Yes, bad to overcharge your batteries.

You have to work at it to damage them with the factory charger that comes with your radio though. If you should forget them and leave them 24 hours or so, it shouldn't hurt them. But with higher current chargers and fast chargers, you can damage them. With the fast chargers, improperly used, you can overheat the cell s causing them to vent the electrolyte. With the higher current chargers, or a standard charger
left on too long, overcharging lowers the peak voltage and tends to reduce the capacity of the cells.

For lots of battery information, take a look at www.srbatteries.com website. There are some pages on their site that will tell you things that you didn't even know you didn't know.

Larry