PTC Thermistor Based Discharge Load

[yshrestha]

Hello,

I am trying to make a circuit that slowly discharges a multi-cell lithium battery to storage voltage. 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 relatively constant power based on ambient conditions (more power dissipation in colder ambient temps, lower in warmer ambient temps) and feature overcurrent and overtemperature protection. 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. I have bench tested this and found it to work great and checks off all of my requirements but I am wary of using a resettable fuse as a power resistor essentially.

- 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.

- I want to stay away from DC to DC converters if possible.

I have attached my proposed schematic. Any help will be appreciated.

[MosherIV]

Hi

The classic way to discharge batteries is with an 'electronic load'

See this
http://www.ko4bb.com/Test_Equipment/DynamicLoad/

You are wanting a cut off at a particular battery voltage. You can do this by adding another opamp which compares the battery voltage against a Vref. Use the output of the opamp to either turn off the power transistor or pull the set point voltage to 0V