A question about battery drain.

[Freewheeling]

I was in a small fender-bender in the parking lot a few days ago, that
practically tore of the front fender. There was no fluid leakage,
however, so I drove it home after going through the accident report
formalities. (I have a 1998 SL2 4 door.) The next day I had a doctor's
appointment so after duct taping the fender sort of back in place I
tried to start the car, but the battery was completely dead. I mean,
dead as a doornail.

Now, it was raining fairly hard the night before and all that day and I
figured that the rain must have shorted some exposed wiring. The shop
did some testing that indicated there was no drain on the battery, so
they concluded I must've left the light on or something. When I parked
the car, however, after the accident, the sun was shining brightly so
there'd have been no reason to turn on the light. Moreover, as I recall
there's an automatic shut off if you turn off the key and leave the
lights on. This mechanic tells me that isn't true, and that if you
leave the lights on the battery will just drain. (All the doors were
shut tight, by the way.)

So, I'm thinking that:

1. The fellow is wrong about the automatic shut off, because that's one
of the features I liked about the car.

2. If they didn't test the car "wet" then how would they know whether
there was a drain on the battery system from exposed wiring that shorted
due to water?

What do you think? Am I on any solid ground here?

--Scott

 

[Dooger]

I was in a small fender-bender in the parking lot a few days ago, that
practically tore of the front fender. There was no fluid leakage,
however, so I drove it home after going through the accident report
formalities. (I have a 1998 SL2 4 door.) The next day I had a doctor's
appointment so after duct taping the fender sort of back in place I
tried to start the car, but the battery was completely dead. I mean,
dead as a doornail.

Now, it was raining fairly hard the night before and all that day and I
figured that the rain must have shorted some exposed wiring. The shop
did some testing that indicated there was no drain on the battery, so
they concluded I must've left the light on or something. When I parked
the car, however, after the accident, the sun was shining brightly so
there'd have been no reason to turn on the light. Moreover, as I recall
there's an automatic shut off if you turn off the key and leave the
lights on. This mechanic tells me that isn't true, and that if you
leave the lights on the battery will just drain. (All the doors were
shut tight, by the way.)

So, I'm thinking that:

1. The fellow is wrong about the automatic shut off, because that's one
of the features I liked about the car.

It's likely he's wrong. But that's a moot point.

2. If they didn't test the car "wet" then how would they know whether
there was a drain on the battery system from exposed wiring that shorted
due to water?

Did the battery charge up and work OK after you found it wouldn't start
the car?

My thought on the exposed wiring...

There should be no current to those wires to "short out" whether wet or
not. The switch inside controls when there is current to the wiring for
the lights and that may work through a relay which will certainly be
inside the engine compartment or another secure place.

I'd be looking for some other source for the problem.

Best, wes

 

[Freewheeling]

It's likely he's wrong. But that's a moot point.


Did the battery charge up and work OK after you found it wouldn't start
the car?

My thought on the exposed wiring...

There should be no current to those wires to "short out" whether wet or
not. The switch inside controls when there is current to the wiring for
the lights and that may work through a relay which will certainly be
inside the engine compartment or another secure place.

I'd be looking for some other source for the problem.

Best, wes

Wes:

Thanks. The battery charged, or at least it charged enough that I could
turn the key off and then restart. I'm virtually certain I didn't leave
the lights on, and the doors were shut tight and locked. I have
noticed, in the past, that sometimes the key wouldn't turn over the
motor, so I'd leave the car in gear and rock it a little, then take it
out of gear and the ignition worked fine. But there was no battery
drain. The ignition switch was just finicky, and they checked the
alternator anyway as part of their diagnostics.

Frankly, that's the least of my problems at this point. But I was
curious why the battery might drain after a collision. If they checked
for, and couldn't find, a drain on the battery then the rain must've had
something to do with it. Either that, or I'm mistaken about the lights
being on. But if there's a timed shut-off switch for the lights then
that leaves some rain-related/collision-related problem, or at least
rain-related/something-related, so the question about the shut-off
switch wasn't quite moot.

 

[BläBlä]

Wes:

Thanks. The battery charged, or at least it charged enough that I could
turn the key off and then restart. I'm virtually certain I didn't leave
the lights on, and the doors were shut tight and locked. I have
noticed, in the past, that sometimes the key wouldn't turn over the
motor, so I'd leave the car in gear and rock it a little, then take it
out of gear and the ignition worked fine. But there was no battery
drain. The ignition switch was just finicky, and they checked the
alternator anyway as part of their diagnostics.

Frankly, that's the least of my problems at this point. But I was
curious why the battery might drain after a collision. If they checked
for, and couldn't find, a drain on the battery then the rain must've had
something to do with it. Either that, or I'm mistaken about the lights
being on. But if there's a timed shut-off switch for the lights then
that leaves some rain-related/collision-related problem, or at least
rain-related/something-related, so the question about the shut-off
switch wasn't quite moot.

Parasitic draw test and Load test. Not much else you can check for at
this point.

 

[Dooger]

Frankly, that's the least of my problems at this point. But I was
curious why the battery might drain after a collision. If they checked
for, and couldn't find, a drain on the battery then the rain must've had
something to do with it.

Assuming the rain has stopped...maybe whatever else may have caused the
battery to drain stopped for a while too. I'm sorry but the chances that
rain caused your battery problem is slim to nil...and slim just bought a
ticket out of town. Do this sometime...take a charged up battery and hook
wires to each post and then lay the other ends in a puddle of rain water
overnight.

You already said the battery/starting system has been flaky in the past.
That's where I'd look first.

Either that, or I'm mistaken about the lights being on.

Things are not always either/or.

But if there's a timed shut-off switch for the lights then that leaves
some rain-related/collision-related problem,

Not necessarily.

or at least rain-related/something-related, so the question about the
shut-off switch wasn't quite moot.

Sure it was. If the wires aren't energized they can't "short" out. And
rain water isn't going to cause enough current to pass between bared wires
or "something" to affect the battery much one way or the other. So unless
you left the lights on and there wasn't a safety shutoff switch there
could have been no "short" in the wires. Distilled water is a very poor
conductor and rain is pretty much distilled water.

Best, wes