In order to measure the amp hour capacity of each battery set (I have my batteries in groups of parallel sets), I used a low cost power meter from Amazon. The maximum current through each of these meters in my setup is 10amp as a worst case design load. In practice the current is less than 3amps per meter.I have parallel sets batteries per meter to allow a low cost metering solution versus a meter per battery. I have saved on cable costs and ancillary hardware (nuts, washers, bolts, lugs). Generally if the amp hour capacity of a battery bank drops below 60% of the original value then it's time to replace. The energy meters per battery sets will provide such monitoring.