ORIGINAL: CustomPC

There are now several manufactures that make switches or devices with built-in mAh counters. These are semi useful for eyeballing the usage but the model has be on the ground to read it. The pilot needs to check the mAh used from the pack before he takes off and decide whether there is enough charge remaining for a safe flight.

Personally, i'm using a telemetry current sensor on my Multiplex radio. The sensor measures current and mAh used and stores the sum of the usage in it's memory between flights. I have a constant real-time readout of remaining capacity and an alarm when the pack reaches a preset usage. The receiver also monitors the voltage and triggers an alarm if the voltage drops.