There are two distinct types of battery gauges: a voltmeter that measures the battery's system voltage and an amp meter that registers the charging system's amperage. Both meters are useful devices to monitor any electrical system that uses a battery. You can install both types of gauges in an afternoon.
Remove both battery cables, positive and negative, from the battery before beginning the installation of the gauges. Install the gauges in the location of your choice for the system. The manufacturer of the gauges may have specific orientation requirements for the gauges--for example, using only their style of bracket or mounting the meter to a metal component for specific grounding purposes.
Wire the voltmeter first as part of the dual meter installation. Use at least an 18-gauge wire or greater. The voltmeter should be installed in parallel with any "on" or "off" switch that you have for the electrical system. For example, if you have a master switch or key to turn the system on, wire the positive side of the voltmeter to the control side of the switch. Do not run the wire from the battery side (feed wire) of the switch, since that will drain the battery when the system is off.
Connect the negative side of the voltmeter using a separate piece of the 18-gauge wire and run to the battery post connection. The type of connectors will vary on the individual installation.
Find the positive wire of the charging system. This can be either an alternator or a separate type of specific charger. In either case, use the wire cutters to cut the wire that is running from the charger to the positive side of the battery. Use the wire strippers to remove approximately 3/4 inch of the insulation.
Cut the 8- or 10-gauge wire to a sufficient length to run from the cut charging wires to the connection posts on the rear of the ammeter. Strip both ends of these wires back the same amount at 3/4 inch. Connect the two wires to the rear of the ammeter using the correct type of connectors.
Twist the separate pairs of the wires together, so one side of the meter is connected to the wire from the charger and the other side of the meter goes to the battery. The wiring for the ammeter should go through the meter to make a series connection. Place a wire nut over each bare twisted wire and use the electrical tape to make a sealed connection.
Connect the battery back to the main cables and test the system by switching it on and observing the meters.
Some ammeters may be labelled "charger side" and "battery side" on the rear side. Not all ammeters are polarity specific. Place the ammeter only through the charger connection since most ammeters are not designed for amperages greater than 30 amperes.