LiFePO4 balancing / voltage limiting

bill von novakbill von novak Solar Expert Posts: 706 ✭✭✭✭
So I just replaced my old AGM bank with a bank of 16 180ah CALB cells.  I bottom balanced them; paralleled them all and brought them down to 2.70 volts.  Let it rest for 2 days and they rebounded to 2.73.  Disconnected the parallel connections for another day and they all stayed at 2.73.

Then I seriesed them and did the first charge while monitoring.  They tracked very closely for the CC portion of charging.  (Charger 20 amps, 56 volts, targeting 3.5 volts per cell.)  When they switched to CV I measured again and got:

3.52 3.45
3.47 3.43
3.48 3.43
3.44 3.45
3.49 3.58
3.50 3.49
3.50 3.47
3.40 3.49

I was a little worried about cell 13 (3.58) but it was still well below 3.65V and I figured charge was almost complete.  I checked 15 minutes later, when current had dropped to 2 amps, and saw:

3.59 3.41
3.45 3.39
3.50 3.39
3.41 3.43
3.52 3.75
3.63 3.40
3.55 3.44
3.36 3.40

So I stopped charge and discharged it until cell 13 was down in the safe range again (3.5 volts.)  I let it sit overnight and now everything is back at 3.33 to 3.35 volts.  Hopefully no harm was done to 13.

So how do I handle cell 13?  Do I have to monitor voltages at all cells and discontinue charging when it hits 3.6V or so? Do I back off the 56 volt charge, go down to 54 volts or something?  Do I terminate when current reaches C/20 or something?  (hard to do with this charger)

Comments

  • BB.BB. Super Moderators Posts: 26,837 admin
    Get a 6 volt brake (or tail) light and clip it on the high voltage cell, and monitor until it gets down to the average pack voltage.

    I would wonder if cell 13 is weak/low capacity/not "matched" to the capacity of the other cells (filling up to 100% state of charge first).

    -Bill "not a Li Ion battery expert" B.
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • bill von novakbill von novak Solar Expert Posts: 706 ✭✭✭✭
    BB. said:
    Get a 6 volt brake (or tail) light and clip it on the high voltage cell, and monitor until it gets down to the average pack voltage.
    I explicitly wanted to NOT do that because then they're not bottom balanced any more.  Right now, in theory, during discharge when they hit 2.7 volts they should do it all at the same time - so that if I set LVD to 45 volts the cells should be safe against overdischarge.  If I discharge that cell then it will hit 2.7 volts much earlier than the rest, and be at risk for overdischarge.
  • BB.BB. Super Moderators Posts: 26,837 admin
    Then that is the new 100% full bank voltage, as understand.

    Unless you replace the (bad/weak?) cell.

    Assuming the bottom voltage was done correctly (not doubting you).

    Have you contacted the battery folks and see what they suggest?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • nickdearing88nickdearing88 Registered Users Posts: 92 ✭✭✭
    Do you have a BMS/balancer system installed or are you doing it manually? I've heard some people who have good results without a BMS of any kind but I wouldn't take that chance.

    I just upgraded to Battleborn batteries but I'll start a separate post as to not derail this one.
    Current test system: 4-100w Renogy panels mono/poly, 1 string of 4 panels in series - 24v 100Ah AGM Battleborn LiFePO4 batteries - Morningstar MPPT40 CC - 1500W Samlex PSW inverter
  • bill von novakbill von novak Solar Expert Posts: 706 ✭✭✭✭
    Do you have a BMS/balancer system installed or are you doing it manually? I've heard some people who have good results without a BMS of any kind but I wouldn't take that chance.

    No BMS - but this makes me think it would be a good investment.  (Although I haven't found a good one yet.)
  • EstragonEstragon Registered Users Posts: 1,543 ✭✭✭✭
    I'm a no-nothing on these, but that seems like a pretty wide range (3.36-3.75) for cells charged once in series from an apparently equal SOC. My understanding is voltage should be pretty much flat between ~10-90%SOC. That implies when the highest voltage cell gets to whatever 90%SOC voltage, stop charging. Discharge until the lowest gets to 10%SOC voltage. The difference is capacity.
    Off-grid.  
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
  • karrakkarrak Solar Expert Posts: 197 ✭✭✭
    The problem is that you cannot have an LFP battery balanced at both the top and bottom unless all the cells have exactly the same capacity which is unlikely. If you have it bottom balanced you have to use a BMS to terminate the charge when the weakest cell hits ~3.45V-3.50V. If you top balance you can set your charge controller to 3.45*16 and all cells will be charged to the same level but you will need a BMS to warn you or shut down the loads when the weakest cell gets close to 0%.

    You are correct in saying that you only need a pack LVD if you bottom balance but this only holds true if the balance does not drift with time. Again this is unlikely and doesn't match my experience and experience of others. Of course the only way you can check if the battery has gone out of balance is to do another bottom balance.

    Because of the very flat charge curve for an LFP battery, you cannot reliably charge an LFP battery to say 90% unless you have a constant current power supply. Solar is not a constant current supply.


    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
     

  • Raj174Raj174 Solar Expert Posts: 461 ✭✭✭✭
    Hi bill,
    I can't say I've heard of a single person bottom balancing their LFP cells, except EV guys. If you want to charge the bank to 55.2 volts (3.45v/cell) without a BMS then top balancing is the way to go. I agree with karrak that the reason for the increasing delta in voltage, as the cells are being charged, is slightly differing cell capacities. I have read that they can vary as much a 5%. So we use the top 80% or the bottom 80%. I believe it is easier to manage charging with top balancing because as the bank nears the end of absorb, all the cells are nearing the same voltage. As they are discharged, they will stay fairly close, probably within 10 millivolts, down to 20% SOC. Could be even lower, however I can't say for sure because I haven't taken mine below 20%. Something to consider is, deeper discharge means shorter life, so I try to keep usage between 70 and 80% of capacity. Charge to 99+% then down to 20 to 30%.
    It's easy to get freaked out when you see the cells coming so close to overcharging. Top balance does away with that concern. Over a year of daily cycling and my cells voltage delta is 25 millivolts at the end of absorb.

    Rick
      
    12 x 300W Renogy PV, MNE175DR-TR epanel modified, MN Classic 150, Outback Radian GS4048A, Mate3, 51.2V 195AH HI Power LiFePO4 no BMS, 4000W gen.
  • nam_battery_vietnamnam_battery_vietnam Registered Users Posts: 1
    hey I will talk this tomorow
    so, I have a few EXP about using LFP

  • PNjunctionPNjunction Solar Expert Posts: 762 ✭✭✭
    If you bottom-balance, you will be slightly ragged at the top.  That is to be expected.  When the first cell of your bank reaches your target voltage, your HVD is supposed to shut off charge.  Some of the simpler setups will note when the first cell hits the upper limit, and take a bank-voltage reading, and use that as the HVD cutoff.

    But you must choose either top or bottom - you can't combine both techniques.  Bottom is great for high-current EV's and other high-draw applications.  But here, as a solar storage application that typicall draws no more than 0.1C or less (to get through many day's autonomy), top balance is the most practical choice, as our relatively low current draw gives us PLENTY of time for a bank-level LVD to react, if it is set conservatively.


  • jonrjonr Solar Expert Posts: 858 ✭✭✭✭
    You can balance at both the top and the bottom - but you need to use a charge shuffling design.
  • YerfYerf Registered Users Posts: 39 ✭✭
    How often do you have to balance? What is the best way to balance a new 50% soc 12.8v lifepo4 battery? https://www.victronenergy.com/batteries/lithium-battery-12-8v

    I have a nominal battery bank of 51.2 v, shall I set the inverter cutoff at 51.2 v to avoid over discharge ? The charger cutoff at 56.8? Is this healthy?
  • YerfYerf Registered Users Posts: 39 ✭✭
    Another question, it gets quite hot in summer. Around 45 c. Would I need to cool the batteries? Would it be a good idea to seperate the charger and inverter from the batteries to reduce excess heat?
  • YerfYerf Registered Users Posts: 39 ✭✭
    How often do you have to balance? What is the best way to balance a 12.8v

    I have a nominal battery bank of 51.2 v, shall I set the inverter cutoff at 51.2 v to avoid over discharge ? The charger cutoff at 56.8? Is this healthy?
  • karrakkarrak Solar Expert Posts: 197 ✭✭✭
    Yerf said:
    How often do you have to balance? What is the best way to balance a new 50% soc 12.8v lifepo4 battery? https://www.victronenergy.com/batteries/lithium-battery-12-8v

    I have a nominal battery bank of 51.2 v, shall I set the inverter cutoff at 51.2 v to avoid over discharge ? The charger cutoff at 56.8? Is this healthy?
    This battery will balance itself if you charge it to 56.8V, I assume you have four of the 12V batteries in series? It may extend the lifespan of the battery to only charge to 55.2V and charge to 56.8V and hold at this voltage for an hour or so say once a month to trigger the inbuilt cell balancers. You may be able to use the equalise charge setting on your charge controller to do this. What model charge controller are you using?

    Are you running this battery with the Victron VE.bus BMS?

    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
     

  • nickdearing88nickdearing88 Registered Users Posts: 92 ✭✭✭
    Yerf said:
    Another question, it gets quite hot in summer. Around 45 c. Would I need to cool the batteries? Would it be a good idea to seperate the charger and inverter from the batteries to reduce excess heat?
    LiFePO4 is less sensitive to heat than LA, but still cooler temps are better if possible. I aim to keep my physical battery temperature between 50F and 75F, which they seem to like.
    Current test system: 4-100w Renogy panels mono/poly, 1 string of 4 panels in series - 24v 100Ah AGM Battleborn LiFePO4 batteries - Morningstar MPPT40 CC - 1500W Samlex PSW inverter
  • karrakkarrak Solar Expert Posts: 197 ✭✭✭
    Yerf said:
    Another question, it gets quite hot in summer. Around 45 c. Would I need to cool the batteries? Would it be a good idea to seperate the charger and inverter from the batteries to reduce excess heat?
    The graph below from this paper http://jes.ecsdl.org/content/145/10/3647.full.pdf+html  gives an idea about how temperature effects the lifespan of LFP batteries.

    We too have hot summers where the temperature gets to 45C. The amount of time that the temperature is above 45C is fairly low and the average daily temperature is in the mid 20s. Under these conditions I don't think the losses will be too severe.

    I agree with @nickdearing88 that you should take whatever steps you can to keep the battery temperature as low as possible but above 0C.

    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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