what a differance a volt makes

[TheCabinetmaker]

my battery died, it had been evidently running on less than all six cells for a while, as my volt meter on dash was always reading low, car cranked over ok, maybe a bit slow, but had been doing this for some time.
got a new battery and noted a volt of so increase on the dash gage. boy howdee, the cars performance woke up, more zip, and runs cooler too per oil temp gage

seems one cell was dead or weak and I was constantly about a volt or two low. drove like that for a long time. not bad enough to cause real notice, until the new battery was installed.

leason learned, make sure the battery is in tip top shape, else you could be robbed of a bit of power

[rvanderpyl]

I would suggest testing your alternator output if replacing your battery makes any difference to how the car runs. When it is running, the alternator should be putting out sufficient amps to run the car, and charge the battery. The battery itself shouldn't be contributing anything to the actual electrical usage, except maybe at idle.

[Fredfox]

I had the same issue with my 928.

[Dave at Pelican Parts]

Well, the charging system does use the battery voltage as a reference voltage, so I can see how a battery with a completely dead cell could make a difference. Even in an otherwise-healthy charging system.

--DD

[rvanderpyl]

Regulator works based on battery voltage? So you couldn't actually run the car if the battery was disconnected. Didn't know that, although I haven't tried it either.

[Dave at Pelican Parts]

You can run it, but the system loses its reference and the voltage can do very odd things. The battery also serves as a buffer for the alternator, smoothing out voltage spikes. Which can fry the more fragile electronics in the car.

So disconnecting the battery with the engine running is possible, but not recommended.

--DD

[TheCabinetmaker]

You need to excite the field coil in the alternator with the car battery in order to get the alternator to work. No current in the field coil and she wont generate any juice.
The small field current is brought to the alternator's rotor by brushes, but unlike generator brushes, carry little current and hence rarely fail. Generators have brushes to carry the charging current, which can cause brushes in generators to wear out. Brushes are a regular maintenance item on generators not alternators. Alternators are considered maintenance free for the most part because of this

the 914 battery is made of six individual cells, each providing about 2 volts of juice, all six are arranged in series to get the 12 volts in a car battery (three cells are in a six volt car battery) If one of the cells shorts out, then you effectually have a ten volt battery instead of a 12. No matter how hard the alternator tries, it cant charge a 10 volt battery(five cells working) to 12 volts. That would be like expecting a good 6 volt battery (three cells working) to be able to be used in the 914 and run at 12 volts, it ain't going to happen.

sometimes cells die as an open rather than a short, in that case the entire battery will be zero volts, as the open cell effectively breaks the series connection of the cells.

PS after an over night charge on a known good home charger the offending battery never got up to 12 volts. Thus it was found to have a dead shorted cell. The new battery is charging fine in the car, the car's charging system is working.