Estimating Long Term Charge Error From Current Sensor Offset

A small current measurement offset can accumulate into a noticeable charge error over a long idle period. A fuel gauge that looks accurate during a short high current test may therefore drift during weeks of low power operation. Evaluate the time integral of the error, not only its percentage of full scale.

Separate offset and gain errors

Identify the current sensing path, shunt value, amplifier or ADC offset, and any calibration applied by the device. The TI article on battery current measurement explains how an offset behaves like a fictitious current that accumulates charge error and describes board offset calibration in its battery monitor example.

Gain error scales with actual current, while offset can matter even when true current is near zero. Use separate terms in the budget so an excellent full scale accuracy number does not conceal the idle error.

Calculate the time dependence

Charge error from a constant current offset is offset multiplied by elapsed time. A hypothetical 100 microamp offset integrated for 24 hours corresponds to 2.4 mAh. Over a much longer interval, that contribution grows unless the system’s measurement or estimation method corrects it.

Compare the error with the intended battery capacity and reporting requirement. Include temperature dependence and operating state rather than assuming a single room temperature offset remains constant for the product lifetime.

Establish a real zero current condition

Follow the selected device’s calibration procedure and verify that the defined current path actually carries no load during the offset measurement. Debug connections, pullups, and auxiliary circuits can create current that is mistaken for offset.

Identify loads that bypass the shunt. A correctly calibrated counter cannot integrate current it never measures. Draw the complete battery current path, including standby supplies and protection electronics, and account for those loads according to the gauge architecture.

Verify low power behavior over time

Compare accumulated charge with an independent suitable reference during representative active and idle periods. Use an observation interval long enough to reveal the offset contribution. Record temperature, sleep states, calibration identity, and any automatic correction events.

Do not assume that charge accumulation alone defines state of charge; battery models and other measurements may also contribute. Save raw current and accumulated charge alongside the reported estimate. The review should demonstrate that the current measurement supports the intended long duration behavior and identify residual error sources rather than relying only on a short discharge test at a convenient high current.