First low temp of the season showing a problem with my LifePo4 / MPPT setup.
Setup: The lifePo4 is DIY 12v using EVE304ah cells and J-K BMS. It has integrated heat pads controlled by a thermostat which is powered by a voltage monitor (thornwave). The thermostat only functions when there is charging voltage present (since I don’t want the heaters on just running off the battery). If charging voltage is present AND the temperature is below 40F, then the thermostat will apply 12v to the heating pads. The whole system is intended to operate during off-grid storage being fed only by solar via 2 panels and a Victron Smart Solar MPPT 100:50. I have the MPPT set at 13.3 v absorption and 13.2 v float. Intention is to hold the battery around 60%SOC over the winter while RV is in storage.
Problem: today the battery temp was under freezing for the first time and the BMS disabled charging as designed. The 12v level out of the battery was 12.4 (internal voltage is 13) with charging disabled. As the solar came up it brought the level up over 13v just for a few seconds, then turned off. The voltage level then kept cycling from 12.4 for a couple of seconds to up over 13 for a couple of seconds and is doing this repeatedly. After looking at everything my conclusion is the the MPPT is switching its output on and off. Everytime it switches off the voltage spikes up over 15 or 16 volts (very quick) and then back down to the 12.4 level coming from the battery.
Thoughts: I’m guessing this results from the battery looking like its disconnected while the MPPT tries to charge it. I’m surprised that it does this since there is still a draw on the circuit. When its over 13v there is about a 1amp draw. The heater is not on at this point because I have a time delay built into the sensor. The voltage has to be over 13.2 volts for 5 seconds for the thermostat circuit to turn on. So the only load initially presented to the MPPT is the quiescent draw of the 12v house circuit.
Request: any insights into this behavior and how to fix it would be appreciated. I’ve looked at all the MPPT settings and can’t see any that can change the behavior. I temporarily fixed the problem by manually turning on the heater circuit which puts a 4amp load on the 12v bus.