It’s important to test your battery and electrical system regularly, not just when it’s starting to show signs of weakness. Proactively testing it (or making sure your mechanic does) at least once a year will help reduce your chances of failure. Refer to your owner’s manual and your battery tester manual for instructions. Review all safety instructions that came with your tester and battery. Remember that batteries contain sulfuric acid that can cause severe burns and hydrogen-oxygen gases that can be explosive.
Fully charged automotive batteries should measure at 12.6 volts or above. When the engine is running, this measurement should be 13.7 to 14.7 volts. If you don’t have a multimeter to tell you the voltage of your battery, you can do a test of your electrical system by starting the car and turning on the headlights. If they are dim, that indicates the lights are running off the battery and that little or no charge is being produced by the alternator. If the lights get brighter as you rev the engine, it means the alternator is producing some current, but may not be producing enough at idle to keep the battery properly charged. If the lights have normal brightness and don’t change intensity as the engine is revved, your charging system is probably functioning normally. If you’ve been experiencing problems with your battery system and the headlight test checks out okay, you should check whether or not the battery is holding a charge, or if something on the vehicle is discharging it.
There are three likely scenarios that could explain the problems you’re having:
A high parasitic draw (“key-off” load). This can quickly discharge a battery and decrease its service life. This may be caused by a trunk light, cigarette lighter, clock/radio, alarm system or any other electrical device. Current drain on the battery can be checked with an ammeter. With the ignition off, disconnect one of the battery cables. Connect one ammeter lead to the battery and the other to the cable. The normal current drain on most vehicles should be about 25 milliamps or less. If the key-off drain exceeds 100 milliamps, there’s an electrical problem that requires further diagnosis. If you don’t want to take your car to a mechanic, the easiest way to isolate the problem is to pull one fuse at a time from the fuse panel until the ammeter reading drops.
A problem with your battery is causing it to not hold a charge. To check this, wait 12 to 24 hours after charging to the full voltage, keep the battery out of the vehicle and measure its voltage. Another faster but less preferable way to do this is to turn on the high-beam headlights for 15 seconds, turn them off, wait five to 10 minutes, then check the voltage. If you measure the voltage of the battery the next day, week or even a month later, the voltage should be close to the max voltages listed above. If the voltage holds when not installed in your vehicle, but drops when it is in your vehicle, see 1 above.
The battery was somehow discharged, and your maintenance charger can’t properly charge your deeply discharged battery. Please see the directions for charging a deeply discharged battery.