We have all seen cells out of balance or had to send bank back to get “fixed”, me more than most for various reasons, all of them part of the learning curve, the Master Doctatorial Degree in “WTF Not To Do”.
- BMS is not suited for daily use on a solar system. Lost a bank because of this.
- Cells not A Grade, nor batch matched.
- Experimenting in mixing and matching different cells in the same bank. Jip, it can work for a period of time. Drop the bank ah to the smallest cell.
- Also mixed and matched different models of 280ah cells. For a limited time it is fine, just watch the cell volts on high SOC.
I have seen:
- Where the ah counter on the BMS reverts backward when charging IF the cell voltages jump around a teeny bit.
- How the BMS jumps to 100% SOC because one or two cells have reached a certain high voltage. AH put back cast aside at this point. Note: Over time, this has a cumulative effect on the actual SOC of the bank.
- How, in seconds, a cell shoots out to 3.65v and BMS disconnects. At the time, BMS SOC hits 100%.
- That “flat charge curve” of Lifepo4 cells, the BMS sits for an hour or more on 80% SOC, happily absorbing big amps, 0.008v Delta, whereas BMV shows 95% on coulomb counting at the time, even 100%.
- Why it is a good idea to discharge and recharge once a day. It helps with the balancing, a LOT!
- BMV will pick up nuances in volts/amps, BMS does not register it even, yet there are still amps going to the bank … and a cell shoots out.
- How, on 280ah cells, it is a good idea sometimes to hit them with as many amps as you can, not exceeding 280a obviously. Same with discharge. The cells appear to handle the Delta better, than on smaller amps charged assuming your cells are in balance.
- How working with 280ah cells, the mA balancing, takes much much longer than on say a 100/120ah bank. So a week of waiting for the BMS to balance becomes a month.
Tools one can use:
- Top balance the bank, even a 2nd time if it goes out of whack is first prize. But, it can be very tedious to strip it out … and dangerous if you go by TTT.
- Keep the bank on Keep Charged for a few days, see the BMS do its thing IF a cell does not run away that is. I do this now with bad weather. Bugger this shiite of trying to recharge it the next day.
- Victron has added under DVCC, Limit Charge Volts, the problem is, it is over the bank, so say you limit to 54v, 15 cells are well below that, one cell shoots out, the bank is still below 54v, so the feature is not “activated”. The banks’ voltage must reach 54v in total. Saw my arse good with this “mistake” I made.
- Limit charge current under DVCC, THAT works the best!
- If you have access to the cells, use “The Bulb”. Tedious, painful, taking great care not to get distracted and walk away. Painful to get that one cell back in line if you have forgotten about it.
- If some cells stay “behind”, charge them individually a teeny bit, and watch the volts, to top them up. Again, take great care not to walk away, stays focussed.
Conclusions I came to, having done this manually myself:
- Keep a track of the banks’ Delta, ideally in software for record-keeping, as it is a good indication that there is a problem brewing.
When a cell, one cell:
2.1) exceeds 3.45v, drop charge amps to 2amps.
2.2) exceeds 3.5v, stop charging completely.
This gives the BMS the chance to balance faster and stop a runaway situation resulting in a potential DC Ripple when the BMS disconnects because of one cell.
- If you are around your system, and the bank goes out of balance on high cell volts, switch on big draw items in the house (stove/oven/geyser), to drain some of the excess volts from the bank.
Focus for Lifepo4 banks should be on the individual cells to pull back the charge current, stop if even, instantly when your pre-set parameters are exceeded.
Hope this helps other DIY’ers … add your tricks here, please.