Deb

Battery voltage down to 10.4 volts and when charger voltage is applied it maxes out at 11.2 volts - the voltage reading on the charger when not connected to the battery runs to 14.3 volts.
I assume that the battery is toast or that it may respond to an equalizing charge?

okbarnett

Battery is bad. Take to store and they have an analizer that will tell. I say a bad cell.

User 4221

The battery has an internal short. Over time the plates corrode internally and the corrosion grows together and shorts.

A car battery is a 6-cell design where each cell sits at a voltage potential of about 2.1V. So.... 2.1 x 6 = 12.6V. short one out and the voltage becomes 10.5V.
The math is close enough for government work.

BTW, if you run the car with the battery in this condition, you stand a high probability of damaging the output diodes in the alternator.

Get a new battery and sleep better tonight.

HTH,
Scott

dr bob

Part of the annual battery service/terminal/cleaning/etc is a check of the specific gravity of the fluid in each cell. Assuming you don't have a maintenance-free (same as-- you can't maintain it) battery, use an inexpensive battery tester to sample each cell's fluid. Weak cells show up early when the gravity of the acid solution in the weak cell is low. The battery testers are cheap (less than $2) and can give you some warning about a weak battery. It's always better to have some time to shop for a batytery, rather than having to buy an expensive "doesn't quite fit right but worked fine for years in the Nova" battery from Bubba's used battery emporium late on a Sunday night whuile traveling home from the mother-in-law's place with the kids.

The tester with the colored ***** is fine, by the way. There are better ones with the calibrated floats, but for almost everyone the ***** are easier to read.


The previous 'bad cell' diagnosis is good. Each cell produces a little more than 2 volts at full charge, so you probably have one cell that's shorted internally. It's time for a new one.

autochicago

Just a comment on battery shopping, too.

Since the 928 battery is located where it is housed and covered, it is very important to select the correct, vented battery. The battery's cells are vented to an outlet, to which attaches a hose. Said hose then runs outside of the battery box and vents (acidic, corrosive, combusible) gases and mist correctly underneath car instead of inside of the enclosed space.

Believe it or not, Sears actually sells a very nice, correct Die Hard battery for these cars, and the more well-equipped centers will often have it in stock and it will come with vent kit. Somewhere around $100 last time I did the job, if my memory serves me correctly.

In lieu of Sears, I always recommend Interstate, which catalogues a correctly-vented 928 battery. However, unless you have a local independent shop that services Porsches and sells Interstate, probably a little harder to find.

And always treat your terminals to service, too! When I was in the Euro repair business, I found that ~90% of problems that could be easily diagnosed as a bad battery ended up actually being repairable by servicing terminals, cables, and grounds.

Good luck.

Your choice of MGB and sailboat are, BTW, also right on the money.

Bill 86.5 928s

Hey Deb....Yupp your battery is toast. Mine went like that just a month or so ago. Replaced it and life was good again. Now I own a good battery and can't drive the car.....Goodluck Deb

Bill

Bill Ball

The Autozone Duralast (rebadged less expensive Bosch) is properly vented and seems quite reliable. I think it's about $50-60. I've had one for 2 years based on a number of recommendations on the 928 email list.

G Man

I just replaced the battery in my 81'. I went to Walmart, but they did not have the one their computer listed as the correct for my car. I used my old battery as a match for a similar sized battery. I wound up with an Everstart #41-4. It fits great and even has a plastic flap that covers the + post. The cost was right too at about $40.00. I don't know if the stock ground strap will reach. I had previously replaced with a longer round cable, which fits with no problem.

kaamacat

I've got the same Duralast that Bill had (1000cca) made for the 928 from AutoZone. Very nice battery, perfect fit!



Bob

Flott Leben

Scott, can you elaborate a little on what symptoms you would have if you have damaged output diodes? I think I may have done exactly what you describe. My battery has drained completley a few times and now my voltmeter guage will sometimes read dead zero (with no light on) and sometimes 14 volts. It may just be a bad guage but I thought I would ask. Thanks.

Flott Leben

Bump.

Scott, are you out there? Thanks for the help.

jayc67

Make sure all these batteries have a vent tube that can be slid through the bottom of the battery box. This is very important in that enclosed area, the gassing of a battery produces a VERY corrosive mist you don't want trapped near that precious metal.....