If you connect a dead battery and a charged battery, so the positive terminals are connected, the capacitance from the charge battery will flow into the uncharged battery. The trick is to mirror the output magnitude so the transfer can take place and to do that you need to learn the impedance ratios. For example, three triple A batteries can charge two double A batteries. After connecting them, seal them with electric tape. Its called electric tape because it protects a current. Leave the batteries connected for a day or until the counter flow of capacitance equalizes. At this point, all batteries will be charged equally and you can replace any or all batteries on one side of the equation to repeat the procedure and recharge the opposite side further. A dead battery can never be fully charged. You can speed up the process by applying a mildly acidic catalyst, like a bit of lemon juice, to the terminals beforehand. This speeds up the transfer.