Charging issue, bad battery or ?
 

I have a red top Optima battery that is about 2 years old. So far its been fine but kinda concerned about things.

I put a digital VOM onto the car and been driving it like this for a while. At idle voltage is down at 11.7, then ramps up to 12.9-13.1 when running. Would rather see the sitting voltage up in the 12's and feel that its a bit low.

Always thought that when running it should be just a bit higher, like 13.4-13.6 or so when running and not carrying high load items.

Engine always seems to crank over just fine and voltage comes up nicely when driving but wondering if my alternator is heading South or the battery is getting old. Engine has 94k miles on it and I bought it 4-5,000 miles ago from a gent here on the forum, and know nothing of its past history. The alternator could be almost new or original to the engine, simply do not know, and the gent I got it from does not know either.

Am guessing that its time to do a load check on the battery and go from there. It came from Costco so hoping that if its getting tired that returning it would not be an issue.

Ideas or suggestions? Anyone have this happen? Oh, yes this is on a 1985 911 with a 3.6 transplant.

Thx, Joe

Digital VOM needs to be directly across the battery posts to know true charging voltage. But it's always a good idea to check voltages near the alternator output, rear fuse panel) AND directly between the 2 battery posts. Any serious difference and you need to find out why.

According to the volt meter in my radar detector mine idles at 13.2 goes to 14.1 - 14.2 while running.
Sounds like yours may need a rebuild.

I had a very similar problem with my 84. I replaced the Batt negative ground, chassis ground and alternator ground (which you should replace anyway if original). These helped but still had the problem.

I checked for any loose connections and found that one of the wires to the starter was not very tight at all. Tightened it up and problem solved. I did get very fast at pulling and installing the alternator... Car now shows 14 volts while driving.

The electrical interconnect for the positive circuit side between the battery/front and the rear/alternator is at the starter solenoid mounted on the starter. The negative circuit side is via the engine-chasis ground strap(s) and the car body.

Good info everyone! Anything else? Will get some more readings when I can and report back.

thx, Joe

Hello Joe, On your 3.6 are you running the single or dual belt fan drive?, Reason I ask is that the single belt setup slows the alternator speed to the same speed as the fan, With the single belt pulley your output at idle will of course be lower,
Also in addition to checking for loose connections at the starter verify the crimp terminal on the output wire coming from the alternator, Have found several that have become corroded causing a voltage drop, the wire will usually show signs of overheating close to the terminal, On the battery, Load test will help rule it out,

Good luck in your hunt, Lou

Lou,

The car has two belts on it but one is only for the A/C compressor, so would say its a single belt for the alternator.

Doing a load check on the battery either tomorrow or Thursday. Just got back from the dentist and a root canal, so working on the car has been pushed down the list a bit! :)

Joe

if you are at 11.7 at idle your gen light should be on. if your belt tension is okay i would think you are due for a rebuild.