Do I Need a New Battery?
You might, but then again you might not. The only way to know for sure is to (1) test the condition of the battery to see if it is capable of holding a charge, (2) check the output of the charging system to see if it is functioning properly, and (3) if the battery and charging system are okay, check for a possible current drain on the battery when the key is off. In other words, if the battery is okay and the charging system is doing its job, then something is draining voltage from the battery and running it down when the key is off.
One way to check the battery is to recharge it, then let it sit for a day with both battery cables disconnected. If the battery holds the charge and doesn't run down, it's probably okay, and the problem is in your charging system or wiring.
To see if the charging system is working properly, start the car and turn on the headlights. If the headlights are dim, it indicates the lights are running off the battery and that little or no juice 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 functioning normally.
You can also check the charging system by connecting the leads of a voltmeter to the battery. When the engine starts, the charging voltage should jump to about 14.5 or higher. If the reading doesn't change or rises less than a volt, you have a charging problem that will require further diagnosis.
If the battery and charging system seem to be working normally, the only thing that's left is the electrical system. If the battery runs down overnight or when the vehicle sits for several days, it means something is remaining on and drawing current when the ignition is turned off. It may be a trunk light or cigarette lighter that remains on all the time, a fuel pump relay or other relay with frozen contacts that's drawing current, a rear window defroster that doesn't shut off, or a short in the radio or other electrical accessory.
All vehicles draw a little current from the battery when the key is off to run the clock, keep the memory alive in a digital radio (so it doesn't forget the station settings) and the engine computer. Alarm systems need current to keep their circuits armed as do cellular phones.
Current drain on the battery can be checked with an ammeter. Make sure the ignition is off, then 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.
Finding the hidden current drain can be time consuming. The easiest way to isolate the problem is to pull one fuse at a time from the fuse panel until the ammeter reading drops. This will tell you which circuit is draining the battery. Then you have to check the wiring and each of the components in that circuit to pinpoint the problem.