What should the voltage be when the car is running?
My voltage gauge (just installed) reads 14V
when ignition is off, it reads 12V

 

You are seeing normal operation!

When the weather is considerably colder, the voltage might run a bit higher, 14.5 or a bit more. This function is built into voltage regulators to help charge colder batteries, which need a bit more of a kick to maintain proper charge.

 

I don't have the MINI voltage gauge, but I have a Pioneer AVIC-N3 headunit which has too many bells and whistles. One of these "whistles" is an onscreen voltage gauge. My readings are usually around 12.7. Is this too low? Please note, this is with the engine on.

 

Different points where you test, different guages will affect readings. Alternators, regulators, batteries will vary some. Temperature will affect the reading. There's a range of acceptable.

The radio, for instance, is measuring after wiring and fuses, and other voltage drops. I have examples of perfectly operating stuff running a 1V drop from the battery or regulator through switches fuses and wires to lights and other high current draw stuff. Also, at idle the alternator might not be at full voltage. Does it go up a bit when you're driving or running the motor a bit more?

What I've always done is figure out "normal" for my car. If something goes wrong you know because it differs from normal.

If you take a good voltmeter directly at the battery with no load, fully charged, it'll be somewhere around 12.6 or 12.7. It's pretty discharged around 12V.

When the car is on (light load, charging, so maybe above idle) you should see something from 13-14.5V measured at the battery.

So with the gauge-I'd verify with a good voltmeter at the battery to see if the readings are the same. When you say ignition off-is that with key on? You'll still be getting some draw on the battery. Your guage doesn't read with the key totally off does it? Then it'll be drawing current and discharging the battery even when off-not real good.

If the voltage is really 12V with the key off (not just not running but key on so the guage reads), then I'd think that's a bit low. Is it turning over ok? I'd check connections, dirty cables, make sure it's getting charging voltage. I'd also find a friend with a voltmeter, or drop by an AutoZone or somewhere. A lot of places will check your battery for free. At least then you'd have more of a baseline for "normal" on your guage.

The 14V says your alternator is running ok.

 

Thanks for the responses!
I only asked because the install instructions from MINI were pretty poor. The install instructions differed from the wiring diagram, and let me tell you removing a splice adapter 3 times and splicing another wire, is not fun!
I get 14V at idle, and running. a little above 12V with the key in turned to accessory position.
So i guess I'm okay.