I'm having charging problems with my CX_5 diesel, battery volts should be showing i believe around 12.6 volts when stood overnight, but i have never seen that, currently its showing around 12.4 volts.

But on two occasions the volts have been down to 12.1 volts, shortly afterwards the car failed to turn over.

I am monitoring the volts with a plug in voltmeter, which does read slightly low compared to my fluke meter, but its near enough to monitor what's going into the battery.

Sometimes the charging volts appear to be what i would expect, other times not, too low. For example immediately after starting i would expect around 14 volts plus, sometimes it never goes above 12.9 volts.

So the fault isn't consistent. Really it would help if i had some criteria to indicate what sort of charge volts to expect.

The volts are always high on the overrun, but not on acceleration, if i put the lights on volts usually also increase.

Any help would be appreciated, mr google doesn't seem to help.