Today’s adventure: a couple of rainy days caused low battery levels, but not too low I thought - still 30% or so; these are lithium batteries and can deep cycle. They are “smart” batteries and if one is full in series, none can charge further - so they should all be at the same charge level all the time. But a couple had gotten out of step somehow and when they reached zero everything shut down.
How to bootstrap it? With no battery output (since a zero battery turned itself off and would not let the battery bank show any voltage!) - there is no way to activate the inverter and let street power run the battery charger. With no battery power, there is no way to turn on the MPPT controller and charge the batteries!
I could rearrange the banks to put four batteries with remaining charge in series because I have an 8-battery system, and get things restarted; but if there had been only four like when I first installed the system - I’d be in trouble.
Another thing that happened. Before I figured out the battery problem, I was trying to switch back to street-power. Because power from the street comes to the inverter first and then the inverter powers the distribution panel, when the batteries are down, I cannot get street power to the distribution panel. I could install a manual bypass, but it is not a commonly needed item and it is a large amp switch. So I removed the inputs and outputs at the inverter and bypassed manually. That worked fine. But in the process of disconnecting or reconnecting, I must have loosened the neutral connection to the inverter. So when the inverter was working again and I checked voltage, I only checked across the two hot legs - yay, 240V. I did not check that each leg was 120v from neutral! They were not: one leg was at zero and the other was at 240. I found this discrepancy fairly quickly after only destroying an outlet strip, the oven control electronics from a very old stove we were wanting to replace, and the controls of an old Sharp microwave oven with the 4, 7, 9 and Clear buttons not working. Woohoo we have a new stove out of the deal!
Thanks! Great info. You are right, I have lifepo4’s -two sets of 4x12V paralleled. There is an app with them to check the internal cell voltages, overall voltage of each unit, state of charge and number of cycles.
You mention the big current that can arise from paralleling batteries - are you suggesting a series-parallel wiring? I have had a lead-acid bank like that previously with cross-connections between series groups at each voltage level. Those cables were usually carrying small currents when all the batteries were in good condition. In a healthy lifepo4 bank with voltages even, could I set that up and would it help? Would a load resistor help (it normally would only see small voltage drops on the order of 0.1 or 0.2 volts, so a load resistor of a few ohms would pass very little current. I have also asked this question to the battery vendor. Awaiting their reply.
You should be able to have a 4s2p setup no problem. It also sounds if you have the individual batteries internal voltages and cycle count you already have the information you need to identify failing batteries… The best indicator is internal resistance of the battery, but cell voltages should with too. Basically, when under load if any individual cells voltage drops more than the other cells by a significant margin you are looking at a battery with issues. The under load part is the critical part… Because just like lead acid batteries the surface charge of a cell can be deceptive. Most good lifepo4 batteries can get at minimum 500 cycles, with some designed for 2000+… And the extra kicker is if you only charge them to 85% you can get another 4-5x that, typically draining then isn’t an issue (as long as you never go below minimum voltage).
As far as the 4s2p, I think that would actually be preferred, but I would defer to the manufacturer. Just like acid batteries, the parallel configuration allows them to support each other for a more consistent experience.
The sweet spot for lifepo4s are better 10% and 70% soc. Usually that isn’t quite enough power for cost which is where the 85% number is used. It sounds like you just got into a bad state where you over discharged one and it’s not actually damaged, but having them in a 4s2p should be better than having the two 4s strings in parallel. Again, just make sure the voltages are even. I wouldn’t use a load resistor. The amperages is based on wire gauges and material. But again, do what the navigate suggests.