what causes my battery to drain

[clemenshess97]

Hey everyone!

My truck has been sitting for 3 weeks and when I first got back into the truck I checked the voltage, it showed 11.8V-12.2V and the truck started up perfectly.
Then I took the truck on a 120km trip.
It then sat for another 3 weeks and when I tried to open it with the key fob - nothing happend, I figured the battery must be drained. I then took the battery out and checked the voltage - it had 5.5V
That battery is done, tried to charge it but it won‘t, 5.5V is just too low to have it come back to life.

I don‘t really have much time at the moment and was wondering if someone has an idea on what could be causing the drain.

What could be powerful enough to drain the battery to 5.5V but weak enough to not drain it to 0 within 3 weeks ?

 

[SnowDrifter]

How old is the battery? 3 weeks is a bit for a car to sit without starting in general.

 

[S33k3r]

Three things come to mind immediately:

1) It's a bad battery
2) There is a parasitic power drain in the system; you may have a loose ignition (it never fully turns off), something could be wrong with the factory alarm, aftermarket radio, a short somewhere, etc.
3) Your alternator is going out

 

[clemenshess97]

Battery came new in July 2020.
Alternator seems to work fine judging by the charging voltage (14.1V-14.5V).

I noticed that maybe 1 out of 20 times the small display in the cluster (the one that usually shows the mileage, Driver 1 or Driver 2, or any other message like „Check Engine Oil“) stays on after locking the car. It doesn‘t say anything, it‘s just completely lit up. I then usually unlock the car, open and close the driver door and lock it back up, it then goes away.
I check it everytime though (if it‘s lit up or not) and it didn‘t light up last time I parked the truck.

 

[clemenshess97]

How old is the battery? 3 weeks is a bit for a car to sit without starting in general.

Battery came new in July 2020

I know…but first time after 3 weeks it was fine, having about 12V left

now the second time all of a sudden it‘s only left with 5.5V

And the drive inbetween those two times should have charged it back up completely

 

[S33k3r]

Sounds like you have a bad door switch.

 

[clemenshess97]

Sounds like you have a bad door switch.

do you mean the entire module with the window controls, door locks, mirror adjuster ?

 

[MassHoe04]

do you mean the entire module with the window controls, door locks, mirror adjuster ?

If that is the issue, the switch mentioned by @S33k3r is part of the latch mechanism. I tells the computer when the door is open or closed, turns interior lights on/off.

The interior lights on my 04 shut themselves off after a while, it does that so they don't drain the battery if doors were left open for too long. It is like 10-15 minutes IIRC.

I am not saying @S33k3r is wrong. But, I kind of think the body control module would shut lights down, if the door latch switch was indicating door open while the door was actually shut. Not sure that switch would do that, if everything else was working properly.

The body control module (BCM) can do weird things, especially if ground connections to the body and frame are not 100%. The BCM is supposed to keep some things "awake" for a period of time, but then put everything to "sleep" after a period of inactivity. If grounds are not good, sometimes the BCM won't let anything sleep and drain the battery down if left too long.

Again, I am not saying @S33k3r is wrong. He is definitely one of our go-to advisors. I am just thinking out loud and wondering if there is a chance it might be the BCM having bad grounds.

 

[clemenshess97]

If that is the issue, the switch mentioned by @S33k3r is part of the latch mechanism. I tells the computer when the door is open or closed, turns interior lights on/off.

The interior lights on my 04 shut themselves off after a while, it does that so they don't drain the battery if doors were left open for too long. It is like 10-15 minutes IIRC.

I am not saying @S33k3r is wrong. But, I kind of think the body control module would shut lights down, if the door latch switch was indicating door open while the door was actually shut. Not sure that switch would do that, if everything else was working properly.

The body control module (BCM) can do weird things, especially if ground connections to the body and frame are not 100%. The BCM is supposed to keep some things "awake" for a period of time, but then put everything to "sleep" after a period of inactivity. If grounds are not good, sometimes the BCM won't let anything sleep and drain the battery down if left too long.

Again, I am not saying @S33k3r is wrong. He is definitely one of our go-to advisors. I am just thinking out loud and wondering if there is a chance it might be the BCM having bad grounds.

the lights always turn off when I close the door, I guess that is working fine.

Do you know which ground is responsible for the BCM ?

Last year I cleaned the two grounds to the frame on the driver side.
One on the front left close to the Bumper.
And I rewired some grounds in the engine bay (new ground from battery to TCM to firewall to throttle body to engine block), I did it to try to fix a throttle body code. Did that rewiring in June/July last year, but the drain appeared only now so I guess they don‘t have anything to do with eachother.

 

[MassHoe04]

the lights always turn off when I close the door, I guess that is working fine.

Do you know which ground is responsible for the BCM ?

Last year I cleaned the two grounds to the frame on the driver side.
One on the front left close to the Bumper.
And I rewired some grounds in the engine bay (new ground from battery to TCM to firewall to throttle body to engine block), I did it to try to fix a throttle body code. Did that rewiring in June/July last year, but the drain appeared only now so I guess they don‘t have anything to do with eachother.

Pretty sure the ground for BCM is the one on the frame under the driver's side door (towards the front). BCMs have been known to go bad... Sometimes they just don't shut things down the way they are supposed to after a certain amount of time.

You may want to see about some more diagnostics, before jumping on a new BCM. They are not very expensive (not sure about Germany) and easy to replace. They do have to match part/serial numbers to work.

One of the stranger issues mentioned recently, related to power drain, was the possibility of a diode inside the alternator shorting to ground. Not only can they not charge the battery when running, they can drain the battery just sitting there.

I think you have some more detective work ahead of you.

Search the forum for "parasitic battery drain" and see what comes up. There were some really great step by step instructions on what to look for and how to check where things might be drawing power when everything else is off.

Hope you find something that is easy to fix. You might get lucky!

Please post any additional steps you take and what you find along the way.