Coloumb counter: "gas gauge" decay

Werner Almesberger werner at
Wed May 13 02:44:05 CEST 2009

Joerg mentioned that not fully discharging the battery in a typical
charge-discharge cycle causes the Coloumb counter to underestimate
the charge stored in the battery, with the effect being cumulative.

I checked the bq27000 data sheet and couldn't find any documented
behaviour that would explain this observation.

However, I noticed that the bq27000 also keeps track of whether its
estimate of the effective battery capacity is correct, and indicates
this in the flag FLAGS.CI.

See also sections "Gas Gauge Operation" (page 10 of the bq27000
manual) and "Status Flag Register (FLAGS) -- Address 0x0A" (page 14).

This flag is not used anywhere in drivers/power/bq27000_battery.c
nor is it or the register containing it exposed to user space.

So an apparent decay of the estimated battery capacity may just be
caused by using the capacity value (or anything derived from it)
despite it being invalid.

- Werner

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