Lithium iron phosphate battery (LiFePO 4) battery for Solar applications ?

Re: Lithium iron phosphate battery (LiFePO 4) battery for Solar applications ?

zoneblue wrote: »

Do you have any documentation on your system at all? Be interesting to see it.

I am not sure what info you would like. My signature gives a list of the equipment I am using.

Here is a brief description, if you want any more information let me know.

I have six 190W, around 35 volts nominal peak power output panels, four hooked up to separate PWM buck circuits and two connected via relay to my battery. All this is controlled by a 68HC12 microcontroller running Forth based software that I have written. Most of this has been at a prototype stage for some time now, maybe i will eventually get around to designing a proper PCB for this...

As a simple visual monitor of the system I have a JLD404 AH and battery voltage display which lives in our living area that has been modified to reset the AH counter when the battery voltage reaches 27.6 volts (3.45 v/cell). I have calibrated the JLD404 to read 0.1A on the current display with zero current going to or from the battery. This 0.1A (2.4AH/day) is enough to take care of battery losses.

For the "BMS" I have a Cellog8 which has been modified to get its power via a switch mode supply from the battery as a whole. The Cellog8 will generate an audible alarm if any of individual LiFePO4 cells goes out of the range 3.6v-3.0v.

I do the cell balancing manually.

On the output side there is the Latronics 4kW inverter, I also run a 24volt power bus around the building which runs all the low voltage equipment like computers, lights etc. This means the inverter is in standby mode for allot of the time which saves in excess of 10AH per day.

This system has worked well for me, the Cellog8 would be the bare minimum monitoring i would recommend for any system with LiFePO4 cells. I would not recommend a simple setup like this for anyone without some electronics knowledge and who is not prepared to monitor the system on a regular basis

Re: Lithium iron phosphate battery (LiFePO 4) battery for Solar applications ?

jonr wrote: »

Is the once per week higher charge done to reset a coulomb counter?

You are correct. One positive side effect of doing this higher charge once a week is that if there is a period of cloudy weather the system may take several days or weeks to get to the higher charge level which will mean that the battery will potentially store more power during cloudy periods which is just what you want.

Re: Lithium iron phosphate battery (LiFePO 4) battery for Solar applications ?

karrak wrote: »

[*]Put some normal (not Schottky) power diodes between the controller and the battery so the controller will see a higher battery voltage than it really is, half a bridge rectifier would work well for around 0.7 volt or full bridge for a 1.4 volt difference. Anyone see any issues with doing this?

I'm about to grab some diodes today for my smaller fixed ac chargers to see if I can tame them with the KISS method.

For controllers that have no temp-comp, then maybe the diode method would work, but I'm only thinking small scale. If they do incorporate temp-comp, in my hot climate that actually helps, but obviously is not an ideal way of fudging it.

Kind of makes me want to get a Genasun lifepo4 controller with the right voltages...

Re: Lithium iron phosphate battery (LiFePO 4) battery for Solar applications ?

PNjunction wrote: »

Picked up the diodes, but it may not be what you want for solar charge controllers.

With a lower voltage overall, my bulk charge is slower even though it will do ok dropping the absorb voltage when it finally gets there. And it may try to hold it there, never dropping to float (which is benign being at a much lower voltage), but it may want to stay there never reaching the controllers true absorb voltage setpoint. We'll see how the charger reacts to that when it gets to absorb. Losing interest in this method fast.

So far the best controller I have on the low end is a Schneider/Xantrex C12 pwm. No temp-comp without the remote sensor. Adjustable voltages, although the pot is VERY sensitive to minute adjustments. Will only hold absorb for a fixed one-hour time limit. Ok. But, if that adjustable pot fails or corrodes it may send my lifepo4's to the junk heap.

Might just end up using a Morningstar prostar-15 pwm set to gel (with massive monitoring) until I can get my act together.

It appears that Genasun's lifepo4 mppt's come with 14.2v standard for nominal 12v systems, but you can custom order the voltage you want.

You can use the RTS port and put a 500K potentiometer instead of a thermistor and control the voltage in a finer range. I think it's the black / yellow & green wires on a rj 11 phone cord. I use it to fake Xantrex fixed voltage algorithm's on their inverter / chargers.

Re: Lithium iron phosphate battery (LiFePO 4) battery for Solar applications ?

karrak wrote: »

You could get around this by placing a Schottky diode in the opposite direction to the voltage dropping diode(s) to power the controller when there is no sun.



When I looked for a 48 volt 30 amp controller fourteen months ago I had allot of trouble finding something that allowed one to set the voltages to the ranges that I wanted, not sure if things have improved.

Sure, having to add circuitry to fool the controller into doing what we want it to do is hardly ideal, but if it means that by doing so we can use a controller that we already have, or we can use a controller that suit all our requirements except that it can't be programed as we would like, is I think a valid reason for doing so. Of course any modifications have to be safe and I wouldn't recommend anyone do this unless they understand what they are doing.

Actually if you try the RTS trick you'll be more than pleased with the outcome. It gives you a remote dongle you can make really fine adjustments with.

Re: Lithium iron phosphate battery (LiFePO 4) battery for Solar applications ?

Using "-0.005 V per ºC per 2 V cell", I get 3.41V per Li cell as the lowest that temperature compensation would bring a 14.4V charger. So I agree, you also need an adjustment to bring the 14.4V down.

Re: Lithium iron phosphate battery (LiFePO 4) battery for Solar applications ?

PNjunction wrote: »

The Iota chargers look suitable too, *without* the IQ4 module, and the dual-voltage jack allows for 14.2v, so maybe for some that won't be too high, although I'd prefer a bit lower.

The IOTAS have a a 10-20 turn potentiometer inside that will give you plenty of voltage adjustment. I never tested the low end voltage, as you said the get it without IQ-4. The high end can be cranked up to 15.5 v with the plug in the jack.

Re: Lithium iron phosphate battery (LiFePO 4) battery for Solar applications ?

I have, and do not do active balancing for a few reasons. I'm kind of scared I brought this up, because it usually provides the seed for an endless balancing thread, bms / no bms, etc. HVC and LVC are different from balancing, and yes, those should be used - at least good monitoring if nothing else.

The major problem with balancing threads for those wishing to use them as house banks, is that they come across 3 different categories of use-patterns, and nearly none discuss the 4th, which is us:

* Category 1: Electric Vehicle users running hundreds of volts, along with 10's of cells, usually in a wide variety of series parallel arrangements. Depending on the owner, they will either top-balance, or bottom-balance the bank, along with extensive cell-level monitoring. Top balance leaves the bottom ragged. Bottom balance leaves the top ragged. Nobody should be doing active balancing in the middle of the curve, nor should they be doing both bottom and top balancing. But that's not us.

* Category 2: Model Radio Control users running a different chemistry than we are, hammering the batteries with high discharge, and quality and accuracy among hobby-chargers vary - not to mention quality of the cells themselves. Quite frequently run overvoltage, or deep into the discharge knee, and upon recharging, hammer the depleted cells too quickly, raising the internal impedance, in addition to bad balance events.

* Category 3: DIY homebrewers (typically ebikes) using hastily constructed packs made out of duct-tape, counterfeit cells, used cells, or abused cells as they get to know what is proper and not when charging. Bad balance issues ensue, also hastened by massive current draws and subsequent high current recharge when well into the discharge knee. Part of the problem here is that they may also be following initial manufacturer specs from 2007, not knowing that they rapidly changed their specs.

That leaves us. Perform a simple balance at first on quality prismatic cells like GBS, CALB, or Winston. Then we don't hammer them with high currents. Most of us will be far less than 0.5C, which is about nominal for Lifepo4. We stay out of the charge discharge knees, not seeking the last drop in capacity to get us home or on the ground. Unless you have a vendor issue, they tend to stay balanced for 100s of cycles. It is wise to use LVC cutoffs, but if one builds the system right, monitoring may suffice. Checking with a voltmeter in this case, is somewhat akin to taking care of a flooded cell with a hydrometer. This is something most don't do every charge cycle with fla. Neither do they equalize every charge cycle. We can do much the same here.

Active or passive balancing, in our kind of operation, may easily be over-engineered and run the risk of additional points of failure with the external balance system, be it blinky-led, or more advanced systems. Basically, for EV's, RC modeling, abusive or low-quality environments, then yes, this would be beneficial.

The thing that ends the endless talk is actual hands on. That is what I've done (for a few years now actually starting with A123 cylindrical cells). So far, I've found that my usage puts me into the 4th category. Since my relatively low current 12v nominal needs vary from about 20ah to 100ah, it is a LOT easier to keep 4 cells in series balanced after an initial balance. I'm doing house-bank power, not motive power.

Grab 4 of these from the vendors mentioned earlier and check it out.

Re: Lithium iron phosphate battery (LiFePO 4) battery for Solar applications ?

jonr wrote: »

If you have high speed, continuous logging and it shows that the batteries always stay within limits with your current usage, then I agree. If you are only checking balance now and then, then you have no idea what is happening on a short-term basis.

Sure we do. In our application, as long as you have performed an initial balance, and are not using junk, then you'll just ride the flat part of the discharge curve until you hit the knee. Proper planning prevents you from hitting that steep over-discharge curve. Or you can run blind and use an LVC, which if you are hitting it frequently, means you didn't do your homework in the first place, or obviously suffered a catastrophic event. Risks you take with any battery chemistry.

When a motor starts up, my inverter draws a lot of current.

No problem! The typical prismatic lifepo4 can handle a constant current of 1C, with little Peukert effect. It's there, but not nearly as bad as the best pure-lead agm like my beloved Odysseys. However, most manufacturers recommend running at 0.5C or less. They can even withstand peaks of 3C for limited amounts of time. Cylindrical cells like A123 can do even better, but in a house bank situation you'd be paying for a feature you'll not really need. High current operations at 0.5C or less is not a balance issue in the flat part of the curve. High current at the ends of the discharge knee, yes certainly. I asked my doctor why it hurts when you go there, and he said to just stop doing that.

Charge controllers (and inverters) already have complicated electronics that could fail. It takes very little additional circuitry to do per cell balancing.

True. But consider that some per-cell blinky-led boards have a tolerance of plus/minus 0.05v. At the extreme ends, that could dutifully take my perfectly balanced battery done with nothing more than a voltmeter out of balance by 0.10v right off the bat.

Put it this way - in our house-bank application, with people that know the difference between a hydrometer and a turkey-baster, a simple initial balance, mild usage out of the knees, and some sanity checks now and then you should be good. If one wants to take it further for peace of mind, knowing the risks of additional POF, then go ahead. I know I wouldn't set my grandmother up with a Lifepo4 bank and hand her my Fluke.

This also brings up a good point - what is balance? .01v as commonly seen?

.01v done at a high soc (like 3.45v after 1 hour rest) is certainly admirable. But that can be obsessive and something no blinky boards I've seen try to achieve with tolerance ratings thrown in. That IS something to obsess over if you are an RC modeler using a different chemistry such as LiCo02, where .01v over the top can start to put the electrolyte into a thermal runaway process.

.05V cell differences would be my indicator that I'd want to start looking into a rebalance with LiFePo4. Since I'm not charging to the top of the SOC curve, I can afford a little imbalance, and obviously I'm not running down into the dregs either. If I'm lazy enough, I might wait until 0.1v.

Thing is, instead of speculating about it, I'm doing it, and finding out that LiFePo4 when operated in a house-bank situation, is actually kind of boring, not the overcomplicated mess that other applications seem to need.

Re: Lithium iron phosphate battery (LiFePO 4) battery for Solar applications ?

If you have a set of series connected batteries and they are all at the same voltage, then you add some number of watt hours, then measure them again, all the voltages will be different (to some extent, depending on how much internal variation they have). You can then discharge them and find that they are back to being the same voltage.

The conclusion - being balanced at some points means little about them being balanced at other points. They might be "close enough", - or they might not me.

Re: Lithium iron phosphate battery (LiFePO 4) battery for Solar applications ?

I'm confident enough that larger capacity cells are on their way, but thought I'd throw in another note about balance to help complete the subject .. I knew I shouldn't have brought it up..

The general rule of thumb for balance when building large capacity batteries that can't be constructed from single cells, is to build up your capacity first by grouping in parallel, and then series connecting the groups to get your voltage.

Ie, if you were trying to construct a 200ah , nominal 12v lifepo4, but only had 100ah cells available, then you would create groups of two 100ah in parallel to get each group to 200ah, and then series connect these groups together to reach your desired voltage, which in this case would be 4 series connected groups. Yes, balance them individually first - and when connected this way, they tend to stay balanced.

While my needs are still met by using single cells with the capacity I need, I have practiced doing it the wrong way with sets of A123 cells, such as merely paralleling two new and freshly balanced 4S packs together. Bzzt. They quickly went out of balance that way. Once reconfigured for capacity first, and then series connected for voltage, all was well and the battery stayed balanced.

I look forward to the day when I actually have the need to do this with huge prismatic cells. That would mean I now own a yacht or rv.

Re: Lithium iron phosphate battery (LiFePO 4) battery for Solar applications ?

PNjunction wrote: »

...top-balance in the upper charge knee (to protect the cells from overvoltage in badly balanced cells), or in some cases with EV's to do bottom balance to ensure that all cells hit the lower discharge knee at the same time. But you can't do both, nor should you do it in the middle of the flat part of the curve.

Continuous balancing is exactly what the best balancing circuits do. They efficiently move charge from the highest cells to the lowest ones and end up balanced at both the top and the bottom.

http://www.artechhouse.com/static/sample/Barsukov-491_CH04.pdf

Re: Lithium iron phosphate battery (LiFePO 4) battery for Solar applications ?

feedhorn wrote: »

Since most RV's and Yachts come with an on-board generator you can get by with a much smaller lithium battery bank by taking advantage of their very fast charge rate.

Certainly agree there. If you can't / don't want to swing LiFePo4, you can get close with "tppl" or pure-lead agm's like those from Enersys/Odyssey, or perhaps even Concorde with their ability to take on huge currents. But then again, you are carrying twice the weight, perhaps even more when you consider that one usually doesn't want to take a Pb battery below 50% DOD, but with LiFePo4, they are rated at 80% DOD for cycle life, and usually at 0.3 to 0.5C discharge, which is quite hefty!

So yes, smaller in overall capacity, weight and size are just 3 of the valuable charecteristics of LiFePo4 if you factor those in correctly.