FoxESS Community - Owners & Installers Forum

What is the lowest battery temp the BMS will tolerate before it gives a maintenance charge? From midnight to 7am today my lowest cell temp went from 17.6 to 15.8 with the BMS kicking in frequently as shown. At 1pm the temp is 16.3 but the BMS seems happy now! Minimal solar generation today and the battery is discharging. I've got the batteries in an insulated enclosure and added a 45w heater inside.

Hi, just wondering do you really mean lowest cell temp or the battery temperature as reported by the BMS ?.

The battery temperature reported in the app is the actual BMS temperature and from experience I have find this to be 6-10C higher than the actual lowest cell temperature.

The latest V2 app (and home assistant if you have it) both show the lowest cell temperature, which typically tends to be the battery nearest the floor.

what was your minsoc at the time this happened? - the BMS will maintenance charge approx every hour if your SoC drops to minsoc

To answer your question specifically,

As mentioned typically there can be 6C between the packs lowest and highest cell temperatures, the BMS always uses the lowest reported cell temperature as the trigger.

Anything below 15C will see a reduced charge current, but the maintenance charges don't usually start until it's less than 10C *and* the SoC is low, if the SoC is > 50% it's more like 5C before it starts trying to warm the batteries.

Hi and thanks. The lowest cell temp I am taking from the v2 app itself and also the data download from the app - 'CellTemperatureLow/C'. The BMS/'battery' temps and SOC were good as shown. 'Battery reserve capacity' is set at 15% and System Min SOC at 10%.

Ok, yes that is the cell temperature the BMS uses as it’s low temperature - so that is a reasonable temp.

But actually just looking at your first trace, that is showing battery discharge as if you’ve got an appliance regularly pulling a bit of power - I get a very similar trace from my aquarium when it’s cold as the heater element comes on and off.

I did wonder if there was something in the house drawing that power but I can't for the life of me work out what would be drawing 700 watts. Is there any way the app data download can tell me if it's the BMS doing it?

JohnH wrote: ↑Mon Mar 04, 2024 5:36 pm I did wonder if there was something in the house drawing that power but I can't for the life of me work out what would be drawing 700 watts. Is there any way the app data download can tell me if it's the BMS doing it?

It won’t be the BMS taking power (other than the 10W it needs to run) it is only a passive device, charging comes from the inverter if it asks for it but this is battery discharge so there must be a corresponding blip if you look at load power which is what is being consumed in the house. It looks cyclical possibly a heater, maybe a small fridge ?