Battery problems. - 986 Forum - The Community for Porsche Boxster & Cayman Owners

nefarious986 · 08-19-2008, 02:50 PM

Hey guys,

My battery died yesterday and left me stranded. I paid for a jump ($55 from AAA) and got the car back home (45 minute drive). This morning it didn't start again, so I walked to Kragen and got a new battery. Its an Autolite 48-84.

I thought 48 was our battery size, but this one was physically a bit smaller than the OEM battery. They didn't have one exactly the same sized as ours, but the guy looked it up though, and said this should work for the car.

Now its installed and cranks, but at idle, the car is not staying at the same RPM. Before, it usually stays at 800rpm flat, with no movement on the needle, but now its fluctuating down to about 600 and then revs back up to about 900 automatically and then drop back down to 600. This would happen constantly.

Car ran strong yesterday after I got the jump start, but haven't been driven since I installed the new battery. The fluctuation rpm worried me and I turned it off after the car idled for about 3 minutes and didn't get better.

Did I buy a bad battery or the wrong battery size ? Or could it be something else ? Should I take it to a shop ?

Thanks.

j.fro · 08-19-2008, 03:26 PM

When you disconnect the battery, the ECU loses all of its memory for the air/fuel parameters for your car. All of the values are reset to the defaults, and the computer needs a bit of running time to find what's just right for your car.

nefarious986 · 08-19-2008, 04:02 PM

I just took it for a 5 mile drive, and it got a bit better. Not much, its fluctuating still but less. Maybe +- 150rpm instead of +-200 from 800. Hopefully it will be better within a day or two of driving.

Thanks.

Lil bastard · 08-19-2008, 04:28 PM

You need to have a load test done on the alternator.

The alternator may have cooked the old battery, meaning that the alt. is the problem, not the battery.

It may just be that you haven't completed a drive cycle for the DME to relearn. Keep an eye on it over the next few days and if no better, check the alternator.

FTD · 08-21-2008, 07:31 AM

How does one test a sealed battery?

Lil bastard · 08-21-2008, 09:20 AM

Quote:

Originally Posted by FTD

How does one test a sealed battery?

Most batteries are more than 12 volts. They have 6 cells, each producing around 2.1 - 2.2 volts. Wired in series, these cells combine to produce about 12.6 - 13.2 volts.

To test the battery, you want to disconnect one terminal and then using a multimeter (DMM), touch a lead on their respective battery post (- or +). If the battery is healthy, you should be seeing voltage in the 12.6 - 13.2 volt range.

Anything under 12.6 or over 13.6 says there's something not right, either low electrolyte, sulphur crytals built up on the plates, a cracked plate or cell. The battery may still work, but is on it's way out and is likely to make the alternator work harder, placing more strain on it's service life and reducing MPG slightly too.

It's also a good idea to test the alternator function while you at it. To do this, replace the battery terminal and start the car. With just the car running, placing the DMM leads as before, you should see somewhere between 13.5 - 14.5 volts (for the oem Bosche alternator) - this is the alternator voltage. Now, turn on accessories such as the headlights, radio, turn signals, fan (but not the AC), this is called a load test. The minimum voltage reading should now should be no less than 13.0 - 13.2 volts. If under this voltage (and assuming the battery tested healthy), or over 14.0 volts, suspect a problem with the alternator, usually the diode pack (one or more diodes may be faulty).

FTD · 08-21-2008, 09:46 AM

Quote:

Originally Posted by Lil bastard

If under this voltage (and assuming the battery tested healthy), or over 14.0 volts, suspect a problem with the alternator, usually the diode pack (one or more diodes may be faulty).

How much over 14.0 = suspect a problem? 14.2?
And how does an overage indicate alternator weakness?
Thanks!

Lil bastard · 08-21-2008, 10:22 AM

Quote:

Originally Posted by FTD

How much over 14.0 = suspect a problem? 14.2?
And how does an overage indicate alternator weakness?
Thanks!

There are a lot of variables at play, idle speed, belt tension/slippage, variances in each individual alternator, battery, and 'load' component.

Plus the alternator does degrade with time, while still having many useful miles left in it. So I wouldn't worry about a couple tenths.

But a wide variance, maybe 0.5 - 1.0 or more volts would concern me.

An large overage is a problem because the alternator alone, without the voltage regulator (diode pack), is capable of producing upwards of 40 volts which would fry the important circuitry in the DME, alarm, radio and other stuff, maybe even cause a fire.

nefarious986 · 08-21-2008, 10:39 PM

Good news guys. Problem is gone. Car is running strong and idling smooth. It was probably the original battery's time to go ( it was 5 years old ) and not the alternator.

The funny thing is that it remembered my seat settings but not the engine settings. LOL.

Weird though because when the old battery died, I was still able to have stereo and lights and gadgets, so it wasn't completely dead, just not able to crank over.

It didn't give any warning though. I started it and drove it just fine 30 minutes before and when I got there, it just wont start again and just died. Oh wells.

Thanks guys.

Boxtaboy · 08-22-2008, 03:35 AM

Quote:

Originally Posted by nefarious986

Good news guys. Problem is gone. Car is running strong and idling smooth. It was probably the original battery's time to go ( it was 5 years old ) and not the alternator.

The funny thing is that it remembered my seat settings but not the engine settings. LOL.

Weird though because when the old battery died, I was still able to have stereo and lights and gadgets, so it wasn't completely dead, just not able to crank over.

It didn't give any warning though. I started it and drove it just fine 30 minutes before and when I got there, it just wont start again and just died. Oh wells.

Thanks guys.

Good to hear all's well. My OEM battery left me stranded on its 5th year too. Same thing too- started just fine, and then after I stopped somewhere for a few minutes, tried to start her up and everything else worked but couldn't crank the starter. Replaced it with a Sears Diehard, and now I'm going to replace on the 4th year just as a precaution so I don't get stranded anymore.