As said a cheap weather station is exactly what you need, they even have max and min recall button that tels you the max and min temp over last 24 hour's just a couple of aa batterys
The problem with a weather station as described is it will only give you the ambient temperature of the van as is or has been. The op was looking for a way to charge a
Lifepo4 battery without damaging it through being too cold? My
Lifepo4 will take a light charge at zero and discharge without any problem well below zero, so I’ve left the
solar mppt charger to do it’s own thing because this time of year when temperatures are low, it will only produce a small current.
Some
Lifepo4 batteries don’t like being charged if their internal temperature drops below 4 degrees. The ambient temperature of a van might be somewhat higher than the internal temperature of the
battery depending where it is and if it doesn’t have a low temperature cut off (like mine) it could be permanently damaged by a b to b charger kicking in at 30 amps/hour or higher when you start the engine?
Mine like many has Bluetooth connectivity and will let me know what the internal temperature of the
battery is, what charge is coming in or being taken out (plus a few other nifty things), but because it has no low temperature cut off, my little thermostat set up will ensure that the
battery can’t come to any harm whenever I start the engine. I do of course check the temperature of the
battery using the Bluetooth before starting up but the thermostatically controlled heat pad is a safeguard, simple to install and uses an insignificant amount of power. I’ve only noticed it kicking in twice since setting it up.
For me it was simply a case safeguarding the cost of the
battery and hopefully get more than the guaranteed 2000 cycles (the Bluetooth registers rolling cycles
)