928 Battery Information

From - Fri Dec 10 11:05:42 1999
Subject: [928] Those of you who care about your batteries, read this: (Longish)
To: "928" (928@rennlist.org)
From: "simon" (simon@do-johodai.ac.jp)

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 substancially 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 
substancially 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 

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

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written: 12/10/99
rev: 05/01/01