You might try a cheap coulombcounter instead.
Battery Monitor Hall Sensor Coulomb Tester DC 9~100v 100A/400A Digital Voltmeter Ammeter Battery Capacity Power Electricity AH Voltage Meter Coulometer for RV,Car,Golf Cart,Boat Amazon.com: Battery Monitor Hall Sensor Coulomb Tester DC 9~100v 100A/400A Digital Voltmeter Ammeter Battery Capacity Power Electricity AH Voltage Meter Coulometer for RV,Car,Golf Cart,Boat : Automotive