Dead battery

Craig Nicol has this to say on how to test your battery and charging system.

Tools

You could have a small drain that's killing the battery, a bad charging system, a bad cable/connection, or a series of bad batteries.

Charge the battery
- Charge the battery with a good charger, then disconnect the battery and let it sit for a week. If it's still good when you reconnect a week later, the battery is OK
Measure the voltage
- With a charged battery, measure the voltage at the underhood terminal while cranking the engine. If it stays in the 10.5-11.0 Volt range, your cables are probably OK.
- After starting the car, again with a reasonably charged battery, measure the voltage at the battery while the engine is running and the lights are ON. It should measure 13.5 to 14.5 Volts. Less than that means a charging system fault.
Check for a battery drain
- Remove one battery cable and secure a 25W, 1-ohm resistor between the cable end and the battery terminal.
- Set your voltmeter to the lowest scale; like about the 1 or 2 Volt scale.
- Remove the ignition key and open and close the driver door to switch off the retained accessory power feature.
- Measure the voltage across the 1-ohm resistor. It should read .01 to .03 volts (max).
- If the voltage is more than .03, then something in your car is ON and draining the battery -- alternator, trunk light, radio, courtesy lamp, etc.
- Pull and replace fuses one by one until you find the faulty circuit and the voltage drops below .03 Volts.

Note: You will have to disconnect the large alternator wire to check the alternator as a potential source - that line doesn't have a regular fuse.

Thanks to Craig Nicol (90 V8Q Cayenne) for the original write up.