12-volt mower batteries are almost identical to 12-volt car batteries except that they are physically smaller. They both run on the same type of electrical system, but on a lawn mower the setup is much simpler, which is why the battery can be smaller yet still keep a lawn tractor up and running for years. The size of these batteries is what makes the real difference here, and you'll need the right battery charger for the correct job.
Locate a battery charger that charges in the 2-amp range with a charging meter. This can be an automotive battery charger with a variable charging feature or even a motorcycle battery charger. Make sure it is rated at 12 volts.
Find the positive battery post on your mower. They are generally marked in red, but every battery post is marked with either a positive or negative symbol.
Attach the positive or red cable from your charger to the positive battery post.
Attach the black or negative cable from your charger to the negative battery post.
Turn your charger on and if this is a variable charger, make sure the switch is set at the 2 amp charging position. Higher amperage might overcharge your battery and could do catastrophic damage.
Check the charging meter every hour or so; once it reaches "zero" your battery is fully charged. You can now unplug the charger and remove the cables.