A short description of how to wire in an ammeter or voltmeter for your car from someone who understands the principles.
The ammeter shows direction and rate of current to and from the battery. 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 on an older car) 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. Charging at too high a voltage will cause a standard lead-acid battery to loose water, it will have a more serious effect on more specialised cells and may significantly shortne the life of the battery.
A voltmeter actually shows more information than the ammeter, and is also safer to wire in since 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. This means that in the case of a fault, the wiring will be at risk of melting if the fuse does not blow quickly. Depending on the fault, a current of 50 A might flow for a few seconds, enough to start a fire or cause a serious burn.
Here's an artistic pic. of how both could be wired in.
A safer form of ammeter is one which uses a shunt load in the main wiring loom. This is in effect a remote sensor and it generates a small voltage which can be routed to a read-out using small gauge wire protected by suitable fuses. If you want to build an ammeter like this, look for a sensitive current meter (about 30mA full scale) and calculate the resistance required to place in parallel with the meter. This will be aproximately 1/1000 the resistance of the meter. Check the voltage drop that this will introduce, and the power dissipated in your shunt load - it needs to be less than a tenth of an ohm (that would give a 3volt drop, and generate 90W of heat!)