Safe LiPo Disposal

[BrianG]

I would size the resistors so it discharges the pack at a slow 100mA. You could get different resistors depending on the size of the pack (2s, 3s, etc), or make an adaptor cable so you discharge each cell individually through the balance connector.

resistor_value = pack_voltage / discharge_current
resistor_power = discharge_current^2 * resistor_value * 3

The "* 3" is a safety margin for the resistor so it won't get too hot. So, to discharge a 2s pack (assume worst case 4.2v/cell) of 8.4v at 100mA, the resistor would be 84 ohms ( 8.4v / 0.1A). Since 84 is an odd size, choose 100 ohms. For the power, you should get a power value no lower than 3w (100mA^2 * 100 ohms *3). You can choose 3w or higher, it doesn't matter - as long as you don't go lower in power.

If you were going to discharge each cell individually through the balancer leads, just use 4.2v in your figures above. You'd need one resistor for each series cell to do the whole pack at once.

Higher value resistors will just take longer to discharge, which doesn't matter unless you are in a hurry. A full 4Ah battery will take ~40 hours to totally dump at a 100mA draw.