completelycharged said:Issue with typical joule / coulomb approach is it's your battery cycle efficiency that limits the accuracy unless you reset the counter each charge cycle [...]
Li-ion has very high coulombic efficiency (typically over 99.5%) so that's not really the worst problem with coulomb counting fuel gauges. Rather, it's correcting for losses not measurable by the gauge, e.g. self-discharge (esp. if it's idle for long periods) and also losses due to loads too small to be measured by the gauge (e.g. BMS quiescent current), which may require a full learning cycle to recalibrate (in the simplest gauges), and such full cycles may occur only rarely (which can be alleviated to some degree by using various methods of voltage modelling). See this Barsukov bookfor much more on fuel gauges.