boxsey911

Trying driving the car and then stop and measure the voltage. Like you I got 13.5 volts with just starting the car up and letting it idle, took it for a run out, stopped, popped the hood and got 13.8 when I re-measured it.

964polar

How long does your CCU fan run after a drive. Should close down after a while, some tend to run for too long.

Cheers

darth

Voltages look a bit suspect, could be a meter calibration issue. Are you using the clamp on meter - I've never used one that measures DC voltages and I'm not sure of their accuracy - did a spec sheet come with it - it may be due to its' accuracy. Can you compare it with another meter with clamp on leadds or check it against a known similar voltage? Try swaping the battery with another and see if you get the same measurments.

Bill

springer3

If the current draw continues with each fuse and relay pulled, but likely culprit is the alternator. Solid state diodes stand off the battery voltage 24/7, and they can get leaky. They can also be intermittent consistent with your problem.

Vlocity

Thats a great suggestion. I'll try that later today and see if the voltage changes.

I don't hear it running at all after shutdown.

I was actually just back to using a good multi-meter, but I will double check. The clamp style meter that I have has a funtion to zero itself before use. I was getting the same measurements with each unit.

The alternator is the original one. The vehicle has 50,000 miles. One other thing is that the typical drive for this car when my wife takes it to work is only 1-2 miles. So a weak alternator may be contributing to the overall decline of the battery since it doesn't have much time to recharge.

I plan to continue to monitor and I am putting in a trickle charger as a bandaid so that I stop killing the battery.

Your thoughts and comments are appreciated.

Ken

Vlocity

Issues remain.

The car will stand on it's own now for a couple of days and still start, but it is obvious that the voltage is still dropping off. (Very slow crank over before starting)

I did drive the car and then put the voltage meter on it, as suggested and that led to a slightly higher voltage reading of 13.6. (double checked with 2 meters)

After shut down the Ma draw is within spec. somewhere below 25Ma after 10-15 minutes. Every time I walk by the car I throw the tester on it and I have not found it with excessive draw. I am assuming that if the alternator diodes are failing that it still would show a draw across the negative cable?

My next observation is that the Optima battery I am using is only rated at 48 Amp Hours while the standard battery is rated at 78 and the battery that the Cabriolet calls for is rated at 88, that is almost twice the stand by time.

Can anyone tell me what the standard Group is for the original battery? I am thinking about ditching the 2 year old Optima and replacing it with an original group size battery in the 88Amp Hour range.

Thanks guys for all of the help and encouragement.

Ken

racemor

Another thing to look at is the a/c control unit in the dash. I was having a similar problem and had my mechanic (Jim Burton of Eurowerks) look it over. He found that the control unit is temporarily spiking to 400ma draw. He says he has seen this before in 964 units. I have not made any repairs yet so can not say for sure if this is the culprit. He and I have talked about adding in a relay to the control unit to make a consistent power draw, or find a new 993 unit. But that's some big bucks if it does not solve the problem.

darth

Ken

"My next observation is that the Optima battery I am using is only rated at 48 Amp Hours while the standard battery is rated at 78 and the battery that the Cabriolet calls for is rated at 88, that is almost twice the stand by time." There is no standard how battery manufacturers rate their batteries and the Optima may in fact be a better battery comparing the two batteries when they are new. As suggested before I would swap it with one of the batteries from the cars in your stable (i.e. I see in your avatar) temporarily before buying a new battery. Immediately after the car has been driven and has a chance to chrge up fully does the engine crank over fast? If it does then it should crank over just as fast a few days later whith the 25ma current draw you're reporting. If not, then I suspect it is discharging internally. You can confirm this by disconnecting it completely after it has been charged up and is able to crank over the engine fast, then try it after a couple days or the period of time you've observed it to have likely discharged by past experience.

Hope this helps
Bill

Vlocity

Bill,

I was not aware that the AH rating was not standardized. Very useful information. After charging/driving etc. the car does crank over as it should. The problem arises after setting for longer period of time.

I just bought a marine grade shutoff switch that I plan to mount over the weekend. That will allow me to fully charge the battery and then do as you have suggested. IE; disconect and determine if there is an internal discharge.

For now I have a battery tender in the circuit, but that's just not acceptable to me.

Thanks very much for your input. I am sure that if I stay after this I will find something definitive.

Regards,

Ken

boxsey911

So you say that after fully charging the battery it's starting to get sluggish again at cranking. Yet you've established that your current drain is minimal and you have a reasonably healthy charge from the alternator. At this stage I would say that the battery is just not holding charge as it should. Changing it for a new one would by next step (and I'd get one rated at 75 Ah at least). From my own experience I've found that 2 to 3 years is about the lifespan on my 964. This is mostly because it's not used regularly. The battery on my boxster (similar rating) lasts 4 years because it's a daily driver.

RichieRoo

Hi Ken

I'm with Boxey on this one.

I had a Yellow Top Optima battery which gave similar symptoms. I made up a cigarette lighter
plug for my multimeter so I could have it plugged in all the time to monitor the voltage as I drove
and when I returned to the car.

My voltages were similar to yours.

It was the battery which was at fault. I suspected everything but the battery because they are
'meant' to be indestructible. I replaced it with the highest Ah battery I could find which would still
fit the dimensions under the bonnet.

So far it has worked well, the car can be left for a week or more with no problems.

Good luck

Rich

Richard Curtis

If you have a battery that's not sealed, you might check that there is adequate water in it. Even for "Maintenance Free" batteries, they still need to have the water levels checked. Use distilled water. I recently noticed that my truck was having difficulty cranking. Checked the water level and it was so low that I added more than a quart (1.5 quarts?). Problem solved.

cmn

for what its worth - had the exam same issue. Was a frayed wire rubbing up against the starter.

DWS964

Yes, I had that issue also on my red '92 C4. The result was an improvement in the battery drain, but still unacceptable over 4+ days. My "new" 93 C4 lasts for 10+ days (haven't let it sit longer than that...) with only a slight decline in battery reserve. I wish I knew the reason for the difference.

Ken, I hope to see you at Mid-Ohio in 2010 !!

Vlocity

I'll check that wire tomorrow. I certainly want to continue to rule things out.

We would love to see you at Mid Ohio. I think we have some of the very best people in the business involved in putting on a great event at a fantastic track.

Follow up Question Is there a consensus that the original battery was a Group 48?

Comment It appears that the Ah (Amp Hour) rating does not appear on most of the new batteries and they have went to a RC (reserve capacity) rating. I found a formula that indicates RC x .06 = Ah. Does that sound about right?

That would mean that I need a 125 minute RC battery to equal about 75 Ah.

Thanks all.

Ken