MPPT behavior when BMS disables charging at low temp

[Visionquest]

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. I'm not using the heat function of the BMS because I wanted full control over temp and voltage. My build 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 seem to have temporarily fixed the problem by manually turning on the heater circuit which puts a 4amp load on the 12v bus.

 

[time2roll]

I run my heaters continuously based on temperature. I suggest doing the same so the battery is warm and ready when the sun comes up. Very possible to pass up more solar than the heaters draw overnight while waiting for the battery temperature to come up and get started. Possibly reduce the delay to 0.1 second.

In general MPPT controllers do not like being without 12v power. Best to keep the battery on even if that means more heat. Otherwise consider disconnecting and just let the battery cold soak until next time it is needed.

 

[Visionquest]

I run my heaters continuously based on temperature. I suggest doing the same so the battery is warm and ready when the sun comes up. Very possible to pass up more solar than the heaters draw overnight while waiting for the battery temperature to come up and get started. Possibly reduce the delay to 0.1 second.

In general MPPT controllers do not like being without 12v power. Best to keep the battery on even if that means more heat. Otherwise consider disconnecting and just let the battery cold soak until next time it is needed.

Thanks I am going to try this and monitor to see how well it does.

 

[Suijkerbuijk]

The bms shutdown but leave the volts open.
But the ampere do not flow in .
It can flow out or the temp is that low that is shutdown.
Still 12volts stay on it.
Thare is not pull from the battery so the mppt normal will not charge on that moment.
And go in Standby.

I have test this with Victron mppt.
And i have test this with a china cheap charger that have off function.
And it go nice off.

 

[Visionquest]

I run my heaters continuously based on temperature. I suggest doing the same so the battery is warm and ready when the sun comes up. Very possible to pass up more solar than the heaters draw overnight while waiting for the battery temperature to come up and get started. Possibly reduce the delay to 0.1 second.

In general MPPT controllers do not like being without 12v power. Best to keep the battery on even if that means more heat. Otherwise consider disconnecting and just let the battery cold soak until next time it is needed.

You were exactly right on the idea to keep the heat circuit enabled. The amount of gain from having the battery accept charge as soon as the sun is up - rather than limiting solar use to heating during the warm up - is overall much more efficient. Keeping the battery warm is also got to be much better than thermal cycling on a daily basis. I've got the internal thermostat set to 45 degrees and even down to 0 F it only uses about 16 Ah overnight to maintain. That's easily replenished even in cloudy weather.

Thanks for the suggestion!