Battery Red Triangle Warning

[dursleyman]

We have been getting the big red triangle battery alarm saying to visit the workshop every time we switch on our 2005 W211 estate. Everything still works fine apart from the SBH brake hold. The car had a new alternator and battery control relay a couple of years ago.
Getting to the local specialist was difficult in these Covid times so I have replaced both batteries because that was an easy option but the problem is still there.
Can I just ignore it for the time being and if so for how long?

 

[sonic]

Put a meter on the battery see what the volts are engine on & off.

 

[jimbg]

Measure the battery when the engine has been off for at least a few hours to allow it to achieve it's rest state.
If you do it too soon you will get a false higher reading.

 

[dursleyman]

The car has been parked up since Tuesday (4 days) so good time to test the batteries.
Disconnected the main battery in the rear was 12.05v.
Auxiliary battery in the front was 12.55v.
I reconnected, started the car and with the engine running for a few minutes, the main battery is now 13.6v but the auxiliary battery is still reading 12.55v.
So does this mean the auxiliary battery is not being charged?

 

[jimbg]

Does that answer it for you? 12.55 is effectively 100% charged

 

[dursleyman]

So the question is why do we have the battery alarm up?

 

[jimbg]

Because you only had a reading of 12.05 volts?

 

[mioba]

measure the v across the batteries with the engine running.

 

[dursleyman]

Yes I did that (see above) 13.65v on the main and 12.55v on the auxiliary.

 

[sonic]

Yes I did that (see above) 13.65v on the main and 12.55v on the auxiliary.

Thats low on the AUX battery, put both battery on charge for 8 hours. But I think you are seeing the end of life of your batteries. Efficiency drops dramatically in winter, short journeys don't help.

 

[dursleyman]

They are both new good quality batteries so I believe the problem must be something to do with the charging system.
Our local specialist replaced the battery control relay shortly after fitting a new alternator so I am looking for ideas what else may be going on.