R320 battery drain
Thread Starter
Junior Member
Joined: Dec 2019
Posts: 22
Likes: 1
2008 R320 CDI
R320 battery drain
Battery was dead. Jumpstarted and started fine. I noticed the battery charger never shut off, tapering off to about 1 amp.
So I assumed there was a parasitic draw, and that the battery charger was powering the drain. I pulled all the fuses in the engine compartment with no change in charge current. Same for the ones in the dash on the passenger side.
Then I moved to the rear. Rather than struggling to pull all the fuses, I started measuring the voltage drop across the fuses to find ones that had current going through them. Fuses have resistance and there are some charts on the fuse rating vs mV voltage drop vs mA. I found two that had >2 mV, which equates to hundreds of mA of current, so pulled them. No change in the battery charger current.
Decided to put a meter in series with the battery rather than relying on the battery charger readout. Turns out the battery is OEM from 2012, so 10 years old. Tests fine surprisingly. After hooking up my meter I measure a 1 amp draw. Hood/door/hatch open and waited 20+ minutes.
Removed all fuses at the battery. No change. Disconnected rear SAM. No change. Took out all front relays and rear relays as a quick test. No change. The two trunk fuses with measurable current were also disconnected.
Before I go an pull dozens of fuses again, is there something I'm missing?
Potentially relevant symptoms:
So I assumed there was a parasitic draw, and that the battery charger was powering the drain. I pulled all the fuses in the engine compartment with no change in charge current. Same for the ones in the dash on the passenger side.
Then I moved to the rear. Rather than struggling to pull all the fuses, I started measuring the voltage drop across the fuses to find ones that had current going through them. Fuses have resistance and there are some charts on the fuse rating vs mV voltage drop vs mA. I found two that had >2 mV, which equates to hundreds of mA of current, so pulled them. No change in the battery charger current.
Decided to put a meter in series with the battery rather than relying on the battery charger readout. Turns out the battery is OEM from 2012, so 10 years old. Tests fine surprisingly. After hooking up my meter I measure a 1 amp draw. Hood/door/hatch open and waited 20+ minutes.
Removed all fuses at the battery. No change. Disconnected rear SAM. No change. Took out all front relays and rear relays as a quick test. No change. The two trunk fuses with measurable current were also disconnected.
Before I go an pull dozens of fuses again, is there something I'm missing?
Potentially relevant symptoms:
- I think last week I noticed the interior fan was on after opening the door; I brushed it off and didn't see it again so not sure if it's an ongoing problem.
- Rear hatch often doesn't latch/lock when closed; I did a bit more testing after the no start issue and had issues with it closing when using any of the buttons; I didn't troubleshoot further as there is currently a consistent draw, and I was afraid of turning it into an intermittent problem that would be harder to diagnose.
- I've seen some codes related to an upper control panel but it wasn't affecting daily use so I didn't investigate/save the code.
Thread Starter
Junior Member
Joined: Dec 2019
Posts: 22
Likes: 1
2008 R320 CDI
