When the lead acid battery is charging it behaves as a set of [electrolytic/voltaic] cells. It converts energy into energy as a [spontaneous/non-spontaneous] chemical reaction is forced to occur due to an external potential difference being placed across two electrodes. To charge a lead acid battery, the lead dioxide electrode must be connected to the [positive/negative] terminal of an external power source, in other words another battery. The lead electrode must be connected to the [positive/negative] terminal of this other battery. This pulls electrons from the [lead/lead dioxide] electrode and forces electrons into the [lead/lead dioxide] electrode. This causes the reactions which had occurred in the discharging reaction to take place in [the same direction/reverse].