If you don't want to spend $100 plus on a Victron battery monitor try the Elite DCM120100C Multi-function Panel Meter. This inexpensive Coulometer measures both forward (Positive) and reverse (Negative) current flows as well as voltage, amp-hours and percent of charge. I bought one but unfortunately couldn't find a good place to install it in my van. Consequently it is for sale and includes 15 feet of 4-conductor wire. For details see Multi-function Panel Meter Model Elite DCM120100C
. I'm asking $30 for the meter and wire (free shipping to contiguous U.S.). New, open-box.