Parasitic drain while locked

[F55MCS]

Edit Edit: when I try to start it and it fails, I always hear the boot locking mechanism making noise. Could that be related or just the usual electrical weirdness that accompanies a dead battery?

Edit: forgot to say that it’s a 2004 V70 D5 manual.

Hi all, hoping you can help me with a battery drain issue that has me stumped. A quick summary of events:

Thursday 20th June: foolishly left side lights on all day and drained battery, bump started

Then went on holiday and it was parked up until 1st July, where it started no problem and I drove my 65 mile each way commute (with a few stops) no problem.

Parked up all day 2nd July.

3rd July, wouldn’t start. Jumped it and drove it for a while hoping it was a one off. It wasn’t...

5th July took it to our friend’s garage. He, rather embarrassingly, found that the positive lead wasn’t properly attached to the battery and was sort of just resting on top. He re attached it, charged it and tested for excessive current draw (with car unlocked) and there was none. Assumed it was the lead, parked it up on the drive and went away for the weekend in a different car.

Got back last night, dead again. Seems to only be doing it when locked. I have had the ‘service alarm system’ message since I got it in November, which I assumed was the backup battery and thought that perhaps it was connected to that. So I removed fuse 38 (alarm siren, 5A) and parked it at the top of a hill (didn’t have time to go for a long enough drive to charge it properly).

Bumped it this morning and drove 65 miles to work, just went out to test it, hoping that it would be resolved and it still won’t start.

I have a cheap Chinese FM transmitter that can also roughly read battery voltage on start up, and it’s showing between 8 and 9 V whenever I go out to it.

Nothing is left on overnight (exterior/interior lights, nothing in any of the 12V sockets etc).

Battery is relatively new (last year or 2).

Alternator is very new.

No idea what could be causing it, any ideas?

[Thassos]

battery terminal resting on top sounds very unusual, could it be that the connection to the battery was intermittent as you were driving?, which i would i imagine play havoc with the system, as the voltage would spike and jump around, if you have a cheap dvm just measure the resting voltage on the battery terminals, then start the engine and see if it increases over 13v, to get an idea if the alternator is actually charging the battery, also after a long run, measure the resting voltage (keys out everything off) as it should be up near 12.8v if its been charging up as you drive.

[F55MCS]

Originally Posted by Thassos

battery terminal resting on top sounds very unusual, could it be that the connection to the battery was intermittent as you were driving?, which i would i imagine play havoc with the system, as the voltage would spike and jump around, if you have a cheap dvm just measure the resting voltage on the battery terminals, then start the engine and see if it increases over 13v, to get an idea if the alternator is actually charging the battery, also after a long run, measure the resting voltage (keys out everything off) as it should be up near 12.8v if its been charging up as you drive.

I think it may have been slightly loose and a pothole I hit on the 1st dislodged it. Never had any loss of electrics while driving or anything weird like that so I doubt it caused any issues, probably just coincidence.

My FM transmitter thing, while probably not massively accurate, does read 13.6V with engine on and 12.something V just after switching off so I think alternator is fine.

[cheshired5]

Battery testing, general parasitic drain testing and alternator diode and parasitic drain testing videos can be found in my YouTube playlist here.

[F55MCS]

Thought I should probably update this, turns out it was the alarm and it wasn’t draining once I had pulled the fuse. It was just that all the discharging had knackered the battery so it wouldn’t hold a full charge anymore. £95 later a new battery is in and it works fine, just with no alarm.

Thanks for the help and suggestions folks.

[oragex]

For others, it is good to test if the siren still works, if it doesn't sound, better to remove the Fuse 38 near the front door because it can indeed slowly drain (and wear) the car battery https://www.youtube.com/watch?v=dMpSlDvaNoU