5KT draining battery

[Aleksander Mierzwa <alex@matrix.com.pl>]

Hi List!

There's definitely something fishy going on with my 5K. Yesterday it
drained the battery for the third time this year. All three times the
symptoms were very similar:
1) The car starts fine, runs fine, no battery problems
2) After being driven in rain and parked overnight, as I come to the car
in the morning the battery is _completely_ dead. I mean about 3 volts at
the battery. Just as I left the high beams on (which I'm sure I did not
:-) )
3) After charging the battery the car runs fine for next 2-3 months.
This time I decided to do a little investigation. I charged the battery.
When fully charged, it showed 12.6v. I installed it in the car. The
interior lights did not come on, the alarm system chirped several times.
I measured the voltage again: 12.3v at the battery, the voltage at the
positive post in the engine compartment was 11.5v, but it seemed to
fluctuate and showed 12.2v several times (maybe it's because the
positive post is dirty/corroded and I used intake manifold as a ground).
Then I connected voltmeter back to the battery in hope that I will see
the voltage rise once I disconnect the component that drains the
battery.
I hand cranked the engine one turn to make sure that's not the
alternator shorting in certain position. No difference.
Then I patiently started to remove each fuse, check the voltage and
replace each one after the check. Removing none of the fuses resulted in
any change in the voltage. I understand I should rather measure the
current, but I don't have a proper ammeter and just FYI, switching one
iterior light on resulted in immediate voltage drop by 0.01v and it
rised back as soon as I turned the light off. I can imagine that
something able to drain 63Ah battery in 20 hours should give significant
voltage drop.
During the test the battery voltage was slowly dropping and after
probably 15 minutes it was 12.21v
The I finally started the car. With the engine running, I got 14.5v at
the engine post, 14.4v at the battery. Needless to say, the car started
with no problems this morning and I bet it will run fine for next 2-3
months.
So, infinite wisdom of the List, any ideas? The problem occurs only once
in 2-3 months and seems to go away as soon as the battery is charged, so
I have little chances of reproducing it. Is it possible that the battery
has a fault that represents itself as a occasional internal short? Or do
some electronic gizmo "wake up" at night and drain the battery?

Aleksander Mierzwa
Warsaw, Poland
mailto:alex@matrix.com.pl
87 Audi 5000CS turbo (mine)
88 Renault Medallion wagon (mom's)
91 mountain bike (just in case both cars broke at the same time :-)