Battery voltage

[renault12ts]

My car failed to start recently...the battery was flat.

Once recharged the voltage reads 13.5v at idle.

At rest the voltage is 11.15v using the Climate control panel diagnostics...however the multimeter at the battery terminal shows 12.5v.

Which one is correct?

 

[BlackC55]

hey both maybe wrong. Have you got a decent meter?

 

[Dieselman]

The meter will be correct unless the BATT symbol is on.
Both the meter and car diagnostics use battery voltage as the comparator, so if the car battery voltage is low, how can it monitor itself?
In addition, where is the Air-con voltage taken from?

 

[whitenemesis]

Which one is correct?

Probably all of them, each correct for the way in which the measurement was made.

 

[ivandraganov]

My car failed to start recently...the battery was flat. Once recharged the voltage reads 13.5v at idle. At rest the voltage is 11.15v using the Climate control panel diagnostics...however the multimeter at the battery terminal shows 12.5v. Which one is correct?

probably the battery is getting too old...

 

[renault12ts]

Battery is 3 years old.

 

[Dieselman]

Mercedes wiring is generally under spec, so don't be surprised at a 1v drop to the A/c.

 

[ivandraganov]

Battery is 3 years old.

Then check the alternator brushes and alternator pulley,if the battery isn't original then there isn't guarantee that is any good,even 3 years old..

 

[grober]

Bill darden

Gentlemen let me introduce the BATTERY ORACLE-- All the questions you wanted to ask about car batteries but didn't dare to ask. Car and Deep Cycle Battery FAQ, Battery Manufacturers and Brand Names List, and
courtesy of one BILL DARDEN I recommend it.

HERE'S ONE TABLE SoC = state of charge

 

[Ted]

Excellent and timely link Grober - maybe one for a sticky?

In reply to Renault12ts' question, most of the above are correct. The best way is to check the voltage across the battery terminals with a decent dvm. I use my old but trusty Fluke 75 for this.
As DM says, there will definitely be some sort of voltage drop between the battery and cluster/climate display.
R12, your voltage does look a little low - personally I would put the battery on charge, and then check the regulated voltage (around 14-14.5) to see if the charging system is keeping the charge up.
Of course, you would also need to check if enough current is being supplied to cover all of the consumables, but battery voltage is a decent guide.

 

[markjay]

Interesting read grober.

The Vauxhall Omega I had some years ago had an insulating sleeve all around the battery to protect it from the heat in the engine bay, which sounds sensible.

I suppose that the battery location on my W203 - opposite side from the exhaust manifold - means it is not required.

I sold the Omega when it was 7 years old with the original battery. Similarly my 2006 W203 is also still on its original battery.

I remember that batteries used to last around 3 years max back in the seventies and eighties.