Battery Voltage

[K3NJW]

Having suffered a flat battery a couple of times recently I bought myself a Leven charger which simply plugs into an adapter, so much simpler than lying in the footwell. Sadly the battery still flatens quite quickly so I put a meter over it to check the alternator was working and got slightly inconclusive results.

The battery is putting out a smidge over 12 volts and the with the engine running it rises to just over 13 when I would have expected higher. Anyone know if this suggests the battery is so shot it won't accept charge, or that the alternator isn't working properly.

On a cost front I'm hoping it's the battery, but it will mean the upside down in the footwell battery extraction routine again !!

[SMB]

A correctly charging alternator is about 13.5-13.7 volts I beleive, the battery voltage fully charged should be around 12.6-12.8 with no load on it.

If the battery has fully discharged you may have a dead or short circuit in a cell, new battery time!

[kend]

With a voltage of 12v the battery is dying quickly, you should check the voltage when everything is off and the car has not run for at least 6 hours any reading less than 12.5V indicates battery is shot. If battery is old, over 4 years then it probably is battery. If battery newish less than 2 years, it would be worth checking the charging system before replacing battery.

>> Edited by kend on Thursday 2nd January 09:51

[K3NJW]

Thanks for your help.... I did suspect the battery, but now I'll get a new one and be done with it.

[RolandM]

I had a problem with mine on Xmas night, although alt was putting out max output it was not making it back to the battery due to a poor pos connection below the starter motor. don't forget to check out the system as a whole instead of isolated unit's (say's the man who waited by the roadside for the AA)

[ribol]

Putting a voltmeter across a battery is not a very good way to test a battery. A battery can have the right voltage in it but still be incapable of supplying the current needed to do the job. The only way to check a battery is to do a discharge test backed up with checking each cell with a hydrometer. A 12V battery will only ever have much more than that in it when it has been recently been charged.

The rate at which the alternator charges is determined by the state of charge of the battery. Checking the voltage across a flat battery with the engine running can read nearer to 14V than 12V as a rough guide.

Just a thought, it is possible that you have a good battery, a good alternator and bad starter/cables/earths. A proper test on the battery/cables/earths would be the first thing to do.

Ivan