Troubleshooting charging systems

[p-cardriver]

I have just started having some difficulty topping off my battery. I use a battery tender (BMW branded but similar to the good ones we all know and love). It is connected directly to the battery posts. In months past it would top off the battery (indicator light on the tender green) in about 12 hours.

This Spring I haven't used the tender much because I drive the car about once a week. There is no short drive where I live - 20 min is the shortest, 30 to work, and speeds/revs are pretty good

Last week I went to move the car (88 911 with 87k miles) and it wouldn't turn over (solenoid clicks, lights, etc all worked). This after just over 2 weeks of not being driven and not on the tender. So I connected the tender and it took nearly 3 full days to charge. Ok, it was pretty dead so although it seemed about 24 hrs long, I can roll with it.

Once charged fully, I drove it to work. No issues, no warning lights, everything worked as much as before (nothing was magically fixed, like say the windshield squirters). Actually I was careful not to use fans, radio, or anything else electrical and didn't need my headlights. So I thought once home it would top off pretty quickly. Immediately put it on the tender, 36 hours later it is still red - not fully charged.

Ambients are high and the garage is particularly warm (an insulated garage in summer isn't all that).

The battery does eventually top off, so I assume its cells are all good. The key is out and no accessories are on (except those I can't control, I suppose).

So either something is causing the battery to drain fast, or the alternator/tender are not able to bring it back as quickly as they should.

Would appreciate thoughts on where to start troubleshooting this one. (sorry for the long post, but details seem to help narrow the possibilities).

Thanks!

[theiceman]

There are a LOT of threads on how to trouble shoot parasitic drain , or checking charging systems. Rather than rehash it all I suggest you
1. Buy a multimeter if you dont have one.
2. Search "charging system" or battery drain"

There will be lots of how to advice. if you have any questions after that come back here .

Good luck.

[p-cardriver]

Iceman,
I've been reading threads all week on this, and I do have two multimeters to work with. I'm just looking for advice on which of the many tests I should start with, or which component I should either focus on or eliminate early. In other words, where experienced hands suggest I begin.

Some thoughts:
parasitic drains: although possible, these seem to be unlikely since there is no reason to think they have suddenly increased enough to cause this.
battery: as I mentioned, it does fully charge, so maybe a weak cell? Never heard of that. Only 3 years old.
alternator: sure, could be failing to charge the battery, allowing it to run down. Shouldn't I get a dash indicator light, though? And I wouldn't expect the same slow recharge from the tender.
grounds: always suspect, but charging from the tender directly to the battery should eliminate them.

[Joe Bob]

12.5v at rest
13+ at idle
13.5-14 at 2500 rpms.

Poorman's load test.....turn on all the accessories at night with the engine idling. Dimming of headlsights means your battery may need replacement.

A voltmeter on the battery and removing fuse one at a time will track down a circuit that has a draw.

[yel911]

I had kinda the same issue with my 993. My tender would go green in an hour or so. But realizing I had a dome light circuit short, pulled the fuse, tender was good again. Then it went red for a day long, battery died. Knowing how old your battery is helps. My was 7.5 years old.

[Scott Douglas]

How long was your battery warrantied for? 36 months?

[p-cardriver]

Battery is an Interstate 85 month warranty stamped into service Sept 2011.

13.1-13.2 vdc at the posts nearly fully charged.

[porwolf]

I would bring the battery to an auto parts store or gas station and have the battery capacity checked. Lead acid battery capacity deteriorates when stored but not fully charged.

[p-cardriver]

Voltages:
Key out: 12.9 dcv
at idle: 14.1 dcv
at 2500 rpm: 14.1 dcv
key out after: 13.3 dcv

Will test the battery with key out tomorrow since it had been on the tender all day. Then, if significantly different I will pull the battery and run it in.

[Joe Bob]

I wouldn't bother....that's a robust battery. Leave the meter on w/o the temder and cehck it every couple of hours. If it drops below 12v, start checking for a parastic draw.

I assume that you have no aftermarket add ons hooked to the battery?

[p-cardriver]

Correct. The only aftermarket mods to this car are a built in radar detector (vintage 1990, turned off and never used anymore) and a Steve Wong chip. Everything is switched off and double checked, and I keep the lid down to turn off the under-frunk light when on the tender, and the key out. So next will be pulling fuses and checking for parasitics in each circuit to see if something has gone amiss in the wiring. After I recheck the voltages over time as Joe Bob recommended.

[T77911S]

just because a battery charges up does not mean it is good. many batteries measure 12.6v and will not start a car.

have the battery load tested and have the charging system checked.
you can have it done for free at places like advance.

[p-cardriver]

I think I've got it!

The driver's side dome lamp was in the "door" position but doesn't light when the door is opened. It does light in the "on" position. Apparently it shorts and is drawing down the battery when in the door position. So it is now off and the battery is on the fast charger, then to the tender to maintain it.

Once the battery is charged I will double check everything to confirm again. Then I have to trouble shoot that light and the door switch.