Cold Voltage Drop

The Porsche wouldn't start for my wife this morning. It was 27 out. Not THAT cold. I checked and the battery was a 11.4V and it would barely but not really turn over. I put the charger on it for 10 minutes and it started right up. 13.6 volts running. Took it for a 20 minute drive and ended up with a non-running voltage of 12.5. Battery is 18 months old.
I know that 964's are battery leaking fools. Sit them for two weeks and you're hosed. The car had only sat for 3 days though, and it's sat longer than that before and been fine. I'm trying to figure out if I'm chasing a car problem or a battery problem or if it was just one of those colder mornings when all of the stars were aligned for a non start and there's not really a problem at all.

trickle charger?

Has the battery gone completely flat before? Maybe it's got a dead cell?

In reply to Giant Purple Snorklewacker:

It hasn't gone flat since I'VE owned the car.
This is the first battery related no start. It could still be bad but the date sticker says 8/12.

Have you measured how many amps the electrical system draws with the car off? I don't like to see more than about a 20-30 mA draw when the car is off.

RossD wrote: trickle charger?

But not for too long. Apparently they smell like natural gas. According to the guy that came buy to check out that "leak".

I think that's about what I would expect. 11.4 is iffy, some cars will still start at that point, most will try and fail. 3 days, cold weather, 18 month old battery, and a no-start makes sense to me. I think you'll be replacing the battery soon unless it's a warm-weather only car.

mazdeuce wrote: In reply to Giant Purple Snorklewacker: It hasn't gone flat since I'VE owned the car. This is the first battery related no start. It could still be bad but the date sticker says 8/12.

Yeah. It was a brand new battery. If it fails a load test it should still be under warranty. I bet the previous owner bought it from a national chain... probably Advanced Auto or Autozone :)

It would be difficult for them to demand a receipt with the date stamped on top next to a big label with 7 YEAR WARRANTY! or some such thing I recall it having.

I would say there's either a current draw that's too big, or the battery is failing. Sitting three days in 27 degree temperature should have no effect, I've let my car sit longer than that in subzero temps and it pops right off. Since the battery is still pretty new I'd be looking for a current draw - maybe a glove box light or a relay somewhere is stuck on.

Is it a gell cell or AGM type of battery? Premature failures of those type batteries are more common than in regular wet cell lead acid batteries, and they will do some strange things before failing completely.

My 911 has always seemed more sensitive to batteries than some of my other cars. I always keep it on a trickle charger now and always run Interstate batteries, which I've had good luck with.

The battery is an Autocraft Gold. I was suprised that the car wouldn't crank at 11.4V. A Porsche thing? The old wagon will turn over it's 390 with less voltage than that.
There doesn't seem to be anything on in the car. All of the lights are shutting off like they should. I'm going to see what it does over the next couple of days before I start chasing electrons. I suppose I need to figure out how to measure load with my voltmeter.

With a battery that is borderline in the cold, it helps to turn the headlights on for a couple minutes, turn them off, and then try to start the car. Turning the headlights on, while it may seem counter-productive, heats the battery up so it will work better.

For some reason, batteries sold in the southern states just don't seem to like sudden cold snaps. The year old DURACELL brand (East Penn) in my Bonneville dropped a cell last Thursday night, it got down to the high twenties for a couple of hours. I had it disconnected while replacing the lower intake mainifold gaskets Thursday and Friday. Just replaced it under warranty Sunday morning. Didn't know about the headlight trick, will have to try it out next time i've got a low battery.

Sounds like it's draining thru the alternator. Put a meter on it cold and see if the voltage drops slowly before your eyes. Then disconnect the alternator and see if it stops.

Charging voltage should be higher. Around 14.3 at 1500 rpm. check for a draw. Load test the battery.

I forget the failure mode, but sometimes a battery will show good voltage, but die out under load. Try a toaster tester.

mazdeuce wrote: The battery is an Autocraft Gold. I was suprised that the car wouldn't crank at 11.4V.

To clarify, was it 11.4 WHILE cranking, or 11.4 static? Those are extremely different scenarios. If it was 11.4 static, I bet it dropped to nearly zero when you put a starter load to it. Could be an isolated incident where something was causing a drain. Could have been that it didn't start on the first crank and wifey managed to flood it and kept cranking for a while before finally calling for help, causing it to go dead just as you'd expect it to.

Lots and lots of what-ifs could create this scenario...but first things first, check for drains with the key off, doors closed, etc. (use a multimeter to measure current between the negative battery post and disconnected negative terminal). If that seems low (<100 milliamps), then give the battery a good, long charge after making sure the water is topped off if it's not a "maintenance free" type. Once you've done both of those, start it up and check the voltage at idle and above idle. Make sure the voltage is 13.8+ (not sure what Porsche spec is) and make sure it's the same voltage on both the alternator post and at the battery post.

These system confirmations will catch the real obvious stuff, take little time, and require no money. Start there (ha...get it!) and report back if you experience further troubles.

Bryce

Nashco wrote:

mazdeuce wrote: The battery is an Autocraft Gold. I was suprised that the car wouldn't crank at 11.4V.

To clarify, was it 11.4 WHILE cranking, or 11.4 static? Those are extremely different scenarios. If it was 11.4 static, I bet it dropped to nearly zero when you put a starter load to it. Bryce

Yeah.^

It was 11.4 static and didn't crank over more than once. I was opening the gate for her and watched it all go down. She wasn't happy. It's at 12.34V right now after sitting all day. Tomorrow I'll embark on trouble shooting things. Thanks very much for the advice.

wouldn't be the first time someone left a door cracked … you usually don't notice it as you're getting in … but when you go to try and crank ….duh … nada