The rule of thumb is that you should never charge a battery at more than one-tenth its rating in amp hours. Meaning that a 20-amp battery should be charged at no more than 2 amps over a 10-hour period.
automotive-type battery chargers, designed to charge a good-sized car or truck battery, can easily overcharge a small motorcycle battery if you’re not careful. Unfortunately, in some circumstances, that may be the only type of charger available. Here’s the solution: Insert a standard test light, or even one built from a taillight bulb, inline between the positive side of the charger and your battery. Typically a test lamp, turn signal or brake light bulb draws about 2.25 amps.