Bad Battery/Alternator = Low Idle??

[DarkWing6]

My E30 is not my daily, so at best I drive it once a week. Since it sits so much I have had to jump it from time to time (after times it has sit for a couple weeks or more).

Recently I have noticed that the idle is a bit low and that it hasn't been too consistent. Probably idling up and down from 350-500. I was going to tackle that with the basic low idle things until I had the idle drop down when parallel parking and die on me. When I turned the key to restart...nothing. I assumed it was battery related because the was no power at all (I didn't have to jump the car to start it that time). I got out of the car, jiggled the battery a bit, got back in, and started the car.

Similar situation happened today after work. Started the car this morning no problem. Low/jumpy idle, as it has been. After parking the car at work all day I went to start it to go home and there wasn't enough battery juice, at first, to start the car. Got out, jiggled the battery, etc, etc it started.

I guess this boils down to...is this an alternator problem causing this low idle and the battery to die like this, or is the battery bad? Generally I would guess battery first because of the starting issues, but the idle makes me think the alternator isn't giving it enough juice.

[jlevie]

It could be both. The battery is suspect if it has some age on it (over two years) and the history of having to jump it "from time to time" would have drastically reduced battery life from sulfation of the plates. If you aren't going to drive the car for a week, connect a battery tender to keep the battery charged. The battery will last lots longer if you do and you won't be faced with a dead battery when you do want to drive it.

Get the car started and take it to most any auto parts house and have the battery and alternator tested. Many times you can have those tests done for free.

[HST]

Auto zone here will test both for free. If you turn on the headlights at idle them rev it up do they get brighter? If so I say alt. If not it is most likely battery. My money is on battery.