battery Management systems (BMS) for LiFePO4 batteries in real situations
    I look at sag a couple different ways. Sag from cabling, connections and Inverter conversion disappear quickly as the load is dropped. Current sag is a little slower to recover the Voltage, batteries in series are fairly fast, but batteries with a lot of paralleling is much slower. The first battery on the positive side will drop much lower than the others and takes longer for all the voltages to adjust as they flow from the higher voltage batteries.. At least thats my experience.
    For the geometry of parallel batteries (there should be no "first" one) take a look at the diagrams on this site:
    http://smartgauge.co.uk/batt_con.html
    The rest of the site is also very helpful on a lot of off-grid subjects.
    SMA SB 3000, old BP panels.
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