bad battery or alternator-- how to tell?

[woody]

i can't tell whether my alternator or battery needs fixing. if my car sits for a few days, it will not start up on its own (need to use a portable car starter i always keep in the trunk).

if i start it up that way, then drive around for about 20-30 min, and then park it, it will start up on its own.

which leads me to believe it's probably the battery not retaining charge and needs replacement. but how can i be sure?

how do i test my alternator (while it's still in the vehicle)? Can i measure the voltage going INTO the battery while the car is running or something?

[jlevie]

Sounds like the battery or excessive parasitic draw that is draining the battery while it is parked. The alternator is apparently working and charging the battery, or else the car wouldn't start after driving.

You can do some simple checks of the alternator. With the battery at full, or nearly full, charge the system voltage at idle should be a bit better that 13v and it should remain above 13v when the head lights and heater blower are turned on. System voltage should rise to a bit above 14v at 2500rpm.

[woody]

can i just attach a voltmeter right to the + and - cables that hook onto the battery?

[Mike325]

^ Yes. You should be getting 14+ volts with the engine ideling.

[jmarch]

Disconnect the negative cable from your battery and use a multimeter (between the disconnected cable and the negative terminal) to measure drain in amps. The number should be very low (less than 1). If it isn't begin pulling fuses to find your drain. Once you pull a fuse which drops the drain significantly - you've found your problem.