alternator K12 Micra 1.2 petrol runs on 7 volts? (55 plate)

[Danage]

Hi all,

Got a bit of a mystery. Micra went in for a service in the local garage in the new town I've just moved to, instead of the usual place in the wife's home town which her Dad is the accountant for ;)

They advise that the battery needs replacing. I think this is odd as the battery was replaced when I was gifted the Micra around 3 years ago. I ignore the garage and drive it home. Once home I put a multimeter across the battery. It registers around 7.2 volts. Slightly concerned I fire up the engine and measure across the terminals again to find out what the alternator is putting out. Voltage measure at around 7.8 volts. Now, if the readings had been 12.2 volts and 12.8 volts I wouldn't have batted an eyelid.

The mystery is that the charge waring light on the dash never comes on when the engine is running... the car is perfectly happy running on sub 8 volts. When turning the ignition, but not fully on, the battery charge warning light is illuminated to show that it does function, just like all the other warning lights.

So there are two outcomes of this story:

1) The battery charge warning system is broken
2) K12 1.2L petrol Micras are designed to run on sub 8 volts.

Can anyone confirm? Does anyone else have a K12 that is happy with a battery and alternator combo running just under 8 volts? It occurs to me that if the system that detects that the battery is under charged is not working, then the system that tells the alternator to let loose with full voltage is also probably broken.

I've still to check all connections to the alternator and measure the voltage drop across alternator casing and battery negative terminal, so I'll do this at the weekend. But I'm thinking it'll pass this test fine.

Danage.

Edited October 27, 2016 by Danage
typo, extra tags, further musings

[Trevor]

I would think that the meter you are using is defective because as far as I am aware there are no 12 volt vehicles that will run at 8.0 volts (except for classic cars running on a dynamo and contact breaker ignition).

If they recommend the battery is defective (as in the surface voltage reading or deep discharge is low) then maybe so, but I would check with another meter first.

[Danage]

I don't have another meter, but have checked it against some other batteries and mains voltage and for those it is accurate. I'm tempted to think it is running on 8 volts because if I put the headlights on, and then the heater at full blast the headlights will dim for a second before coming back up to full brightness. I've worked through the service manual trouble shooting for the charging system and it says to replace the alternator.

Terribly tempted just to do nothing. Tomorrow morning it's meant to be minus 4 degrees outside. If it still starts when it's that cold then there's no way I'm shelling out for a new alternator!

D.

[Trevor]

That sounds fair enough...minus 4, that's extremely cold....we're only at 1 degree and that feels bad enough