Hybrid coulomb-counting fuel gauges measure both battery voltage and battery current to estimate percent State-of-Charge (SOC). The effect of current sensing measurement errors on fuel gauging error depends on the type of measurement error. As an example, measurement errors can be modeled using a formula such as:
I_measured = (1+Gain_error)*I_actual + Offset_error
As an example, take the case where Gain_error = 0.1 and Offset_error = 0; this implies a system with 10% gain error and zero offset. The MAX17xxx family of fuel gauges report the SOC as percentage of total capacity, and one way they measure total capacity is by monitoring the total charging current it takes to move the battery from 0% charged to 100% charged. For example, if a battery needs an actual charge of 1A for 1 hour to go from 0% to 100%, this would imply an actual capacity of 1A-hr. A measurement Gain_error of 0.1 would make the fuel gauge measure that it took 1.1A for 1 hour to charge, so it would measure 1.1 A-hr of capacity. However, since the discharging current would have the same gain error, it would also look like it discharged 1.1A for 1 hour to go from 100% to 0%, and also have 1.1A-hr capacity. The 50% SOC would still happen after 30 minutes.
For the case where Gain_error = 0, and Offset_error = 0.1A, an actual current of 0A would be measured as 0.1A. If a 1A-hr battery was charged at 100% and had an actual current of 0A for 1 hour, the fuel gauge would measure 0.1A being discharged for 1 hour. In this case, the fuel gauge would estimate SOC to be 90%, which would be a 10% error. The error would increase by another 10% after another hour. In actuality, the ModelGauge algorithm will bound this error because it also uses battery voltage information, but this type of offset error should be minimized by careful schematic and PCB design. Typical causes of offset error are improper current sense resistor layout that has additional currents that do not flow into or out of the battery flowing through the sense resistor, and improper Kelvin-sensing of the current sense resistor that can appear as additional error current not actually flowing through it.