The charge controller circuit inside them simply shuts off electricity flowing into the battery when it detects that the battery has reached maximum safe charge level. In super simple stuff like solar garden lamps there is no such circuit and the batteries actually do get overloaded, but the energy input is so low that the batteries can survive it well enough by shedding the extra energy as heat (this is dependent on battery type – don’t try this on Li-Ion, but Ni-MH can survive it at very low power).
They cant get past the “full charge”. There is electronics on them that control the charging process and will stop charging when you hit certain limit.
Fun fact – its not optimal to charge batteries up to 100% (simplifying a little here). So a lot of the phones will display 100% charge, while the actual charge is at optimal limit to extend battery life.
Fun fact no2 – you can see example of overcharging when charging car battery with some of the older chargers that don’t check for charge level. If you leave it on long enough, acid inside the battery will start boiling, which can lead to a nasty accident if you allow it to happen..
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