A battery ground post can loosen over time. When the ground or positive posts become loose, this can cause problems such as a battery that won't start or battery fluctuations that cause the vehicle light to flicker. Checking and re-tightening these bolts is a simple process that can be accomplished with basic hand tools, though care must be taken when using metal tools around your battery.
Park your car on level ground, and open the hood. Make sure the engine is cool. Locate the battery. It is typically in the front of the engine, either along the driver side or passenger side. Some vehicles, such as the Mazda Miata, place the battery in the trunk. If cannot see your battery, check the owner's manual to confirm location.
Check the battery terminals. There will be two large battery cables that connect to the positive (red) and negative (black) terminals of the battery. In addition, there will be a secondary black wire that runs from the negative terminal to a grounding point, which is a piece of metal in the engine bay. With the engine off, check to see if they are tight, or if they rotate or slide off the battery posts. The terminals and the ground wire will typically be held on my a bolt that is 10mm to 14mm in size.
It is a good idea to clean the terminals at this point. Loosen and remove the terminals, them use a steel brush to clean the battery posts so that there is no oxidation or debris. Do the same for the cable terminals. Spray a dab of dielectric grease on the terminals, then re-connect the terminals and tighten down the bolts.
Tighten down the ground wire, which will be the single black wire. It will typically be secured at the grounding point with a 10mm to 14mm bolt as well.
