Yes
It sounds like you have nickel-cadmium batteries, since 7.2 volts is an integral multiple of 1.2 volts. You should notcharge a nickel-cadmium battery with any charger that is not specifically designed for it. The issue is that overcharging these batteries will damage them, and that the battery voltage alone is not enough to determine state of charge. (You have to measure the slope of the voltage, and detect the inflection point which occurs at full charge. This is a very specific thing, so the battery must be matched to the charger.)
A NiCad charger has different sensor values than NiMh. I would find a charger that has a switch to select either, or specifically a Nickel Metal Hydride battery charger to prevent damaging and shortening the life of your batteries.
Yes, you can charge a 500mA battery with a 1300mA charger.
You cannot charge a battery without a battery charger or having it charged by the alternator in your vehicle while it is running.
There is no only one way to charge it and that is with a battery charger. A 15 amp battery charger will charge it about as fast as it needs to be charged.
sure, you can buy a battery charger that will charge any car battery.
No, the battery charger has to match the voltage of the battery it is to charge.
If it is an automatic charger let it charge until the charger shuts off.
A trickle charger is a car battery charger that is designed to be left on a car for very long periods of time to re-charge that car battery by slowly adding charge to the battery.
No, you can not charge a 3V battery with 12V charger.
You do not charge a 7.2 volt battery with a 9.6 volt charger period unless you want to ruin the battery. You charge it with a 7.2 volt charger.
No, you cannot charge an 8 volt battery with a 6 volt charger. The voltage of the charger must match the voltage of the battery.