fROM THE INTERNET:
Wiring for Ammeter or Voltmeter
A short description of how to wire in an ammeter or voltmeterfor your car.
The ammeter shows direction and rate of current to and from thebattery. Under normal conditions, the meter should show a (small) charge. This will be higher after starting, but probably never more than 10A, and often about 1A. If you see a discharge reading (other than maybe at tickover) you're running on borrowed time.
The voltmeter also indicates whether the battery is being charged. A reading of ~12V indicates that there is no current flowing into or out of the battery. Less than 12V indicates discharge (typically 8V when cranking the starter) and 13-14V is a healthy charging voltage. More than 15V when charging indicates a faulty regulator on the alternator.
A voltmeter actually shows more information than the ammeter, and is also safer to wire insince it takes no current and this allows the use of a small (1A or less) fuse. The ammeter needs to take the full normal load. This is 30A or so for a Seven, but add a few extra lights etc. andit could be more.
Here's an artistic pic. of how both could be wired in.
