When I bought the car 18 months ago it was very slow to start and voltage would drop to 11-12 while driving. I replaced battery, alternator and starter starter/alternator cable and voltage was now good at 13.5.

It’s been fine till recently when I worked to re-wire the headlights (nightmare) as I wanted to insert LED’s and realized all the wire coatings disintegrated so I had to repair all that.

I completed the repair and put the headlights back in and all lights are working perfectly (Auxito LED’s) and now I see my battery is draining again, down to 11.9-12.1. But the LED’s work great and I’m not blinding anybody.

So I suspect the lights and after recharging to 13.5 with my CTEK I have removed all fuses for lights (A 1,2,7,9,10 and B6) and am monitoring for battery drain. Will add them back in one at a time to see if any of these are the root cause.

I’ve seen the procedure for parasitic drain tests. I guess I’ll do that next if I can’t prove the recently repaired lights. But I don’t want to disconnect the battery to run the tests. Instead I’m using a Konwei battery tester to take my measurements.

Any suggestions for things to check if the lights don’t turn out to be causing drain?

Any obvious or well known battery drain culprits?

I’ve heard the alternator/starter cable is often the cause but I already had it replaced.

The CTEK is kind of a life saver as it allows me to at least get the battery charged after it drains to an unhealthy level.

 

Your plan is methodical and well thought out. You could always just pull out the light assemblies overnight to see if that makes a difference. I would also check under the center console to see if anything is connected to the communications cable.

 

Electrical Engineer here. I've been following this thread but haven't figured out a good diagnostic test except for the one you are using now. I suspect something went wrong when you installed the new lights, but that doesn't add much. The starter/alternator cable will not draw current when the engine is not running. That cable will only draw current when it is old and corroded, but that only happens when the car is running, not when it isn't. It would not be a source of battery drain in any case.

 

Thanks for your comments. I’m not sure if drain is happening while the car is running or asleep. So today I’ll try and confirm if there is drain while the car is asleep. If she stays at 13.5 after all the fuses are back in I’ll go for a ride and see if there is any loss during driving. I can confirm that the voltage does not drop any further when I activate the lights, the radio, the fan or the A/C.

I did just complete a long 350mi round trip to Maine. I think the level dropped over the course of that trip. So that would indicate the drain occurs while driving.

Rennlist is great. It’s like having your own personal Porsche therapist! :-)

 

Did you put in a brand new alternator? Still could have bad diodes. Let the car cool down to stone cold, open engine lid and place your hand on the alternator. If it is warm to the touch the diodes are bad.

 

Get one of these ...https://www.aeswave.com/Fuse-Buddy-P...Kit-p8907.html

It is the most useful tool you can have for testing for battery drain .Normal battery drain should be less than 30 milli amps..

You can test at the battery for the overall drain or at each fuse for individual drain..

 

I don’t yet have the meter to check milli amps but today I observed about .02 volts drop every hour on a rested and sleeping car. I added back one fuse at a time and the .02 is consistent. Is this normal?

 

so I had the "duh" version of this problem, it was kind of a reverse of a "is it plugged in problem"...
Turned out one of my kids had turned on the passenger reading light in the center of the wind-shield
and it was aimed enough to the right and back you could not see it was on from the driver seat..
If I didn't lock the car the light didn't turn off..

I found it by accident when I came out to my garage in the dark and noticed a faint glow on the passenger seat.
I was running a battery tender full time for a week or two before I found it.

 

FYI that tester really has a higher resolution than I like ( .1A =100MA) ...but has all the adaptors/and alligator clamps... The Fuse Buddy 302M has a better resolution ( 0.01A=10MA) but you will have to use jumper wires to test at the battery, but it only costs $20 ...

I can't say, I've never tested for drain/draw that way...I've only done it the proper way by testing the actual amperage ( milli-amps) because we have specs for this..

 

Another clue here:

Car starts fine despite battery slowly draining a little each day while idle. When cold it reads a normal ish 12.5v. When I start driving I see the meter drop to 12.0 and sometimes it goes down to 11.0, usually when in traffic or stopped. Then it hovers around 11-8v 12.0v for the rest of the drive.

With new alternator, starter, battery and cable what else can I suspect?. Somebody mentioned another ground cable from trannny to frame?

 

Put an external meter ir guage on it and verify the voltage.

 

Yeah, check all grounds. Most cars have one or more ground straps on them. Not sure where the 996 ground strap is.

For the voltage to drop that low, you either have a very large current draw (like a short), or a bad connection somewhere.

Does your battery die or is your car slow to start ever, at all? I mean is it possible your voltage gauge is going bad? You can buy a cigarette lighter voltage gauge and verify voltage while driving.

 

Further to this issue I have confirmed the battery draw only happens when the car is being driven and up to temperature.

My battery tester says the SOC = 10% and SOH = 100%. It’s a 1 year old Interstate. So definitely draining and after a charge she is good once again for a few weeks.

I went for a long drive and while moving it site at 12v or just under. If I stop for any period of time the meter drops to under 11v which is alarming.

if I shut the car off for a few minutes it starts OK but I can hear it struggle a bit to get going. Starter and alternator also brand new. After restart the voltage once again reads 12v.

So I can live with this but would prefer to find and fix the root cause. BTW the Y cable was replaced 3 years ago by the prior owner. Maybe it’s a cheapo and needs to be replaced with a Porsche cable?

Any clues here that can explain the behavior?

The car is now tucked away for the winter and on the CTEC battery maintainer

Appreciate all the comments so far. Thank you.

 

After you start your car, the voltage needs to be 12.5 or higher. Normally we see close to 14V at least once, and then after awhile, the voltage may drop down to 13 or 12.5. But that’s as far as it should drop.

If you don’t see 13-14 volts, then it sounds like your alternator is not working.

I would start your car, and check voltage at various locations on the vehicle to see where your issue is.

 

tbrodeur421, since the alternator and battery are new, the classic sign of a bad Y cable is that the voltage drops to 12 or below when the car is warmed up. While it may be only 3 years old, it could still be bad. One way to tell is to visually inspect it. Take a look at mine. You can see where it was drawing so much current the insulation melted - circled in blue. This is the end of the cable that attaches to the starter. It is from my car, and I had the voltage drop to those levels when warmed up. If you can get in there to do a visual, it might help make the decision.