VOLTMETERS show electrical pressure, to verify when batteries are fully charged or are very discharged, but they are less precise at indicating mid-levels of charge.
AMP (short for amperes) METERS show current flowing at the present time. Amperes is a rate of flow, like gallons per minute. Turn on a light and 2 amps begin to flow. Turn on a second and the flow increases to 4 amps so long as both are on. We use a digital ammeter to show total amperes being used. If it reads higher than expected, we look for lights or appliances accidentally left on.
AMP – HOURS is the total energy stored (or used) over a period of time, just like total gallons of gasoline available in the tank (or used on a trip). Amp-Hours is figured by the number of amps flowing multiplied by the number of hours they flow. A light using 2 amps, lit for 6 hours, uses 12 amp hours. If lit 30 minutes, it uses 1 amp hour.
A BATTERY NET AMP-HOUR METER starts at a full battery and counts amp hours passing into and out of the battery. It keeps track of the running total so you know how much charge is left in the battery. This is the closest thing to a fuel gauge yet invented for batteries. Batteries, like leaky gas tanks, loose a little power internally, where the meter can’t measure it. To compensate, Battery Amp-Hour meters “fudge” the count lower 10 to 15% to match the actual battery condition. These meters must be carefully set up to match your battery. (See Tri-Metric meter)
METER SHUNTS are calibrated brass blocks that measure very large currents, up to 500 amperes, and send a small signal to a digital Ammeter or Amp-Hour meter. The signal can be sent on small wires, so the meter may be located some distance from the shunt, often in the living quarters for more convenient checking. All current going in or out of the battery passes through the shunt and is counted.