How to Install a Boat Battery Monitor and Shunt

Equipment and information required

Choose shunt-based, not voltage-based
LiFePO4 sits at a nearly flat voltage across most of its usable range, so a voltage-reading gauge cannot tell 70% from 30%. Only a shunt counting amp-hours in and out gives a usable state of charge.
Fit the shunt in the negative, at the bank
Battery negative to one side of the shunt, everything else to the other. The shunt has to see every amp that leaves or enters the bank.
Move every stray negative to the load side
This is the step that goes wrong. Any negative still landed on the battery side of the shunt — a bilge pump, a stereo memory, a solar controller — is current the monitor never counts, and the reading drifts steadily wrong.
Set the monitor's battery parameters
Enter the bank's actual capacity and the charged-voltage and tail-current thresholds from the pack's specification. Defaults are usually lead-acid figures and will never register a full charge.
Calibrate at a known full charge
Charge the bank until current tapers to the manufacturer's tail figure, then synchronise the monitor to 100%. Without this the monitor is counting from a guess.
Re-synchronise occasionally
Amp-hour counting accumulates small errors. A full charge every few weeks re-anchors it, and a monitor that never reaches 100% is usually telling you the charging is not finishing rather than that it is broken.

Leaving one negative on the battery side of the shunt.

Everything except the battery goes on the load side, with no exceptions.

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