Either something is wired around the shunt so its current is never counted, or the monitor has not seen a full charge recently enough to re-synchronise, or its capacity and chemistry settings do not match the bank. All three produce a plausible number that drifts.
A monitor that counts correctly and reports wrongly is the most common complaint on this shelf, and the cause is nearly always outside the instrument.
The wiring cause
Every load and every charging source must pass through the shunt. One cable left on the battery terminal — a chassis bond, an alarm, a factory accessory — makes its current invisible, and the error grows every day in one direction.
The synchronisation cause
The monitor counts amp-hours in and out and needs a reference point to reset against. It takes that from a detected full charge, defined by voltage, tail current and duration. A system that is never charged fully, or whose criteria are set unrealistically, drifts without ever correcting.
The configuration cause
Capacity, chemistry, charge efficiency and the full-charge criteria are all entered by hand at installation. Defaults from another battery produce a percentage that is internally consistent and unrelated to the bank fitted.
The order to check them in
Compare against a clamp meter first, because that separates counting from reference in one measurement. Then look at whether a full charge has been detected recently. Then look at the settings — and change one at a time, because two changes make the next reading uninterpretable.
What people ask before they order
How do I know it is a wiring fault rather than drift?
A clamp meter on the main negative, compared with what the monitor reports at the same instant. A steady discrepancy means current is bypassing the shunt; agreement means the counting is fine and the reference is not.
What counts as a full charge for a monitor?
A combination of voltage and tail current held for a set time, all three of which are configurable. If those criteria are never met, the monitor never resets and the error accumulates indefinitely.
Last reviewed 16 September 2026