Why does my battery charger not turn off sometimes? Is this a potential safety issue?

If the battery charger has been charging the battery for longer than expected but the LED on the battery is still red, the charger is still trickle-charging the battery (the final 10% or so of the charge cycle, which can take a disproportionately long time compared to the first 90%). If you were to remove the charger and connect the battery to the motor, it may even register as 100% charge. This is normal and helps protect the battery. Eventually the charging LED will turn blue and current will stop flowing to the battery entirely.

​The battery has a safety mechanism—an electronic charge management system—that prevents it from being overcharged.