Yes you can overcharge a battery, it all depends on the charger your using. Most chargers are single stage chargers. Most scooters come with a single stage charger. My scooter has a built in single stage charger What this type of charger does is output a constant 24v @ 4amps.The problem is batteries amperage requirements decrease as the battery charges. An overcharge will occur when the battery is charged. It requires very little amperage to maintain a charge on a fully charged battery. A single stage charger like mine will output 4amps regardless of the state of charge of the battery. Batteries bring charged with single stage chargers loose their electrolyte. What happens is overcharged batteries break down the electrolyte into oxygen and hydrogen. So overcharging will cause a battery to "boil-out" its electrolyte. Batteries don't work without electrolyte. In the old days there were only single stage chargers. That was the state of the art back then. So to cope with the constant overcharging batteries had refill ports where you could replenish the electrolyte using distilled water.
Scooter batteries are far and away different from the lead acid of the 1970's. Scooter batteries are sealed, they are deep cycle, they use a glass mat soaked in gelled electrolyte. These glass mat batteries optimize the electrolyte used so they can use allot less of it. These newer batteries have a lot more power in a lot smaller package but they have a draw back, they must be charged properly. You can absolutely can not charge these batteries with a single stage charger. Actually you can charge with this device but you will loosed electrolyte each time you charge.
I use a three stage charger 24v @ 5amp. This charger s different from a single stage. Its what they term a "smart charger" it monitors the state of charge of the battery. As the battery charges it turns down the amperage. When the battery is fully charged the amperage is turned low enough were the charge is maintained. The entire charging process is designed to preserve electrolyte. In short this type charger will not overcharge the battery.Click here
to see the charger i currently use.