Have Lithium Ion batteries come of age for off-grid systems?

2

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  • karrakkarrak Posts: 307Solar Expert ✭✭✭✭
    The efficiency is great but again  a few extra solar panels and that issue is moot. It really comes down to maintenance vs cost vs reliability so far.
    To me some of the main advantages of LFP batteries are the hidden advantages. In particular the ability to accurately know how much power you have left in your battery even when it hasn't been full for over three weeks. It has made it easy to operate our system without a generator. If you want the security of a generator  you can get away with a smaller one than you would need with a lead acid battery as there are no minimum restrictions on charge rates and you don't have to charge up to 100%, you just have to put enough charge in the battery to last you till the sun comes up.

    Without a BMS,  I remember a client in Baja who got hit with lightning after a hurricane. His charge controller shorted the PV in to output and for 3 days the battery had close to full PV in on it. It just boiled off the water in that case.

    Yep, that is the fault that worries me as well. I went to the trouble of having a relay between the switching FETs and the solar panels in my own MPPT controller for just such an eventuality. It might also offer some lightning protection as well, as much as the distance between a set of open relay contacts would provide ...

    I wonder what sort of protection the modern commercial MPPT controllers give one, especially the 150V and higher units. The thought of 150Vdc + at the output terminals of the MPPT controller does not appeal to me.

    Simon
    Off-Grid with LFP (LiFePO4) battery, battery Installed April 2013
    32x90Ah Winston cells 4p8s (24V), 4kW Latronics Inverter, 1160W of Solar Panels, homemade MPPT controller
    Homemade BMS https://github.com/simat/BatteryMonitor
     

  • karrakkarrak Posts: 307Solar Expert ✭✭✭✭
    Raj174 said:
    Sure Simon, here's a link to where I bought it. I think they are based in Hong Kong.

    http://www.dhgate.com/product/high-accuracy-battery-management-system-bms/375809910.html

    This is the manual.
    Thanks for the information, that is a nice looking unit. When you get it I would be interested to know how it performs. One feature that would be nice that this unit does not appear to have is the ability to store and download battery data to a computer.

    I do like the ability of being able to check my battery via the web when away from home and being able to access past battery data to create the graphs that I have published earlier in this thread.

    Simon

    Off-Grid with LFP (LiFePO4) battery, battery Installed April 2013
    32x90Ah Winston cells 4p8s (24V), 4kW Latronics Inverter, 1160W of Solar Panels, homemade MPPT controller
    Homemade BMS https://github.com/simat/BatteryMonitor
     

  • Dave AngeliniDave Angelini Posts: 4,008Solar Expert ✭✭✭✭
    With a flooded battery one can measure SG at anytime so SOC is not a valid point to me. I had my last generator 10 years ago and so I get what you are saying but I really think it gets down to maintenance, cost, and is this going to be as reliable as what I and my 97 offgrid clients have now. Several of them are wanting to ditch the maintenance and are happy about the Schneider all in one system being an option. If you are charts and graphs man it is all there, anywhere anytime.

     I would much rather just see a green light and go outdoors :)

    The MPPT failure was a brand that is no longer making that model. I did the beta testing for the Schneider 600v mppt and the failure mode and effects analysis (FMEA) I saw could not do that failure. Same with the mppt-150.  There is a way that if they were flooded with water it could happen but this is true with almost anything with an input and output.
    "we go where power lines don't" Sierra Mountains near Mariposa/Yosemite CA
     http://members.sti.net/offgridsolar/
    E-mail [email protected]

  • mike95490mike95490 Posts: 7,901Solar Expert ✭✭✭✭
    karrak said:....................................I wonder what sort of protection the modern commercial MPPT controllers give one, especially the 150V and higher units. The thought of 150Vdc + at the output terminals of the MPPT controller does not appeal to me.
    Simon
    They should be robust enough to not short the PV input to the battery output, there's a transformer and switching FET's in the middle, and without it functioning well, nothing would get through, unless a catastrophic event happened. (full of conductive slag from a lightning strike)
    But they still need voltage surge protection, especially on the PV input side.  Either Delta or Midnight (or both) Surge Protection.
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

  • BB.BB. Posts: 27,882Super Moderators, Administrators admin
    I am not sure that 150 volt and less charge controllers use transformer isolation. Typical buck switcher is a "switch" (FET/Transistor) between the the array and an inductor (energy storage device)--The inductor (more or less) connects to the battery + terminal.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • mike95490mike95490 Posts: 7,901Solar Expert ✭✭✭✭
    BB. said:  I am not sure that 150 volt and less charge controllers use transformer isolation. Typical buck switcher is a "switch" (FET/Transistor) between the the array and an inductor (energy storage device)--The inductor (more or less) connects to the battery + terminal.    -Bill
    Years of mil and space design, and I forget about the shortcuts !
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

  • Dave AngeliniDave Angelini Posts: 4,008Solar Expert ✭✭✭✭
    mike95490 said:
    BB. said:  I am not sure that 150 volt and less charge controllers use transformer isolation. Typical buck switcher is a "switch" (FET/Transistor) between the the array and an inductor (energy storage device)--The inductor (more or less) connects to the battery + terminal.    -Bill
    Years of mil and space design, and I forget about the shortcuts !
     I got to see a few satellite charging systems in my day.  The Keyhole series is going to have a few new "eyes" soon.

    A United Launch Alliance Delta 4-Heavy rocket will deploy the massive payload for the U.S. National Reconnaissance Office in a mission known simply as NROL-71 from Vandenberg’s Space Launch Complex 6.

    They and others can protect from a solar input to output short!
    "we go where power lines don't" Sierra Mountains near Mariposa/Yosemite CA
     http://members.sti.net/offgridsolar/
    E-mail [email protected]

  • MangasMangas Posts: 547Solar Expert ✭✭✭✭
    edited February 2017 #39
    Interesting article.

    Until they can get the cost equal to or less than my FLAs' with the same reliability and capacity demonstrated over one battery lifecycle, I wouldn't risk making the technology change on a battery bank as large as ours $$$.

    Dave's point about the importance of same manufacturer networked communicable inverter, cc, ags and bridge components.

    Looking at say 8 years until li-ion's are economically mainstream? Sooner?



    Ranch Off Grid System & Custom Home: 2 x pair stacked Schneider XW 5548+ Plus inverters (4), 2 x Schneider MPPT 80-600 Charge Controllers, 2 Xanbus AGS Generator Start and Air Extraction System Controllers, 64 Trojan L16 REB 6v 375 AH Flooded Cel Batteries w/Water Miser Caps, 44 x 185 Sharp Solar Panels, Cummins Onan RS20 KW Propane Water Cooled Genset, ICF House Construction, all appliances, Central A/C, 2 x High Efficiency Variable Speed three ton Central A/C 220v compressors, 2 x Propane furnaces, 2 x Variable Speed Air Handlers, 2 x HD WiFi HVAC Zoned System Controllers
  • Marc KurthMarc Kurth Posts: 446Solar Expert ✭✭✭✭
    Changes are coming fast and I have no doubt that Lithium battery technology is going push lead-acid designs into the back ground. I see the potential of it being a multi year process to gain acceptance. Right now, people tend to think about Lithium battery longevity and cycle life of their cell phones, laptops and other portable devices.

    Once they see them deployed in off grid systems at a low enough cost, for enough years, acceptance will come quickly.
    I always have more questions than answers. That's the nature of life.
  • MangasMangas Posts: 547Solar Expert ✭✭✭✭
    Agree Marc.
    Ranch Off Grid System & Custom Home: 2 x pair stacked Schneider XW 5548+ Plus inverters (4), 2 x Schneider MPPT 80-600 Charge Controllers, 2 Xanbus AGS Generator Start and Air Extraction System Controllers, 64 Trojan L16 REB 6v 375 AH Flooded Cel Batteries w/Water Miser Caps, 44 x 185 Sharp Solar Panels, Cummins Onan RS20 KW Propane Water Cooled Genset, ICF House Construction, all appliances, Central A/C, 2 x High Efficiency Variable Speed three ton Central A/C 220v compressors, 2 x Propane furnaces, 2 x Variable Speed Air Handlers, 2 x HD WiFi HVAC Zoned System Controllers
  • jimmyazjimmyaz Posts: 114Registered Users ✭✭
    Have anyone actually try to use one of the Tesla Model S module with the current standard 24V/48V inverter?  Doesn't work? because of the lower nominal voltage rating?

    http://www.ebay.com/itm/Tesla-Model-S-battery-module-24V-250Ah-5-3kWh-444-Panasonic-18650-3400mAh-/262333679871?hash=item3d144e48ff:g:OSkAAOSwh-1W5acD&vxp=mtr

    Fully charge is 25volts, recommended to stay at 80% SOC, meaning 24-24.5 max.  Say discharge to 50%, wonder if the voltage would drop too low for the Inverter causing the Inverter to shut off too soon?

    I was thinking to get two and turn it into a 48V 250AH pack = 12kw.

    Any input guys?





  • HorseflyHorsefly Posts: 310Registered Users ✭✭✭✭
    Doesn't look like that EBay listing includes a BMS. I don't think I would mess with it without a BMS.
    Off-grid cabin: 6 x Canadian Solar CSK-280M PV panels, Schneider XW-MPPT60-150 Charge Controller, Schneider CSW4024 Inverter/Charger, Schneider SCP, 4 x Vmax XTR12-155 12V, 155AH batteries in a 2x2 24V 310AH bank.
  • jimmyazjimmyaz Posts: 114Registered Users ✭✭
    Horsefly said:
    Doesn't look like that EBay listing includes a BMS. I don't think I would mess with it without a BMS.
    Let put that BMS topic on a side for now and not arguing over whether or not to use a BMS or not.

    Let discuss about norminal voltage and capacity, amp draw, voltage drop vs standard 24/48v inverter first.


  • PNjunctionPNjunction Posts: 762Solar Expert ✭✭✭
    edited February 2017 #45
    Jimmyaz - Ok, let's review the basics from a single-cell standpoint then.  You can refine it from there and do the math for your desired series connected voltages.  I'll divvy this up into several messages to keep things from going off the rails. :)

    First - know the differences between your lithium chemistries:

    LFP, aka LiFeP04 is the *only* lithium chemistry that is a "nominal 3.2v" cell.  Don't charge with voltages any higher than 3.6v.

    ANY OTHER lithium chemistry, such as those from the Tesla pack or those commonly found in laptops which are NOT lfp, are "nominal 3.6v" cells.  Don't charge any higher than 4.2v per cell.

    Keep your lithium chemistry voltages in mind!!  LFP = 3.2v nominal, 3.6v max.  If you don't keep this in mind, you'll overcharge and burn up an LFP, and merely undercharge a non-lfp cell.

    LFP has discharge rates of about 1C maximum.  3C in short bursts.  Very suitable for our solar housebanks, which when sized properly for reasonable discharge rates, are about .05C or lower max!  Basically LFP cells laugh at our application, being tickled by very slow charge and discharge rates, provided of course one is making a housebank of suitable capacity.

    Non-lfp cells, can usually take up to about 10C recharge!  Suitable for vehicular charging / regenerative braking and the like.  Sounds great right?  Not really - for us.

    Using / charging Tesla packs has already been discussed elsewhere, such as on the DIYELECTRICCAR forum:

    http://www.diyelectriccar.com/forums/forumdisplay.php/batteries-and-charging-35.html

    BUT, the problem is that while yes, you CAN use these, as long as your voltages are correct for this NON-LFP chemistry, is the fact that under solar housebank use, which never needs a 10C discharge / recharge capability (not to mention USED cells in most instances), only means you'll be paying for a high-rate current capability you'll *never use*.

    Thus, LFP with it's lower nominal voltage per cell (makes it easier to construct multi-cell batteries to your desired voltage), along with it's already high-enough discharge / recharge current capability, makes more financial and DIY simplicity for a solar housebank.

    TBC..

  • PNjunctionPNjunction Posts: 762Solar Expert ✭✭✭
    edited February 2017 #46
    How do I charge a single cell LFP to full?

    Using a cc/cv charger (be it a dedicated LFP single-cell charger, or a programmable bench supply if you have one and know how to use it), set your absorb (CV) anywhere from 3.45 to 3.6v.

    Do NOT use any sort of temperature-compensation!

    3.45v per cell is the minimum needed to achieve a full charge, provided you allow for enough absorb time.

    When absorb current naturally drops to C/20 (aka 0.05C), you are considered fully charged.

    The time it takes to absorb down to 0.05C takes longer at 3.45v than it does at 3.6v.

    How much current to supply?  The *ideal* current is 0.3C.  You can go higher and lower, but for longest life, try not to exceed 0.5C.

    Thing is, a solar housebank with capacity properly sized, will never even reach 0.5C under most conditions!  But yeah, you can do so if you want.

    Like most batteries, a minimum amount of current is needed to recharge, but to do it from an efficiency standpoint, you'll want to have at least .05 to .1C available from your array.

    tbc
  • PNjunctionPNjunction Posts: 762Solar Expert ✭✭✭
    Full charge vs 90% charge:

    An initial FULL charge is needed upon receipt to make sure that all the materials inside are truly active.  If you don't do this, even if charged somewhat from the factory, you can create inactive areas, and thus "hot spots" can form.

    NORMAL USE:  don't always charge to full!

    We've seen that a full charge is an absorb voltage anywhere in the window from 3.45 to 3.60v, but it is the end-amps, or absorb current that determines the full state of charge.  So voltage is not a good determination of full charge, but end-amps is - provided you are between the 3.45 to 3.6v window.

    Thus, to charge to say 90%, don't allow for a full absorb.  It's as simple as that.  You can merely cut off charge when a voltage is reached.  Or you can allow the cell to reach full voltage, but stop absorb half-way.  Heck, you can even stop charge well before 3.45v, but of course your overall capacity is reduced.  You choose your stopping point - based on the knowledge of 3.45 to 3.6v and an end-amp current of .05C is the generic value for a full charge.

  • bill von novakbill von novak Posts: 783Solar Expert ✭✭✭✭
    LFP has discharge rates of about 1C maximum.  3C in short bursts.  Very suitable for our solar housebanks, which when sized properly for reasonable discharge rates, are about .05C or lower max!  Basically LFP cells laugh at our application, being tickled by very slow charge and discharge rates, provided of course one is making a housebank of suitable capacity.

    Non-lfp cells, can usually take up to about 10C recharge!  Suitable for vehicular charging / regenerative braking and the like.  Sounds great right?  Not really - for us.

    This is backwards.  LiFePO4 chemistries are typically good for much higher charge and discharge rates than LiCo chemistries.  This is cell-specific, of course - but generally, LiFePO4 are capable of much higher charge and discharge currents.  You CANNOT push a standard LiCo cell (like the NCR18650B) anywhere near 10C.

    People often think LiCo is capable of more power because of the Lipo packs used by RC enthusiasts.  (Note - Lipo means lithium polymer, which is the electrolyte; LiCo refers to the cathode, which is lithium cobalt oxide.   Most Lipo cells are LiCo.)  However, these are NOT standard cells, and are far more dangerous than LiFePO4.  RC enthusiasts use them because LiCo are higher energy density and that is very important to them - and they are willing to pay the price of the occasional fire.

  • PNjunctionPNjunction Posts: 762Solar Expert ✭✭✭
    edited February 2017 #49
    The holy grail voltage:  3.5v

    A lot of newcomers to LFP are in search of the holy-grail value for the CV voltage, thinking that voltage is the determination of full charge values.  But now you know that it can vary between 3.45 and 3.6v, with the absorb current being the REAL value.

    However, why do we see all over the place to keep your overall CV voltage low, like 3.45 to 3.5v max??

    It is because we are dealing with series-connected cells, each with their own slight differences in overall capacity and internal resistance, and we don't want our neighboring cells to go sky-high when doing a two-terminal (pack level) charge.

    Assuming we bought from reputable sources, new and not used trash, the cell capacities and internal resistance should be close enough for use to merely keep our CV voltage in the low-to-mid area of the 3.45 to 3.6v spec.

    When you choose an overal low-to-mid CV voltage spec, the slight differences in cell characteristics will not be enough to drive one cell over 3.6v.

    Thus, provided you have done an INITIAL individual cell charge prior to placing high quality / matched cells together, a CV charge of 3.5v max gives you a little leeway.

    Ie, if you have a 4S set of cells to make a 12v battery, have done your initial individual charge, for normal use, set your CV to no higher than 3.5/cell, or 14v overall.

    Because you set your CV voltage no higher than mid level, (3.5v per cell) during charge you may see something like this at the end when you reach the canonical .05C current drop in absorb:

    Cell 1: 3.48v
    Cell 2: 3.43v
    Cell 3: 3.52v  << You still are under 3.6 and have a bit of headroom.  This is also the lowest capacity / highest resistance cell.
    Cell 4: 3.49v

    This is perfectly fine!
  • PNjunctionPNjunction Posts: 762Solar Expert ✭✭✭
    edited February 2017 #50
    Re: LFP charge rates:

    Bill - check out the specs on most LFP CALB / GBS etc *prismatics*.  The one's we'll be using.  The max is about 1C, with about 3C bursts.  UNLESS you are talking about Headway LFP cylindricals, which were specially designed for high-rates.  Not that you can find anything but old-stock in headways these days - the little 10-15ah cylindrical types.

    In the end, even the lowliest prismatic LFP cell (CALB / GBS, Winston etc) has capabilities usually far beyond what we need in a solar-housebank battery sized appropriately.

    Brings up a good point - there is no need to go to specialty LFP types.  There are some prismatics in the CALB line for instance, with higher rate capabilities, but we don't need it.  Why pay for that capability if you aren't going to take advantage of it in the real solar housebank world?

  • PNjunctionPNjunction Posts: 762Solar Expert ✭✭✭
    edited February 2017 #51
    How low can I go - my LVD or low-voltage disconnect setting ....

    Generally, you don't want to take any LFP cell below 2.8v resting which equates to a 100% discharge for the most part.

    In practice, that means using a bit more conservative values under load.

    Generally, about a 90% discharge under the typical housebank currents from an appropriately sized battery means not taking the cell below 3.1v under load.  3.2v is a nice conservative value.

    For the 12v battery example above, that means an overall pack level LVD of 12.8v  (3.2v * 4) under load.  This is assuming you have quality cells that are reasonably matched for capacity and internal resistance, AND have had an initial full charge upon receipt.

    HOWEVER - if you want to refine it somewhat, the better thing to do is this:

    1) Discharge your bank at the normal level.  Watch the voltage of the cells.
    2) When the **FIRST** cell reaches 3.1v, measure the overall voltage of the pack.
    3) Set your programmable LVD to this overall pack voltage.

    EX:
    Cell 1: 3.18v
    Cell 2: 3.20v
    Cell 3: 3.10v   <<<  This is the trigger cell being the first to reach 3.10.
    Cell 4: 3.12v

    Add up the voltages.  3.18 + 3.20 + 3.10 + 3.12v = 12.6v LVD.

    At this end of the discharge curve, there is not much left.  Choose your level of conservative values.  You could have watched for the first to be 3.2v for instance.

    Keep your application in mind.  We are different from an EV, drawing huge currents just to get back up the driveway, where this simplistic LVD could lead to a cell-killing reversal.  This is far different from the "bottom balancing" technique so often discussed for EV's.   Because our current draw is very minimal compared to an appropriately sized bank, we have this luxury of time for the LVD to trip properly without allowing for a cell reversal using "pack level" techniques.

    The other reminder is that we are assuming you are starting out with high-quality cells from a reputable vendor.  Anything else, like used trash cells from the dump or waste recycler requires a whole different strategy.
  • PNjunctionPNjunction Posts: 762Solar Expert ✭✭✭
    edited February 2017 #52
    Beware benchtop hacks:

    A LOT of confusion arises from the use of benchtop hacks and used / trash cells where one has to go to extreme lengths to protect the battery.  Usually what we are talking about is someone trying to make a 200ah bank out of used trash hobby small cylindricals.

    Basically what WE are dealing with here are large capacity PRISMATIC lfp cells.  CALB / GBS / Winston etc etc.  We are NOT talking about small capacity cylindrical types.

    In the lead-acid world, nobody in their right mind would build a bank using batteries that are of different capacities, age, new/used combos and the like.  You buy from reputable dealers with matched sets for the most part, give them an initial charge watching your hydrometer / voltages very carefully at the start.

    This is no different in the LFP world - or ANY battery bank regardless of chemistry.

    Ie, don't play with TRASH, unless you KNOW what you are doing and are willing to accept the compromises in cost / safety / maintenance.

    Most DIY lithium battery projects are based upon trash, and the results are poor, especially when there is a lot of trash to sell to those who don't know.

    If a guy rolls up in a van down the street, and wants to sell you a 48v bank of Rolls-Surrette with unknown characteristics, would you buy it?  I think not.  Same with LFP. :)

    DON'T GET DISTRACTED by benchtop hacks.  Oh sure, we all love our Arduinos, Raspberry Pi's, and other little hackable stuff where one can custom program the GPIO's to do things.

    What I'm dealing with here is the off the shelf stuff that the common solar housebank user is going to use, not the benchtop fun stuff.
  • PNjunctionPNjunction Posts: 762Solar Expert ✭✭✭
    edited February 2017 #53
    Obsession with top-level charge voltages:

    We've kind of tried to steer clear of balancing issues to get to the meat of the basics.  So I'll keep this somewhat short.

    A lot of DIY guys go nuts trying to keep their cell voltages during the end of charge at *exactly* the same voltage.  They freak out if there is a .001v difference between the cells.

    In our usage, a difference of 0.1 (zero-point-one, aka 100mv) or less is acceptable to most manufacturers.  Of course, don't exceed 3.6v on any cell.  Normally quality cells, that have had a proper initial charge to full, end up much less, like 0.07v (zero-point-zero-7, or 70mv) difference or better.  AND it all depends on which part of the final charge curve you measure them at!

    They are playing a detrimental game trying to make all their cell voltages line up exactly when reaching the CV pack voltage, which can NEVER be in their natural state due to slight cell differences in overall capacity and internal resistance.  Once again, we are assuming that each cell has been individually charged to full upon receipt, not a hodge-podge of individual cell SOC values slapped on a charger.

    Oh, you can play this game, but TIME is going to hurt you in the end.  They never think about TIME.  What do I mean?

    Let's put the bleeder-boards on the 12v battery as discussed above.  Assume something like 100ah cells, with only 500ma bleeder boards.

    The user gets them from the distributor, and each cell as received in reality is thus:

    Cell 1: 40% charged
    Cell 2: 50% charged
    Cell 3: 80% charged
    Cell 4: 58% charged

    "I'll just slap my bleeder boards on this thing and charge away".  Guess what happens:

    Cell 3 reaches the high voltage limit of 3.6v and stays there waiting/bleeding forever for cell 1 to catch up.  Could take DAYS.  TIME is coming after you now.

    After several days of bleeding, during normal use, all these cells are programmed to do the "lets make all the voltages line up exactly" dance.  Again, this takes TIME.  That is, time spent sitting at fully charged waiting for the others and the blinky-light dance begins.

    Basically what I'm saying, is that for a simplistic LFP bank, made from *quality* components, you don't need to do this dance every cycle.  A difference of no more than .07v (70mv) between each cell during charge can be quite acceptable.  Of course, less is better, BUT take into account that your cells have slightly differing capacity and internal resistances which lead to their natural SOC voltage, which may differ a bit.  As long as you don't exceed 3.6v, you should be fine.

    Any difference larger than 70 to 100mv means something else is wrong:
    1) Cells were not individually charged to their full capacity initially.  (3.45 to 3.6v CC/CV w/end amps of .05C)
    2) You were sold a low capacity / high resistance cell.
    3) Your wiring infrastructure is poor - corroded/oxidized terminal connectors, loose wiring, OVER-torqued bolts etc.

    The question to ask is that will my cells have a longer life if I spend TIME each cycle doing the blinky-light dance trying to make the voltages exactly the same, or will small and reasonable differentials in voltage, with an overall lower amount of TIME reaching my full-charge limit, be better in the long run?

    It is the latter method that actually results in the longest lifetime.  But, there are a lot of blinky-light boards to sell to the unknowledgeable DIY'er to feel-good in the short term.  And of course to those who never want to deal with a proper *initial* charge to each cell upon receipt.

    This is akin to a guy making a 48v bank of 12v batteries of unknown characteristics, slapping them all together, and wondering why one battery is gassing upon first charge. :)

    So yeah - would I give an LFP diy bank to my Grandma?  Heck no.  To experienced battery guys on this forum?  Sure!

    EV'ers and benchtop guys are in a different application than us, and have different needs.  So when discussing LFP, keep your solar-application in mind, otherwise you can end up in the weeds / tangents of discussion.

  • jimmyazjimmyaz Posts: 114Registered Users ✭✭
    Thank you for all the information. 

    Maybe I didn't write my question clear enough.  The reason why the Salvage Tesla battery interest me is because you would expect the cells were very equal to each other (or as much as it can) from the beginning and checked by Tesla and through out it life so far it has been using with a BMS system to keep it in check.  Yes, there's a risk of unclear of how much life is left in the battery.  But the worse... at least 1000-2000cycle.  At these salvage price, still much cheaper to buy them, than making your own from old laptop batteries....

    My question here is, let say you go with the 12s config = 43.2volts nominal and charge to only 4volts X 12 = 48V.  But isn't most standard 48v inverter nowdays lowest cut off point is minimum 44volts?  Or at least the Schneider Conext stop inverting at 44volts.   My worry is that charge to 48v and cut off at 44v, that's not enough wiggle room. 

    This is why I was asking to see if anyone had try this before... can reference as too how much of a voltage drop is there base on that model, 24v 250AH.


  • PNjunctionPNjunction Posts: 762Solar Expert ✭✭✭
    Ah, ok kind of a different arena, one-off scenarios.  Sure, anything could be done.

    In the old days, it was ripping apart DeWalt tool packs for the A123 LFP cylindricals.  Then of course you also have crashed/salvage Nissan Leaf batteries that were the rage for awhile.  Now salvage Tesla stuff.

    The keyword here is "salvage", where safety / performance / maintenance is unknown.  Keep your wallet shut, because the "salvage" guys purposely target the benchtop hobbiest, who don't know what a prismatic LFP, along with the intended application is all about. :)

    Somebody else here might have more info for you - though to be sure see the EV forums for additional notes.

  • PNjunctionPNjunction Posts: 762Solar Expert ✭✭✭
    De-rating your LFP cell capacity by 30% !

    Aside from being overjoyed at being able to discharge to 100%, most don't want to do so, and also takes you down into very dangerous areas from the DIY standpoint using simplistic LVD's and such.

    Instead, the least conservative target is to run no more than 80-10% DOD for longest life.  That means for a 100ah LFP cell, derate that down to 70ah usable for your normal calculations.  Don't forget your days of autonomy.

    Sure, you could eek out a *little* bit of additional capacity beyond 80% DOD, but there is more than one reason to use that as a stopping point!

    For our low-current housebank reasons, that means about no more than 3.2v per cell under load.  Personally, I have mine set at 3.19v to just tick over as the trigger.


  • jimmyazjimmyaz Posts: 114Registered Users ✭✭
    De-rating your LFP cell capacity by 30% !

    Aside from being overjoyed at being able to discharge to 100%, most don't want to do so, and also takes you down into very dangerous areas from the DIY standpoint using simplistic LVD's and such.

    Instead, the least conservative target is to run no more than 80-10% DOD for longest life.  That means for a 100ah LFP cell, derate that down to 70ah usable for your normal calculations.  Don't forget your days of autonomy.

    Sure, you could eek out a *little* bit of additional capacity beyond 80% DOD, but there is more than one reason to use that as a stopping point!

    For our low-current housebank reasons, that means about no more than 3.2v per cell under load.  Personally, I have mine set at 3.19v to just tick over as the trigger.


    I have not research far into the LFP stuff, so no idea.  But I know A123 (of of the Dewalt battery) is probably the safest and best battery there is for a lot of application (because I have seen guys at my RC club using it for RC plane and purposely drain them dead and they keep on living.  That if they were real and made by a trusted manufacture.

    With the LFP nowadays..... Which supplier is the trusted one?  Or do they even know what they are selling is quality battery or just another cheaply made one.   I'm not trying to be sarcastic, I am asking if I were to buy LFP, where would I buy it from if I am in North America?




  • PNjunctionPNjunction Posts: 762Solar Expert ✭✭✭
    OOPS, I over-discharged, now what?

    Yep, you took your cell(s) below 2.5v or so and now things are getting seriously bad.  First, find out why your LVD didn't go off at 3.2v / per cell or so .

    Second, you CAN recover provided you do the following things:

    1) You didn't actually send the cell into total reversal.
    2) You get to it ASAP.  Not a week from now.
    3) You RECHARGE SLOWLY.  NO MORE THAN C/100 current until you get higher up into the knee at about 3.2v, where you can start to apply the normal higher charge current.  But yes, you gave that cell a heart-attack.

    Not following the rules, especially #3 for many early diy-EV'ers meant that they *could* have recovered, but their recovery process damaged the cell even further, but wasn't really noticed until it became an issue down the road. The cure was worse than the symptom.

    In fact, many enjoyed taking their cells deep into the "discharge knee" beyond about 85% or more, opened the garage, and BLASTED their LFP's with normal charge current.  Bad, unless they had a smart charger that knew about this.

    Basically, using the 80-10% DOD rule keeps you "in the knees" so to speak.  No worries about hitting the bank with a full current charge.

    Going well beyond the 80% DOD and into the steeper part of the discharge knee, where voltage is collapsing fast, means that when coming OUT of the steep discharge knee with a recharge, you do the opposite - charge slowly with reduced current!

    There are reasons for this that I won't get too deep into here, such as intercalation ion-storms (an angry mob with nowhere to go), and blasting more than ions from the anode/cathode into the electrolyte etc.  Basically degradation occurs when you go into the deep part of the knee, and recharge out of it too quickly.

    Caution:  dumpster-divers trying to sell you trash may use this low current charge technique (C/100) to partially revive it.  It may even look good from a voltage standpoint.  But a few cycles later, you know the truth. :)  Don't buy trash.




  • karrakkarrak Posts: 307Solar Expert ✭✭✭✭
    edited February 2017 #59
    jimmyaz said:
    Have anyone actually try to use one of the Tesla Model S module with the current standard 24V/48V inverter?  Doesn't work? because of the lower nominal voltage rating?

    http://www.ebay.com/itm/Tesla-Model-S-battery-module-24V-250Ah-5-3kWh-444-Panasonic-18650-3400mAh-/262333679871?hash=item3d144e48ff:g:OSkAAOSwh-1W5acD&vxp=mtr

    Fully charge is 25volts, recommended to stay at 80% SOC, meaning 24-24.5 max.  Say discharge to 50%, wonder if the voltage would drop too low for the Inverter causing the Inverter to shut off too soon?

    I was thinking to get two and turn it into a 48V 250AH pack = 12kw.

    Any input guys?
    Most 24 volt inverters will work down to 21 volts which equates to 3.5V/cell on the 6S74P battery pack.

    As with most things the devil is in the detail, if the cells are Panasonic 3.4Ah 18650Bs at the 1C rate the capacity at 3.5V is around 60%SOC

     
    If you are only charging to 80% that give you a usable window of only 20%SOC. The price per usable Wh is ~$1.22/Wh (1300/(5300*0.2)) compared to ~$0.50 for new LFP (LiFePO4 batteries) assuming a usable window of 80%.

    Simon
    Off-Grid with LFP (LiFePO4) battery, battery Installed April 2013
    32x90Ah Winston cells 4p8s (24V), 4kW Latronics Inverter, 1160W of Solar Panels, homemade MPPT controller
    Homemade BMS https://github.com/simat/BatteryMonitor
     

  • karrakkarrak Posts: 307Solar Expert ✭✭✭✭
    Full charge vs 90% charge:

    An initial FULL charge is needed upon receipt to make sure that all the materials inside are truly active.  If you don't do this, even if charged somewhat from the factory, you can create inactive areas, and thus "hot spots" can form.

    NORMAL USE:  don't always charge to full!

    We've seen that a full charge is an absorb voltage anywhere in the window from 3.45 to 3.60v, but it is the end-amps, or absorb current that determines the full state of charge.  So voltage is not a good determination of full charge, but end-amps is - provided you are between the 3.45 to 3.6v window.

    Thus, to charge to say 90%, don't allow for a full absorb.  It's as simple as that.  You can merely cut off charge when a voltage is reached.  Or you can allow the cell to reach full voltage, but stop absorb half-way.  Heck, you can even stop charge well before 3.45v, but of course your overall capacity is reduced.  You choose your stopping point - based on the knowledge of 3.45 to 3.6v and an end-amp current of .05C is the generic value for a full charge.

    Can you post a link to the articles on inactive areas and hot spots, sounds very interesting.


    Your charging scheme works great if you are charging with a bench power supply but does not work if you are charging with solar. With reference to the above graph which is the discharge curves of a Winston prismatic if you charge to 3.45V/cell with a fixed charge rate  of ~C/3 (0.33C) and terminate the charge with no absorb you will end up with an SOC of 90%. If you absorb and terminate the charge when the current decreases to C/20  you get a final SOC of ~98.5%.

    With solar, maximum charge rates are usually in the range C/3-C/10 (My maximum charge rate is ~C/10). With a large solar array which has a maximum charge rate of C/3 under ideal conditions with no absorb you could get a final SOC of 90%. On an overcast, hot day solar output could easily be down to less than 20% of the C/3 figure (~0.05C). Under those circumstances the end SOC could be around 98.5% or even higher.

    Lets try a charge voltage below 3.45V, if we pick 3.40 volts as an end voltage with no absorb and a charge rate of 0.2C we could end up with a final SOC of ~70%, same 3.40V and charge rate of 0.05C and we end up with an SOC of ~96%.

    There are a number of people who charge their LFP batteries using solar energy to an SOC of ~99% and float at this SOC who post on a number of forums who have batteries that are up to 6 years old that have detected little if any charge in their battery capacity.

    Simon


    Off-Grid with LFP (LiFePO4) battery, battery Installed April 2013
    32x90Ah Winston cells 4p8s (24V), 4kW Latronics Inverter, 1160W of Solar Panels, homemade MPPT controller
    Homemade BMS https://github.com/simat/BatteryMonitor
     

  • karrakkarrak Posts: 307Solar Expert ✭✭✭✭
    jimmyaz said:
    With the LFP nowadays..... Which supplier is the trusted one?  Or do they even know what they are selling is quality battery or just another cheaply made one.   I'm not trying to be sarcastic, I am asking if I were to buy LFP, where would I buy it from if I am in North America?
    Voltronix http://voltronixusa.com/products/ is selling the Winston batteries under their name in the US, these are the batteries I use.

    Simon
    Off-Grid with LFP (LiFePO4) battery, battery Installed April 2013
    32x90Ah Winston cells 4p8s (24V), 4kW Latronics Inverter, 1160W of Solar Panels, homemade MPPT controller
    Homemade BMS https://github.com/simat/BatteryMonitor
     

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