Although incredibly useful, a multimeter’s voltage display only provides a snapshot of what’s happening with your batteries. While voltage does correspond to state of charge, it’s a moving target. The ebb and flow of current into a battery affects battery voltage greatly. So unless a battery has been “at rest” without any charge or discharge for one to two hours (a rare occurrence, typically only accomplished by intentionally shutting off all power sources and loads), voltage will not accurately reflect SOC. A battery monitor, however, literally counts energy (amp-hours) in and out, providing a more reliable SOC estimate.