There is a "Track battery cycles" flag for each device that lets you decide if you want to track average battery life and remaining battery life.
The GPS logger is already keeping track of how long it's switched on, all that you need to do is note when you change or recharge the batteries.
Based on that, GPSLog Labs can compute the average time your batteries last and you can use that to determine if there's enough life in them to cover your planned activity. No more losing valuable logged data because you forgot when you last charged them, and no more having to change the batteries just in case.
GPSLog Labs assumes each file is created during a single battery cycle (i.e. if you change batteries, a new file will be started) so for each log file you can choose which cycle it belongs to on it's "edit" page.
You can exclude battery cycles from the average calculations if they are unusual (e.g. you deliberately changed the batteries early, or forgot to not when they were changed.)
There's a warning on the home page when your device reaches 75% of it's average battery life.
Graphical display of Device's battery cycles. Excluded cycles are shown in red, and the current cycle's time is shown as a green line.