I'm not as smart as you think, I only have the one charger for this type of battery (a block of 4 18650s). Normally, I can get 3 weeks out of a charge but it has been 2 weeks, 1 week of commuting (last week) and most of this week in the garage (lows of -15C/5F to this morning's -26C/-15F) so I decided to check the battery condition today for this afternoon's ride home in dusk and nearly dark conditions.