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Re: [bmw] New battery failed?
John,
It sounds like your car had a major battery drain. You checked out the
current drain with everything off and found it to be 2mA, which is
acceptable. That certainly won't drain a new battery overnight. So, unless
a power-using component was left on (exterior lights, dome lights, trunk
light) or if there's a problem with door locks, door lock heaters, heated
mirrors (if you have these), it sounds like a problem with the charging
circuit. The charging voltage (voltage across battery with all accessories
turned on) should be higher than 13.4 volts. It should be in the range of
14.2-14.5 volts. You need to have the charging system checked out.
To measure charging current, you need to break the alternator circuit and
insert an ammeter. Since the charging current can be very high (enough to
fry a standard multimeter), the current is normally measured by inserting a
low resistance shunt (for example, 0.01 or 0.1 ohm) and the voltage drop is
measured across the shunt. I once had a multi-purpose car tool that
measured dwell, rpm, etc., and also had a shunt to measure charging current.
You can find these parts at a better auto parts store. Your best bet is
to take the car in and have it checked out.
The battery light on the dash illuminates if the alternator field current is
not flowing. So, since the light is off, it sounds like that circuit is
okay. Without current for the alternator field, it won't be able to
generate any electricity.
Also, check and perhaps replace the ground lead for the alternator. There
should be a short ground wire that grounds the case of the alternator to the
engine block. The alternator bolts are mounted in rubber bushings, so you
need this ground wire to firmly connect the alternator case to the engine
block. If the ground is bad (it can look okay visually), you won't get
proper current flow from the alternator, and the alternator won't properly
charge the battery.
Besides, when the car is running, it's really relying on the alternator to
provide it's electrical needs. The battery serves to start the car, and to
provide power for accessories when the engine isn't running.
-rb
Ron Buchalski
BMWCCA #76387
1995 E36 M3
1993 E34 525iT
1999 Mazda Miata
Date: Wed, 04 Feb 2004 16:10:02 +0000
From: john <j.s.smith@domain.elided>
Subject: [bmw] New battery failed?
Sorry for this long post...
My '89 E30 325i failed to start last night - flat battery :-(
The thing is I bought the battery new from a BMW shop only 6 weeks ago, to
replace the existing
battery which had been in the car for 7 years! My problem is now that I
don't know whether this new
battery has actually failed in some way - or is there a fault on the car.
btw I only replaced the old
battery because that ran flat too.
What I have done so far:
1) Measured the current drain with ignition OFF and everything electrical
in sight turned off. The
current was about 2mA so I think that's ok
2) Measured the battery terminal voltage with engine running at 1000 rpm
and most every electrical
thing turned ON. The reading was 13.4 volts - so looks good to me.
3) The red battery lamp on the dash goes out as normal when I start the car
so I *guess* is charging
the battery?
The way the battery went flat is also strange... I used the car the
previous evening, no problems. It
was standing overnight, weather was warm. Next evening it was completely
flat.
Also, the new battery is a sealed type and has a led indicator built into
it. The label says
green=ok, black (no light)= needs charging yellow= replace battery. I've
had this thing on charge
for 10 hours and led is still off - although the battery voltage is 12.4V
and I can take take several
amps from it to drive a latge test lamp.
Is there an easy and reliable way to tell the charging current when engine
running?
I don't want to look a right idiot by taking the battery back and then them
telling me it's ok - so
would appreciate some help with this.
Thanks
ps When I bought this battery from BMW it had a minor scuff mark on the
side and wasn't in any kind
of box - I just had the feeling maybe it had been lying around a while.
Also the labelling on it says
it's sell-by date is 4th April 04. Mmm
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