Asked by: Daniel McLaughlin, Pudsey
A battery uses a chemical reaction between two materials to produce electrical charge. What you're seeing are its last gasps as the energy from the electrochemical reaction peters out. When you disconnect the battery, a small amount of charge trickles to the terminals. It's enough for a momentary surge of voltage when you reinsert it. But, by now, the dying cell is too lacklustre to sustain the flow of charge.
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