pcBlackDog

I have an Optima red top 75/35-925 battery in a 1996 porsche 911 c4. I recently took my car out of storage after leaving the battery on charge through a cigarette lighter style charger during the Winter. The car started fine and has been running ok but seemed a touch weaker when starting. Then I had a new Alpine BT138 stereo head unit installed and I have had some difficulty starting the car since then but it could be coincidence rather than the cause. I am trying to isolate whether this is an old battery issue or one caused by the stereo install. I have contacted Optima for questions about the part number as it has been discontinued.

When I took voltage readings recently, it appears the polarity may be reversed. Could this be possible and the battery still charge and the car still run?

I did some testing. I was able to drive the car and it maintained a charge of 12.45 for 24 hours. I started the car and the voltage stayed above 10v. When the car was running it was 13.8v. All seems good according to the "993 Essential companion". I went out of town for 5 days. I left the hood open and then couldn't start the car. The voltage had dropped to 9.5v. I'm slowly charging the battery and plan to check the static current by removing the ground and bridging the gap with a multimeter once I get above 12v. That way I will know what the drain on the battery is when stationary. I wish I had done all of this before the stereo install.

What actions would you recommend I take?

Thanks!

cmat

Above 10V while starting and 13.8V while running are both good. Leaving your hood open so the hood light is on will easily discharge the battery in 5 days. Once the battery is charged, the static current draw should be around 30ma. Good luck.

95_993

How old is the OPtima?

No HTwo O

If you leave your hood open, manually close the latch, and the light will go off.

If you need a new battery, I recommend an Interstate MTP-91. Direct fit and the proper vent tube for our 993's. About $120. I've seen as low as $90 in Costco.

pcBlackDog

I'm not sure how old the battery is but do know the part number has been discontinued. I'm certain it is at least three years old and probably older.

When I left the hood open, there was not a light that came on. I'll have to investigate whether the bulb is out or if has been disconnected. Assuming the light was disconnected, is there anything that would draw current due to the open hood? Thanks.

Falcondrivr

The immobilizer senses the open bonnet also. I'm not sure that it would affect the amperage though. When my bonnet is open, the interior lights stay on.

DanL993

Where is Alex? He loves to weigh in on these issues with really good info.

pp000830

Trunk light, engine compartment light, dome light, leaving a turn signal on, parking lights any of these on will run down your battery overnight. I got fed up with the turn signal parking light issue and added a chime for when the ignition is off and the lights are on. Battery runs down a lot less now.
13.9 volts - Altinator good, Holds charge with ignition off over 12.5 volts ok 13.5 volts better.
You can always take the bulbs out of your trunk and engine compartment lights. They are not bright enough to be useful anyway.

Coyote

This thread just saved me a lot of time and aggravation! Was able to narrow my problem down to a bad battery with a few quick tests. Replaced and then tested alternator. Good to go! This forum is fantastic!

NC TRACKRAT

Correct me if I'm wrong but, even if you remove the light bulb in the trunk, and then close the latch, if the microswitch is out of adjustment and remains on, the system will still draw current. Measurement with a digital multimeter is the only way to be sure.

993/907

Opening the hood on my '95 will turn on the interior dome lights....you may not have noticed yours were on?

I like Interstate batteries; just replaced 13 year old Interstate a few months ago....that's a loooong time for a battery to last in my experience.

Gordo