We place the batteries in a computer-controlled test rig that charges them, then discharges them at a rate equivalent to the demand of a high-drain device — like a remote-controlled car. Once the cell voltage reaches 1.0V, equivalent to empty for most power-hungry devices, the rig charges them again.
- We judge each battery’s long term endurance by measuring how many cycles until its capacity falls to half its initial value.
- The battery’s average capacity over the amount of charge/discharge cycles before failure gives our battery life per use score.
- Rechargeable batteries gradually lose charge while sitting idle. We measured this rate of “self-discharge” for each battery to get our charge retention in storage score.
- We also look at the ruggedness of each battery’s construction by placing them in a rotating cylinder (a bit like a gentle tumble-dryer) for 24 hours, before taking them out and sorting them from least to most damaged.