Aux battery - can you trickle charge it?

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mauchincloss

New Member
Joined
Nov 22, 2020
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2
Location
Edinburgh
Car
CLS 350d AMG sport Shooting Brake
During lockdown, in common with many people I assume, I put our cars on a trickle charger as they were not being used as much as normal. So far so good. But! In my 2013 CLS 350d SB the stop/start function stopped working. The same was true of my BMW M140i when the battery was low but charging the battery in that cured the problem.

I understand in the CLS it is the AUX battery in the boot that is crucial for the stop/start. Is there anything to stop me putting the trickle charger on that battery?
 
Presumably you charged the main battery via the battery terminals?

I expect you are meant to use the jump start terminals and that would have "allowed" the car electronics to manage both?
 
Well, one was on the battery and the other was a negative terminal located on the inner wing. Are there other jump start terminals?
 
You would have to check for your particular model, remember in some cars the aux battery is just a capacitor!

No idea about the CLS, sorry.
 
Well, one was on the battery and the other was a negative terminal located on the inner wing. Are there other jump start terminals?
Did you attach directly to the battery?

The correct procedure (according to Mercedes manual) is to attach the + cable to the positive post beneath the red plastic cap/flap/slider (not the battery) and the earth post mar the wing as you describe.
 
The aux battery is normally behind a relay so the only way to charge if you are not running the car is by charging it separately and directly.

I don't think it's necessary to run a trickle on it personally as an ongoing maintenance requirement.

I would think by now if you are on the original aux battery, it's probably time for it to be changed.

If you've had the main battery changed, ideally they should both be done at the same time.

If it were me, I'd try running a proper charger on the aux (unless your trickle can do that as some do both) to see how much it can be improved and take it from there.
 
The aux battery is normally behind a relay so the only way to charge if you are not running the car is by charging it separately and directly.

I don't think it's necessary to run a trickle on it personally as an ongoing maintenance requirement.

I would think by now if you are on the original aux battery, it's probably time for it to be changed.

If you've had the main battery changed, ideally they should both be done at the same time.

If it were me, I'd try running a proper charger on the aux (unless your trickle can do that as some do both) to see how much it can be improved and take it from there.
During lockdown when my Smart wasn’t used much - and when it was used those journeys were very short - the stop-start system was disabled.

After using my CTEK battery conditioner - connected to the charging posts, and not the battery - the stop-start system immediately started work.

This has happened a few times during the course of this year, and as a result I assumed that charging through the charging posts charged both batteries.

Maybe the Smart is different?
 
During lockdown when my Smart wasn’t used much - and when it was used those journeys were very short - the stop-start system was disabled.

After using my CTEK battery conditioner - connected to the charging posts, and not the battery - the stop-start system immediately started work.

This has happened a few times during the course of this year, and as a result I assumed that charging through the charging posts charged both batteries.

Maybe the Smart is different?

Yeah could be.

I only know for sure there is a relay between main and aux on a 222!
 
On the SLK R172, the Aux Battery is isolated from the main battery when the engine is switched off and requires a direct connection to charge it. I have fitted two sets of CTEK charging leads (with the 3 colour led indicator) so that I can gauge which battery requires a top up charge.
 
I'm presuming that as the S has them separate, the majority of the other models do and if the SLK is the same than that theory could hold.
 

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