Do you mean overnight or within an hour or less? If it happens in a very short time, most likely the battery has a shorted cell and must be replaced. If it needs overnight to go dead, suspect a drain from something that isn't turning off. Glove box lights and trunk lights used to be common, but now cars have so many computers that cause trouble too.
The place to start is by measuring the battery voltage with an inexpensive digital voltmeter. With the engine stopped, a fully charged battery will read close to 12.6 volts. At 12.0 volts, the battery is good but discharged. If you find around 11 volts, a cell is shorted.
Next, with the engine running, the battery voltage must be between 13.75 and 14.75 volts. If it is low, there is a problem with the generator or voltage regulator. It is not recharging the battery after starting the engine.
If those voltages are good, suspect a drain. A common way to verify that years ago was to remove one battery cable as soon as the engine was stopped, then reconnect it the next morning. If it still was charged and started the engine, you knew the battery was okay and something in the car was draining it. I DO NOT recommend trying that now because there are so many computers on the cars and things lock up and stop working. Volkswagens are the worst offenders. Disconnecting the battery or running it dead is guaranteed to result in the need to have the car towed to a dealer. I don't know what will happen to a Saab if you disconnect the battery.
I should ask too how you know the battery is dead and how do you get the engine started? What exactly are the symptoms? Does anything work such as the radio or head lights?
Sunday, June 12th, 2011 AT 6:11 AM