There are a few things you can do for your battery if it is low, before you go to the length of replacing it. If your car battery is totally out of power, you should jump the car battery. Make sure you use the jumper cables appropriately and match first positive to positive, then negative to negative (on the good battery), and finally ground the other end of the negative cable on the frame or engine of the vehicle that has the dead battery.
Start up the car that has a working battery and run it for a few minutes. If your battery is holding a charge, this should put you back in business. If, however, there is still something wrong, you will need to go to the next step.
Once the car is off, take the battery caps off and look at the levels. You can add distilled water if they are a little low. Corrosion is a common problem for the terminals. You can clean those off using a toothbrush and some water and baking soda.
Scrub off the terminals carefully and wipe the area down, being sure to dry off any moisture before trying to start the car. You can use anti-corrosion on the battery terminals to prevent it in the future.
Now, use a voltmeter to check the voltage of the battery. It should be between 12.6 or 12.8, if it is holding a full charge. Then, using a hydrometer, you can examine the electrolyte cells, looking at the sulfuric acid concentration. If it is under 1.215, you should probably replace the battery. Just be sure that you buy a battery that is compatible with your vehicle’s opening and electrical specifications.