Post #9605

7)Charging methodology. When charging an LFP battery our goal is to try to get all cells over 3.45v and balanced. But once the cells are balanced and very closely match each other's voltages there is no need to keep the cells at such a high voltage. The typical charging profile is that the BMS will ask for 56.8v and if the temperature is above freezing advise the inverter or MPPT chargers to send all the power they can. This is called "bulk charging". When the battery voltage reaches 56.8v (3.55v per cell), the BMS will usually wait 2 hours. This is called "absorption" (the word is a hold over from lead acid battery days, in our case it should be called "balancing"). After two hours, we assume the battery is balanced and there is no need to maintain such a high cell voltage. The BMS will advise the inverter or MPPTs that the battery now wants to stay at 55.2v for the rest of the day. The inverter will draw down the battery to 55.2v and maintain it at that voltage. 8)State of Charge. Because LFP cells have such a flat discharge voltage profile, it is impossible to tell state of charge by simply measuring the pack voltage. Good BMS's will measure amps in and out of the battery to keep an "odometer" of sorts. When a battery is full, the BMS will reset this "odometer" to say 100%. This is subject to small errors so the more days you run without hitting 100% the more uncertain the BMS will be about the actual SoC. We like to have a battery hit 100% every week or so if the BMS is measuring the amps in / out. An exception to this is if you use an external Victron Smart Shunt. This device is so accuate that you could go a month without hitting 100% and still be within 5% of actual SoC. Take aways.... LFP don't catch fire. BMS keeps battery cells within a safe range. Try to design a system that can fully charge your batteries at least once a week, weather permitting.