In the negative line, between the battery’s negative terminal and everything else, so that every amp entering or leaving the bank passes through it. Anything connected on the battery side of the shunt is invisible to the monitor and skews every later reading.
A shunt monitor measures well and is trivially easy to install in a way that guarantees wrong answers for years.
What the shunt is
A precisely known low resistance in the current path. The monitor measures the tiny voltage across it and calculates the current from it, then integrates over time to count energy in and out.
The one rule
Every cable that used to go to the battery negative goes to the load side of the shunt instead. Every charging source too — the solar controller, the DC-DC charger, the mains charger. The only thing on the battery side is the battery.
What gets forgotten
The one connection that was already there: a chassis earth, an alarm, a factory-fitted accessory. On a vehicle this is where the error usually enters, and it enters silently.
Setting it up honestly
The monitor has to be told the bank’s capacity, its chemistry and the criteria that count as a full charge. Those settings are what make its percentage correspond to something real, and every maker documents them. A monitor installed correctly and configured with defaults is still guessing.
What people ask before they order
Why does one forgotten connection matter so much?
Because the monitor counts what it sees. A load or a charging source wired around the shunt accumulates an error that never corrects itself, and the reported state of charge drifts further from reality every day.
Why does a monitor need a full charge occasionally?
To re-synchronise. Counting current accumulates small errors, and the monitor uses a detected full charge to reset its reference. Systems that never reach full drift.
Last reviewed 16 September 2026