Suppose you have three batteries, with capacities of $30,20,$ and $15$ Ah $($Amp$-$hours$),$ respectively. The $20$ and $15-$Ah batteries are fully charged $($containing $20$ Ah and $15$ Ah$,$ respectively$),$ while the $30-$Ah battery is empty, with $0$ Ah$.$ You have a battery transfer device which has a $$source$$ battery position and a $$target$$ battery position. When you place two batteries in the device, it instantaneously transfers as many Ah from the source battery to the target battery as possible. Thus, this device stops the transfer either when the source battery has no Ah remaining or when the destination battery is fully charged $($whichever comes first$).$ But battery transfers aren$$t free! The battery device is also hooked up to your phone by Bluetooth, and automatically charges you a number of dollars equal to however many Ah it just transferred.