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Battery Monitor
battery monitor, state of charge monitor, SOC meter, battery shunt, coulometer, Victron BMV-712, Renogy battery monitor, amp-hour meter, battery percentage gauge, 500A shunt
A battery monitor measures the state of charge (SoC) of a leisure battery by tracking current in and out via a shunt resistor (typically 500 A / 50 mV). It shows remaining capacity in amp-hours or percentage, voltage, current draw and estimated time to discharge — far more accurate than voltage alone.
Shunt-based monitors (Victron BMV-712, Renogy BT-2) are the most accurate. The shunt is wired into the main negative cable; the monitor display or app reads the data. The monitor must be configured with the correct battery capacity and type (AGM, LiFePO4, gel). Victron BMV-712 includes Bluetooth and feeds into the VictronConnect app.
FAQ
Why can't I just use a voltmeter to check battery level?
Voltage is a poor indicator of SoC: lead-acid voltage varies with temperature and load; LiFePO4 voltage is nearly flat across 90 % of the capacity range (13.1–13.3 V). A shunt-based monitor counts every amp-hour in and out, giving a reliable percentage regardless of voltage behaviour.
Where exactly is the shunt wired?
The shunt goes in the main negative cable, between the battery negative terminal and the negative busbar. All negative cables in the system must connect to the load side of the shunt — not directly to the battery — otherwise current bypasses the shunt and readings will be wrong.