system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do coin batteries seemingly die, then after a days rest return with renewed vigour?
gpt
Battery voltage depends on how much load is being placed on the battery. One that's 3.0V at rest will fall to, say, 2.9V with a light load. Under a heavy load, it might fall to 1.5V. 

The reason is there is a rate limit to how fast the chemical energy can be converted to electrical energy. If a battery is placed under a heavy load, the reaction goes as far as it can before it "falls behind" demand. You're basically stripping off the "surface charge" and have to wait for the "deeper" charge to replenish the "surface". If you then let the battery rest for a while, the chemical reactions can "catch up" and restore voltage. 

The voltage drop is also influenced by the battery's own internal resistance (which is always more than zero), so if you put a battery under load the voltage will instantly drop some (due to resistance) and then slowly fall more as you deplete the surface charge.