warmfuzzies

The biggest battery I could find was 80A/hr. I manufacturer called Banner. Bosch were 72A/hr silver plus. Could not find a larger one for love nor money.
I would suggest Adrian that Both yourselves and Loren are looking at this with differeing methods, If I scope an Alternator the voltage will have around 300mA or more ripple easy. If I measure with a DVM or similar they are not quick enough to respond to the frequency changes and therefore average out the readings, or more often just hang on to the last one registered.
The quality of output is dependant though is it not on the sophistication of the diode bridge and quality of components, easy enough to vary through manufacturing I would suggest.
My own 964 only ever reaches 13.9v o/p and does not have an issue with the battery, even after standing for 3 weeks recently, does this not suggest that 13.9v o/p is perfectly ok in real life.

Kevin

Lorenfb

Needless worrying is going on here!
The Porsche 964 alt. output at the battery of 13.5 to 13.9 is O.K.
Let's stop over-solving a non-existent problem.

If there's a problem, again:
1. measure the current drain, < 50 to 100 ma
2. check the voltage, 13.5 to 13.9
3. measure the source current from the alt. (not the charging current
going into the battery) using the lead from the alt. It should source
over 100 amps.

Actually, having a lower output voltage is better from the standpoint
of lower alt. faliure rates because of excess charging current demands
of large (monster) batteries. My '88 3.2 has a Paris-Rhone alt. which
outputs 14.5 at the battery. This voltage when using a large battery
with large charging current demands can damage an alternator.

I DON'T use a large (928 type) of battery in my 3.2 for that reason.
Remember, an alternator is a voltage source device with essentially
a non-limited current output, i.e. it can and will be destroyed with
excessive loads whether it's the result of an electrical short or a battery
which demands excessive charging current.

Bottomline: Always use the SMALLEST battery based on a good cranking
situation. The alternator is the one that supplies the current when the
engine is running and NOT the battery. The battery only gets charged
and "goes along for the run" until the next cranking/starting of the
engine.

Have Fun
Loren
'88 3.2

DavidI

Kevin, I recently replaced my Bosch Silver Plus (66Ah) with a Varta Blue Dynamic rated at 74Ah. Other than the rating, badging and colours, these batteries seemed identical - I said as much to the chap at the auto factors and he reckoned Bosch and Varta batteries were made at the same premises. For what it's worth I get 14.3v at idle.

http://www.varta-automotive.co.uk/co...e-dynamic.html

Doug H

New battery won't increase altenator voltage. A battery is only 12.4 volts fully charged. You can get batteries that are less drain on alternators such as Optimas due to low ESR. This allows your alternator to work less diligently at charging your battery so it can allocate its resources elsewhere.

Lorenfb

I think you mean higher ESR (equivalent series resistance). The lower the
ESR the greater will be its cold cranking rating and the greater will be its
charging current.

Good Luck
Loren
'88 3.2

Adrian

The Porsche battery capacity recommendations are contained in a number of their documents. If you are being told that a cab and coupe/targa have the same battery capacity you are being misinformed.
The larger capacity batteries provide exactly that. Larger capacity to handle the load. The cab roof is an additional electrical load so therefore a larger battery capacity is required. Your battery MUST be capable of handling a specific load for a specific period of time on its own. This is a certification requirement. Capacity is the ability to deliver X amount of current for X period of time. You cannot judge a batteries capacity by its outside appearance either. It is all in the internal construction.

The measurement of an alternator output directly at the alternator with a cro or Fluke 87 style meter will always show an AC ripple. The alternator creates 3-phase AC which is then full wave rectified. For those who know their correct theory of operation understand that the DC output from the diode pack is always rippled. This is why it is called ripple DC. However when this voltage is applied across a serviceable battery, the battery removes the ac component and supplies smooth DC to the car systems. This is why you DO NOT run any car on just the alternator. Like some people suggest. You never disconnect the battery with the engine running to test the capability of the alternator.

This thread has become awfully confused and complicated. Electrical problems like the one with how this thread started are easy to solve yourself by following simple steps. Forget this nonsense about AC ripples and just work on the electrical system. You start at the battery. Simple voltage checks tell you where to go next. 13.5V at idle is too low. This indicates a problem.

I am also struggling with an Audi with electrical problems. Nothing is open until Monday so I have to try and keep it going until then.

Winter months kill batteries. This is the season when they fail.

Ciao,
Adrian
964C4

Loren do you have an address for your Porsche repair facility?

Lorenfb

Since this thread is on the topic of alternators, it should be pointed out
about errors which exist in the new Porsche book (Enthusiast's Campanion):

1. Pgs 172 & 383 picture non-964 alternators as 964 alternators

2. Pg 173 states; "If the load on the battery increases (more demand by
electrical consumers), the voltage regulator increases the voltage to the
alternator rotor."

This is incorrect. The regulator only varies the time the voltage is applied
to the rotor and NOT the voltage which is constant.

3. Pg 173 states; "The output of the alternator is limited. If the output
reaches its maximum possible output of 22 volts (regulator at maximum
voltage) the engine will appear to surge and in some cases will stall."

This is incorrect. The output of the alternator is not limited by the
regulator to 22 volts. Furthermore, if the alternator exceeds 15 volts,
some of the electronics may be destroyed, e.g. the ECU and air bag
units.

Have Fun
Loren
'88 3.2