How to Test Lithium Boat Battery Capacity in Amp-Hours

Equipment and information required

Charge to a genuine full
Charge until current tapers to the manufacturer's tail figure and the monitor synchronises at 100%. A test that starts from an assumed full charge measures nothing useful.
Apply a steady, known load
Something that draws a consistent current — a resistive heater or a fixed inverter load works. A varying load makes the result hard to compare against the rating.
Keep the discharge rate reasonable
Rated capacity is usually quoted at a modest discharge rate. Testing at a much higher current will return a lower figure through no fault of the pack, so note the rate you used.
Count amp-hours to the cut-off
Record the amp-hours the shunt counts until the BMS opens or the pack reaches its specified cut-off voltage. That total is the usable capacity you actually have.
Compare against the label, honestly
Expect somewhat less than the label, especially on a warm day and at a higher discharge rate. A pack well below its rating, tested properly, is evidence worth taking to the supplier.
Note the temperature
Capacity varies with temperature. A test result without the temperature recorded is not comparable to the datasheet or to a retest later.

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