Auxiliary battery getting hot and venting!? - Mercedes-Benz Forum
 
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post #1 of 6 (permalink) Old 06-24-2019, 12:00 PM Thread Starter
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Auxiliary battery getting hot and venting!?

I have an issue with the auxiliary battery (in the trunk) getting hot after a long drive (2hrs plus).
I have had it tested and it seems to be functioning fine with no issues. After a long drive it starts to smell and if I stop and look under the car I can see vapour coming from the vent tube from the battery and when I touch the battery it is hot !

- The terminals look fine and I have checked they are secure.
--The charging also has been checked and is the voltage regulator appears normal?
- Should I change the battery, is it on the way out? Is it something else?

Thanks
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post #2 of 6 (permalink) Old 06-24-2019, 12:09 PM
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when a battery gets really hot and starts venting, that is usually a sign that it is getting overcharged. I would have your charging system checked and make sure the regulator is working properly.
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post #3 of 6 (permalink) Old 06-24-2019, 12:14 PM Thread Starter
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Originally Posted by tonyjroy View Post
when a battery gets really hot and starts venting, that is usually a sign that it is getting overcharged. I would have your charging system checked and make sure the regulator is working properly.
I got this done today, the charging seems fine at 13.5V, however if I was to change the voltage regulator, is it on the alternator and easy to do?

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post #4 of 6 (permalink) Old 06-25-2019, 08:20 AM Thread Starter
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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.


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post #5 of 6 (permalink) Old 06-27-2019, 10:09 PM
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I'm not sure, but will SDS capture an intermittent overcharging issue?
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post #6 of 6 (permalink) Old 06-28-2019, 04:08 AM Thread Starter
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Update today.

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
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