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Today for the first time I noticed that steam vapour was coming from the auxiliary battery in the trunk via the small tube that vents through the floor, the battery felt hot to touch too and smelt weird.
Now it is worth mentioning I drive a lot and my weekly commute is 400miles taking 7hrs without stopping, so could it be that the battery is just being used for too long a period at a time?
The alternator may be overcharging the battery due to a failed voltage regulator. I'd measure the voltage across the battery terminals with the engine running. A typical reading is around 13.8, though both Mercedes' and the battery manufacturer's specifications may allow for c. 15 volts or more -- I'd have to check.
I checked the voltage with the engine running and I'm getting a constant 14 volts with load, this would rule out the voltage regulator at least.
I then looked closer at the breather pipe and found that due to the age of the car it had flattened and I am now wondering if the battery just couldn't breathe properly and as a consequence got hot over a 7 hour drive.
I've now replace the breather pipe and also checked the contacts on the positive and negative terminals which were fine, incidentally the battery is an original Mercedes battery.
Is there any corrosion building up on the positive battery post? Overcharging will cause this to happen over time due to the battery gas leaking out around the post.
Is there any corrosion building up on the positive battery post? Overcharging will cause this to happen over time due to the battery gas leaking out around the post.
Was the battery venting gasses when you measured 14 volts with the engine running? The only thing that I know to cause the battery to get hot and vent gasses is rapid charging or rapid discharging. If it is not overcharging then there must be something that was putting a large load on it causing rapid discharge.
Or is it heating and gassing all of the time that the engine is running?
Was the battery venting gasses when you measured 14 volts with the engine running? The only thing that I know to cause the battery to get hot and vent gasses is rapid charging or rapid discharging. If it is not overcharging then there must be something that was putting a large load on it causing rapid discharge.
Or is it heating and gassing all of the time that the engine is running?
Mike.
No the battery behaves fine on short journeys, I have my 7 hour commute tomorrow morning, so will see.
I had a similar issue and it turned out that my passenger side SAM was faulty. They can drain a battery fast when they fail. I have a MB II scanner from iCarSoft which failed to detect the fault. It didn't show up until I took it to dealer for SDS. Nice MB BTW.
I had a similar issue and it turned out that my passenger side SAM was faulty. They can drain a battery fast when they fail. I have a MB II scanner from iCarSoft which failed to detect the fault. It didn't show up until I took it to dealer for SDS. Nice MB BTW.
Thanks for that !
Did you get the SAM fixed and did it resolve the hot battery?
I would peel back the trunk liner above the battery and sneak a peek at the battery control module since there is ample evidence that these can fail catastrophically burning up the car. Any smells when you first open the trunk?
I have read a few comments on the battery control unit, I pulled back the liner and had a look to see if there was anything obvious as water can ingress there, but all looks good, Tonight I may take the unit apart and check inside.
I have had the battery analysed and all checked out fine with good charging at 13.5V and the batteries are both healthy.
I have now run a set of cables from the + and - to the front and connected my multi meter to monitor the charging volts, so far over a 30min drive it remained constant at 14.1V.
I have also added a thermocouple to monitor the temperature of the battery whilst driving to see when it starts to get hot.
Checked the battery control unit and all looked in order, no evidence of burning not water leaks.
After checking that the charging voltage remained constant I decided to buy another battery to try out on my long 6 hour drive home.
I set up a thermocouple probe on top of battery and connected my meter directly to the battery terminals to monitor from my seat.
All the way home for 6 hours the voltage remained at 14.1 VDC, which is perfect. The temperature did not change much and only sat 2 degrees above ambient which would be expected being under the rear tray with bags etc. Therefore today was 26 degrees C and the battery was 28 degrees C.
Conclusion I think is that the previous battery had a bad cell, i recall the owner jump starting from this battery as the car was sat for ages, I think this may have damaged it as it clearly states do not jump from this battery.
Lastly I bought a Bluetooth remote temperature monitor that I fitted next to the battery to keep an eye on it once in a while from my phone
06-20-2019, 03:24 PM
