The Wirecutter explains why predicting rechargeable battery capacity is hard.
http://thewirecutter.com/2016/07/why-your-phone-dies-when-it-claims-to-have-battery-left
Their AA battery charger review explains why slowly recharging batteries is a good idea.
http://thewirecutter.com/reviews/best-rechargeable-battery-charger/
— Rex
