Lithium LiFePo4 and keeping warm in winter.... (diy heater)

[activecampers]

We've a 200Ah LiFePo battery, lives in a wooden box in the uninsulated garage.
We have a 12v compressor fridge, conveniently centre of the van, and thus have extra thermostatic fans that in summer blow the warm air into the garage, but in winter diverts to go through the battery box. In addition a second thermostatic switch on the battery forces the fans on if it goes <8' and the source of this is back of fridge and the habitation area which should be heated, and ergo keep the battery warm,

The above has been fine and kept battery warm at freezing and slightly below.

Next trip may see <10'C or less, and its not proven if this will be enough.

So I've made a heater which I'll detail below if anyone is interested!!

Its an aluminium plate that sits under the battery (on foam, so the components can recess)

2x 15W 12v heater pads in parallel (eg 30w) stuck to plate at 25% +75% distance
1x Digital thermostat, set to heat, set temp variable, and thermosat probe centre of plate.
1x thermostatic switch, 25'c normally closed - as a secondary fail-safe if the plate goes above 25'c it just cuts power to heater.
Plus a few wires and 12v connector.

Anyway, works like a charm!
Setting to 15'c the heater cuts in and out, on maybe 20-25% of the time, then allowing gentle heat soak into the battery for gentle warm up.

I could've wired differently to reduce the heater output and less on/off switching, but I don't know if I need the full 30W heating when its -20'c lol - so thought I'd not really care.

Total cost c. £12 (top cash back and codes with other purchases etc)

• Digital thermostatic relay and box £1.79 https://www.aliexpress.com/item/32952729454.html (I use loads of these for different things)
• Safety thermal switch 25' normally closed - £1.24 for 2x https://www.aliexpress.com/item/32978219671.html
• 2x 15w 12v heating pads @ £1.89 ea https://www.aliexpress.com/item/1005007586342911.html

Then just some wires and connectors

• Hella connector £1.24 (better than cigar socket) https://www.aliexpress.com/item/1005006875248528.html
• And wire https://www.aliexpress.com/item/1005007328808885.html

And of course an inline fuse which is UK spec'd as I don't trust chinese fuses.

(plus stock glue, connectors etc)


Today, its cold outside, air in battery box is 8' - battery was 8.5/8.0 (2 internal sensors)

Turned it on set to 15' (needs a temperature differential of course)

Battery internal sensors are now 12.1/10 (assume one sensor is higher than the other!) but obviously warmed up. Air temp in battery box 8.5. (outside temp is 6').

All in all a success - so far. And I'd say whilst it pulls 2.5A when running, today, its around 1A/hour.... (obvs will be dependant on temperature and needs)

 

[activecampers]

Pics below
(app pics from battery BMS)

30m after turned off, battery lost 1' of heat - so 10.9/10 (though air in battery box warmed to 9' - outside air 6')

So certainly does seem to warm the battery and its environment which is what I want.

(Oh - van is "cold" - no heating/fridge air to interfere with the test)

This is just testing, obvs, probably would set it to 10' which should give it enough differential to keep above freezing. Though hopefully the free warm air from the fridge will do most of the work!