You can overcharge a lawn mower battery if you use a battery charger.
Leaving a battery charger on the lawn mower battery for too long can cause the battery to overcharge and cause the battery to go bad.
A lawn mower battery should fully charge with a battery charger within a few hours to 8 hours at most.
You should not leave a battery charger on the lawn mower battery for longer than 8 hours because if you do the battery could overcharge and battery acid could leak out.
That happened to me when I had put the battery charger on the battery for the lawn mower and then I went in, had dinner, took a shower and went to bed.
The next morning I remembered I had left the battery charger on the battery and it destroyed the battery.