An amp-hour meter is similar in concept to a water meter. A water meter determines the amount of water used by calculating the flow rate of the water through a pipe and then multiplying the flow rate by the amount of time that the water flows through the pipe. An amp-hour calculates the amount of electrical charge used by multiplying the flow rate of electrons (the current in amps) by the amount of time electrons flow through the wire.
Amp-hour meters often just consist of two probes. Current flows into the amp-hour meter on its input probe and flows out of the amp-hour meter on its output probe. An amp-hour meter often has a digital display. When current flows through the amp-hour meter, the numbers displayed continually increase. And that's because an amp-hour meter measures current continually and in the process continually sums the current that has passed through it.
An amp-hour meter sums the amount of electrical current that flows into an electronic device over a given time. It does this by calculating the average current over a period of time and then multiplying average current by the time elapsed. It displays the amount in units of amp-hours. If an electronic devices requires, on average, 2 amps of current to operate and the device is connected to an amp-hour meter, the amp-hour meter will display 0.5 amp-hours after the device has been operated for 15 minutes, because 2 Amps multiplied by 0.25 hours is 0.5 amp-hours. After one-half hour it will display 1 amp-hour.
Different types of amp-hour meters are needed to measure different types of electrical currents. Amp-hour meters can be designed to measure direct current, the type of current found in battery powered circuits; alternating current, the type of current generated from electric wall sockets; or pulsed currents, the type of current found in data communications wires. More expensive amp-hour meters can measure the amp-hours of all three types of current.
Designs of amp-hour meters vary. However most are based on the design of an ammeter, an electronic instrument used to measure current. An ammeter can be used to make a simple amp-hour meter. Simply read the current measurement of the ammeter. Then multiply the amount of time (in hours) it is at the level of current. Every time the current reading changes, start measuring the time again. For each change in current level you should have a time in hours that corresponds to the time the current was at the specific level. Multiply the current and time together for each current level. Then add the sum of these calculations together. This is the number of amp-hours.
An actual amp-hour meter automates the latter process. It incorporates timing, multiplication and summing circuits in addition to an ammeter circuit.