From - Fri Dec 10 11:05:42 1999
Subject: [928] Those of you who care about your batteries, read this: (Longish)
To: "928" (
From: "simon" (

For all of you who didn't get the battery info, here's the LONG (but edited) version of a whole lot or really useful info I got from a truly international bunch of guys.  Thanks to Wally, Bob, Jim, Glenn, Jonathan, Phil, Graham and Rex, or for country watchers, Australia, New Zealand, England and, of course, the States. I figured there was too much good info in here, some of which came to me directly, so I thought I would share the collective wisdom.  Their tips,
not mine.

85 AH batteries are recommended by several people.  Guaranteed starts  even after several weeks of no use with alarm on.  Capacity is everything.  The additional cost should be thought of as a saving in wasted time - well worth the extra cost.

Referring to non-spec batteries, auto electrician seem to often remark that the battery appears too small for the car.  928's are power hogs.  Even the old cars have high current draws when they have all the stuff operating. Even after fixing or replacing alternators, your battery still may not hold charge properly.   A number of auto electricians have diagnosed high resistance in the wiring.  Easily fixed by fitting the correct battery.  Some people will argue that smaller capacity batteries are overtaxed by the 928.  Don't take the chance, especially, if you live in the cold. Also, in cold cranking conditions, the battery loses capacity quickly, since it's ability to hold charge is based upon a chemical reaction. A larger battery will extend the cranking time pretty significantly.  That said, batteries age and lose capacity - a 90 amp/hour battery can deteriorate quite a bit more than a 60 amp/hour battery before each becomes inadequate, so the larger battery will often have a longer service life. Again, if you're not in the cold, you may be able to get away with less, but if you are moving into winter, bigger is better.

Under normal RUNNING conditions, the amperage used is provided by the alternator, not the battery, and the capacity of the battery (within reason, say 50 to 90 amp/hours) makes no difference. If you spend a lot of time in the dark, in the rain, with the the HVAC system on high, and idling, the alternator output is marginal, and a larger battery acts as a bigger buffer between bursts of higher speed charging operations.

In normal cranking conditions, the battery must be able to furnish about 180 amps at 10 volts for long enough to crank the car. If the car cranks quickly, even a small battery is adequate. A larger battery just adds a longer cranking cycle before the available voltage drops below the required voltage and the brain shuts down.

Does your battery seem to have trouble holding charge?  Recharged batteries mean nothing if your alternator is on the way out.  Watch for the tell-tale alternator light.  It seems that on the older cars, the insulation of the internal wiring may deteriorated to the point where it shorts, and the alternator may end up drawing three amps on its own.

Here's a suggestion for future troubleshooting.  Buy yourself an inexpensive VOM (Volt Ohm Meter).  You want one that measures amperage to about 10 amps, too.  Then, while your car is running, measure the battery voltage at the battery.  It should measure around 13.8 to 14.2 volts at the battery.  If it's substantially lower than that, you probably have a charging problem.  I.E. bad connections or possibly a bad Voltage Regulator in your alternator.  The VR is inside your alternator and not a separate unit on ALL 928 cars.  Next, disconnect the negative side of your battery and place the amp meter in series with the battery post and the battery cable.  Then, close the hatch switch to turn off the inside lights.  Your meter should show about 20 milliamps with nothing on.  If it's substantially more than that then you have a problem somewhere and you'll have to look for it. Start with a fuse pulling session in such cases to isolate the system where that is drawing the excess current.  You pull a fuse and recheck your current meter.  If you do these tests, you won't be guessing about your electrical system, you'll KNOW if there is or isn't a problem.

Maintenance is the key.  8 years on the same battery!  How?  Plug in a battery maintainer every time you get back home if your car is not a daily driver.  It may be a week, or months before you drive it again, and it's always hot and ready to crank. You can use the same battery for 8 years or more, so it really really extends your battery life.  If your car is not used during the winter, you also won't have to worry about your battery freezing, because a charged battery won't freeze.

Batteries don't like being fully discharged and usually after about 6 times give up. If you have access to a good battery charger, remove the battery from the car and remove caps. Charge the battery at about 30 amps for 30 minutes then reduce to 5 to 10 amps and let the battery gas off for an hour. The electrolyte in each cell should be
gassing at the same amount. If a cell has failed its all over and time to drag out the credit card.

Parts importers are cheaper than dealers.

If you ever get caught in cold weather, or if you suspect that you may be running out of battery capacity, you can help a 928 crank much more quickly. Just hit the starter for very short bursts rather than holding it on. The fuel pump runs for only a very short time until the engine cranks. By using short bursts, you allow fuel pressure to build up, greatly
improving the chances that the engine will start quickly.

Earlier cars had the battery box attached to the gearbox/diff assembly, and used its weight to help dampen drive train vibrations.  Not relevant for newer cars, and it seems that Mark A is using  a Westco gelcell to save weight.  I wonder if he still has the HVAC system connected . . .

Watch the height of those non standard batteries!  The battery box lid is NOT made of plastic.  If the lid is latched down hard on the battery terminals you're putting a lot of trust on that black rubberized positive terminal cover (if you still have it).

Well - that's about it.  Thanks again for those who took the time to either write to the list, or me individually.  There was probably some new info in there for most of you, right?

Simon in Sapporo
84 928 S2 with new 85AH battery


Ian wrote:

I had to deal with a flat battery even after I put a new Bosch Silver in - I left my car with the hatch not shut fully and  the alarm/interior lights drained it over a day. I tried jump starting off the post in the engine bay, but it's quite hard to find a good negative (ground/earth) connection with regular jump lead clips, especially if you don't want to scratch paint anywhere. With a fully charged BIG battery I couldn't start it, just got the usual clicks and a bit of engine rotation. Even off the battery directly connected it wouldn't go. In the end I only got it going by jumping it straight off my Wife's car, and leaving the connections on for a couple of minutes with hers running before I tried the 928.

I suspect there's a LOT of resistance (or something) between the post and the battery, and between the battery and the motor. I assume these are the same wire though.

It is possible that you do have high resistance somewhere in the battery circuits on your 928, and this should be checked.
There is another possibility, however. There are few consumer items sold in the USA that have a higher percentage of pure crap than jumper cables. I saw a set in a major discount store this week that were good looking, heavy cables - but the conductors were 14 gage wire! That isn't heavy enough to run your headlamps, much less crank the car. It is common to find jumper cables with 10 or 12 gage conductors.

A good set of jumper cables with long 2 or 4 gage conductors and good clamps is expensive, bulky and heavy - but they WORK, and are a very good investment.

>I plan to buy a boost charger and use the cigar lighter if I ever need to do >this again - unless anyone knows that this is a bad idea!

I suspect that this is another case of words changing meanings over borders. In the USA, a "boost charge" is equivalent to a "jump start", a high-amperage, fast charge to get the car going NOW. A "battery charge" is usually a two to eight hour slow charge to recharge a dead battery, or top off a low battery, when there is no particular hurry. A "trickle charge" or "battery maintainer" is used for long-term battery maintenance. They are not the same thing in most cases, however. A trickle charger usually can and will overcharge and damage a battery over extended periods, while a proper battery maintainer is safe for indefinite usage.

I believe that a "boost charge" to Ian is equivalent to our trickle charge or maintainer charge.

In any case, as several members have pointed out, the 928 cigarette lighter isn't suitable for any of that. We sell the Porsche Battery Maintainer together with the adapter kit, which includes wire and connectors to hook the maintainer to the battery circuits. The jump start terminal is suitable for connection of a battery charger or maintainer.

Wally Plumley
928 Specialists


hi all,
sorry about the length, but i need some more grey matter than i have for this one.
recently noticed that my voltmeter has been showing much lower than usual. 
this has coincided with the hotter weather and use of a/c.  normally has shown +/- 13 now drops to 11 if sitting idling under load. tightened the alternator belt as per wallys suggestion.  also went through the checklist wally posted recently and cleaned all the contacts as described.  had my battery checked at autozone and its ok.
I've also noticed while driving (freeway long distances) with a/c on, radio on, voltmeter gradually drops to around 12 and a/c seems not to work as well (fan seems to slow down quite a bit) and the temp gauge goes from middle to
just about the second white line. went out today and did some measurements with my multi meter.

1) car off
battery:  12.59
starter:  12.56
jump points:  12.57

2) car on at idle with no other loads
battery:  13.5
jump points:  13.75
voltmeter in car:  13.00
sorry but didn't want to crawl under with it running  since it was off the jacks

3) car on in park with a/c on, fan on max, stereo and lights on
battery:  12.2
jump points:  12.07
voltmeter in car:  11.8

4) car on in gear with a/c on, fan on max, stereo and lights on
battery:  12.02
jump points:  11.70
voltmeter in car:  11.00

seems to me that it charges ok except when under load at which point it runs off the battery.  is something wrong here (alternator/voltage regulator) or is this to be expected?
don't want to break on the way to sitm.
89 s4

The alternator should put out 14.1 volts. Less than that indicates diodes going south.