Battery voltage - Mercedes-Benz Forum

 
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post #1 of 8 (permalink) Old 08-30-2010, 02:10 PM Thread Starter
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Smile Battery voltage

I've seen several post's regarding low battery voltage.

What would the minimum voltage be for an auto battery - at what battery voltage will the electronics begin to malfunction and cause problems/false indications?

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post #2 of 8 (permalink) Old 08-30-2010, 02:20 PM
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The first problem to occur with low voltage will be failure to turn the starter motor.

Anything past that isn't too relevant, I'd think.

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post #3 of 8 (permalink) Old 08-30-2010, 04:08 PM
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From what I've read on here, lower than optimal battery output voltages can cause various electrical gremlins and /or trouble lights and might even set diagnostic trouble codes too. And if I remember correctly, these symptoms can occur when the battery still has enough juice to start the car. How low the threshold has to be is anyone's guess but I suspect it's somewhat different from one model to another and dependent on what type, and how many, power consuming options it has.

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post #4 of 8 (permalink) Old 08-30-2010, 06:54 PM
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Originally Posted by lkchris View Post
The first problem to occur with low voltage will be failure to turn the starter motor.

Anything past that isn't too relevant, I'd think.
This should be correct for the cars mentioned at the OP's profile but dual battery cars like the W211 can give the low battery warning from a bad AUX battery while the car would still start fine.
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post #5 of 8 (permalink) Old 11-20-2012, 01:09 PM
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I would like to know what the optimum voltage really should be. I've read that a weak battery can cause w251 power tailgate problems. My tailgate motor is not running and I'm reading 12.8v. I'm thinking that's a little low, but would like to know for sure before replacing the battery.

Last edited by Clambake; 11-20-2012 at 01:59 PM.
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post #6 of 8 (permalink) Old 11-20-2012, 02:07 PM
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Originally Posted by Clambake View Post
I would like to know what the optimum voltage really should be. I've read that a weak battery can cause w251 power tailgate problems. My tailgate motor is not running and I'm reading 12.8v. I'm thinking that's a little low, but would like to know for sure before replacing the battery.
With the engine off your battery should be between 12 and 14 volts. If it stays around 12 it is probably time to replace it. With the engine running it should be at 13-15, optimally 14. If it's 13 or below and the engine has been running for a while it might be a bad alternator.
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post #7 of 8 (permalink) Old 11-20-2012, 04:09 PM
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Engine off with good battery reading should be 12.3-12.5 volts
Engine running actually loaded with minimal accessories 14-14.2 at idle
Same 14.2 when above idle and everything on.
12.8 not running is high but fine and the problem is probably the gate motor.
Load test would cover all bases
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post #8 of 8 (permalink) Old 11-20-2012, 04:51 PM
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Originally Posted by 1carguy View Post
I've seen several post's regarding low battery voltage.

What would the minimum voltage be for an auto battery - at what battery voltage will the electronics begin to malfunction and cause problems/false indications?
I just picked up a brand-new Varta battery today, which is fully charged. The voltage at the terminals is 12.68v.

I know that if the battery voltage is 12.1v it is 50% discharged, and at 11.7v it is fully discharged. However, the alternator will maintain the terminal voltage at 14.2 v while the engine is running.

The starter may still work with the battery at 10.0v - 11.0v. At 7.0v the ECU will inhibit cranking.
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