I am trying to make a circuit that slowly discharges a multi cell lithium battery to storage voltage. I already have a comparator circuit that can turn on or off discharging. I want to support 2s to 6s batteries so we are talking about voltages ranging anywhere from 7.6v to 25.2v. I am shooting for 500mw of power dissipation so I can achieve my form factor requirements for the PCB (11mm x 15mm). Ideally, the same circuit can discharge any cell voltage with constant power. So far I have tried/considered:
PTC Thermistor load. This will essentially run the PTC perpetually in its "tripped" state. I have an NTC thermistor in series to handle the inrush current until the PTC heats up and protects the circuit. This best captures what I want. However, I contacted a support engineer and they say the use case is not specified and could negatively impact the life of the device. https://www.murata.com/en-us/products/thermistor/ptc/prg
BJT with an NTC thermistor switching a resistive load on or off. I would preferentially avoid this due to extra components necessary.
I would like to stay away from MCU controlled constant heaters for simplicity sake.
Buck or Boost converter is not practical for form factor and cost.
Any help will be appreciated.
- I am creating a battery discharge circuit small enough that can be integrated into a battery connector (like an XT60 adaptor).
- Users may inadvertently connect a higher cell count than selected or thermally insulate the device so I want overtemperature and overcurrent protection.
- Users can choose cell count with a DIP switch so I want a constant power load.
- I also want to decrease the BOM cost of course, so one device that does it all would be wonderful :). So far a PTC thermistor fits the bill but I am wary of leaving it in a tripped state for extended periods.
Edit: One last requirement that I overlooked (I apologize). The device will be "plug in and forget" so it needs to have a very low quiescent current so it can be left on the battery for months or even years without over discharging the battery (which will damage it). The solution I have right now has a leakage current in the microamp range.