Amp hour meters are used to give detailed information about a battery bank including voltage, amps going in or out, and charge remaining. Most also save historical information which is extremely useful for troubleshooting.
The Victron meters shown here can be hooked up to a computer or to a GSM link for remote monitoring. The Blue Sea meter shown here monitors AC current and tank capacities in addition to battery capacity. The Balmar Smartgauge uses a totally novel way to measure battery capacity
For more information see our detailed article on Monitoring a Battery State of Charge