RE:<E36> Battery drains

[aaron lung <alung@xxxxxxxxx>]

[snip]
> The dealer is claiming the detector is killing the battery. 
> Especially as the battery is weakened with age (like about a year) it 
> is being dragged down by the detector until it is dead.  Claims that 
> BMWs are more sensitive to battery level than other cars.  Also has 
> checked the car several times and claims there isn't any other drain 
> when the detector is disconnected.

"BMW's are more sensitive"??  That definitely sounds and smells
like a typical dealer hogwash response. I'd challenge the 1yr age statement...
But there's probably *some* truth in the rest of it

Gotta remember--A BMW is like any other car--there's nothing special
or mystical about the electricals (or anything else other than $$$$
for that matter)...so don't let that fool anyone :-)

There are the usual FAQ's/theories on why batteries die due to
leakage current, dirt, corrosion, bad wiring, phantom loads, etc, but
in more cases than not, the problem is always more fundamental :-)
"Cold" weather like now doesn't help either.

Lead-acid batteries have inherent self-discharge characteristics.  ie,
if you let it sit long enough with no load, it'll lose charge itself in
the order of some percent per day . And they don't like being left
in a discharged state which has a huge bearing on its life

This of course assumes the electrolyte hasn't been neglected and
allowed to drop below the top of the plates too...

I'm not a chemistry person and won't claim to be -- supposedly
the longer a battery is in a depleted state, the more stuff precipitates off
or grows on its plates until the thing doesn't work anymore
(I forgot what the scientific term is--someone jump in here! ).  Car
batteries are built with thin plates that don't necessarily last
as long as deep-cycle Marine batteries which have thicker plates to
specifically deal with deep discharges between recharges

Battery brand makes a difference too.  Johnson Controls is one of
the better battery manufacturers (note mfr, not brand name).  Sears
Diehard occasionally OEM's their batteries, as does Costco; Costco
batteries are half the price and just as good.  Avoid the cheapy no-name
megatorque-amp-wizz-bang brand specials.

Could be a bad alternator, but typically unlikely.  Easy to check
in anycase... as long as it's coughing up about ~13.8V when the
engine is running, it should be charging the battery OK.  Either
that, or the car isn't being driven long enough between trips to
fully recharge the battery, not to mention replenish the additional
charge lost in starting the engine and running anything else.

I don't remember what the V1's active current draw is (I have one) but
it's probably in the hundred+ milliamp range if I'm not mistaken.
Multiply that by days of the car not being driven, and you'll
come up with many amp-hours/watt-hours.  Not a necesarily a huge big deal, but
the fact is the battery is sitting in a more-than-typical discharged
state for days at a time.  The quescient drain from other stuff such
as clocks, OBC, etc is "negligible"...I'm more than sure the BMW
Engineers designed  them with all of this in mind, and you can
measure it yourself to prove it either way as Alex suggested (except
with an ammeter, not voltmeter) :-)

Bottom line is we don't want to be continously flattening a lead-acid
car battery too often for too long.  They're not designed for that.
More than likely, the fact that you're leaving the detector with
all the other stuff running all the time has a cumulative affect
over time in killing the battery plates

I'd move that V1 and any other accessory to a switched circuit..or
simply turn it off when not driving.  That'll get rid of the obvious
and see if there isn't something else sucking current (like a seat heater
that's stuck-on as an extreme example)...

good luck!!!

aaron

88 528e 81K.  On my 2nd battery since I bought the car in 88 :-)

------------------------------