# How do safely discharge Li-Ion batteries to check full capacity

I regularly have to test old phone batteries in my mobile phone refurbishment business. I need a safe solution to discharge 3.6V batteries. When I buy batteries they come from old stock so I need to discharge them them check how many mAh it still takes. This is difficult and time consuming. In the past I've used a 3.6V light bulb but the process takes forever. I was considering a 3.6V motor but that would probably get warm and break pretty fast. Any suggestion? I know I'm not supposed to completely discharge Li-Ion batteries but these are phone batteries with built-in protection so it should be fine.

• Just to be clear, you want to quickly measure the capacity of the battery, right? The easiest way to rapidly discharge the battery would be with an appropriate resistor. But if you want to measure the capacity, then it is better to discharge with a current sink. Just because the math is easier. Time * current = capacity. With the resistor method, current changes over time so you need to integrate. Commented Mar 19, 2021 at 23:35
• Probably you should buy a battery cycler. Commented Mar 19, 2021 at 23:35
• Are the "3.6V" batteries Lithium? A Lithium battery cell is 3.2V when almost dead and is 4.2V when fully charged. If you discharge below about 3.0V then it is ruined. See it at www.batteryuniversity.com . Sometimes the phone has a protection circuit that disconnects the battery when its discharge voltage gets too low. Commented Mar 20, 2021 at 0:33
• @Audioguru it is an exaggeration to say that if you discharge it below 3.0 it will be ruined. In general the best way to discharge it for testing purposes is according to the cell manufacturer's specification. But a single deep discharge will not destroy the cell. Commented Mar 20, 2021 at 4:19
• You can buy capacity measuring gizmos from the usual absurdly cheap sources. Connect a big resistor (or that bulb) to the output, program the cutoff voltage, and wait. When the voltage falls below cutoff, it stops discharging and displays the capacity.
– user16324
Commented Mar 20, 2021 at 14:23