The only way to properly maintain a 12v car battery is to regularly test it to see which parts need maintenance. You will have to make sure the battery looks right from the outside as well as verify that the lead plates are in good condition, and the electrolyte level is appropriate. A car battery tester consists of two tools called a voltmeter and a hydrometer, and the following list of tips will help you use them properly.
You will need to use a car battery charger to make sure that the battery is fully charged before you test it. An uncharged battery will give lower readings than a charged one. Next, turn on your high beam headlights for a few minutes to remove the surface charge, or charge collected on the positive plates of the battery. These can affect voltmeter readings. Finally, disconnect the battery leads before testing the battery.
If the battery is maintenance accessible, you should pry off the cell covers with a screwdriver to refill the electrolyte cells. Add distilled water until they are full.
Squeeze the rubber top of the hydrometer and stick it into one of the electrolyte cells. Release the rubber top to fill the hydrometer with electrolyte. It should read 1.265. If the reading is 1.225 or lower, you need to replace that cell. Repeat for the rest of the cells.
Set your meter to the 20-volt scale, and attach the positive lead to the positive terminal, and the negative lead to the negative terminal. The reading should be about 12.7 volts. If it is any less than 12.4 volts, the battery may need to be replaced.