2

I'm trying to design a circuit to be used with some 12V LiFePO4 battery packs which don't have an integrated heater, so I can add thermal protection.

The circuit would sit between a 65W solar charge controller and the battery's existing BMS.

enter image description here

It would sense temperature and during detected freezing conditions it would:

  1. disable charging using an n-channel MOSFET
  2. enable a heating pad when charge current is available
  3. prevent the battery from discharging into the heating pad
  4. not prevent the battery from discharging into the load

I won't have access to the load terminals so my circuit sees the charger and load together. The load is about 2W.

I'm stuck on how to achieve #2 and #3 together without disconnecting the load. I need some way to sense that there is charge current.

I'm currently exploring a strategy involving the comparison of battery negative voltage to the drain voltage of the charge control MOSFET. This approach seems promising as, during charging, the drain voltage is expected to be lower than the battery negative terminal voltage.

enter image description here

Falstad simulation here of charging scenario.

This approach works in the simulation, but:

  • My model of the battery and charger are very crude and possibly wrong. How should I model these?
  • The rds_on of the MOSFETs are unrealistically high, so the potential difference I'd need to sense would be much smaller, possibly too small to be reliable?
  • I'm unsure what would happen at edge cases where there's just a bit of charge current available. For example, some kind of hysteresis is probably needed for current sensing but I'm not sure how to add this.

Many BMS systems boast built-in thermal protection capabilities. Given this, I assume my goal is attainable. Is my general approach sensible, or are there more effective alternatives to consider?

davegravy
  • 319
  • 2
  • 8
  • Bedtime in NZ. I may provide an answer in a day or few if nobody else does - but they probably will. || BUT: A useful 'trick' can be to insulate the battery and provide it with a thermally linked water jacket. Standing it in water may be viable. With relatively modest insulation using polystyrene you can get some days at 0 degrees C when external temperature is say -20C or lower. I have some related calculations on this site from a few years back but will not try to dig them out at present. Advise if more information wanted. – Russell McMahon Aug 26 '23 at 13:06
  • I lied :-). See https://electronics.stackexchange.com/q/467518/3288 . || Also some relevance https://electronics.stackexchange.com/questions/527325/feasibility-of-sub-zero-peltier-cooler/527341#527341 . – Russell McMahon Aug 26 '23 at 13:11
  • @RussellMcMahon thanks there is some great info and ideas in these links. Note that at the very least I need to design a circuit to disable charging so I may as well try to integrate self-heating if I feasibly can. In terms of the battery box design I'm somewhat limited in size for adding a water tank and thick insulation. It's ok if the load shuts down from cold sometimes (the less it does this the better). I can't have battery packs failing. – davegravy Aug 26 '23 at 14:21
  • I don't know how long you need to protect it for. If you need weeks or indefinite water is not suitable. Days, it may be. – Russell McMahon Aug 28 '23 at 02:10
  • @RussellMcMahon indefinite – davegravy Aug 28 '23 at 15:28

0 Answers0