maemo.org - Talk - View Single Post - estimate battery wear

shadowjk	2012-01-15 , 20:18
Posts: 1,258 \| Thanked: 672 times \| Joined on Mar 2009	#5

Originally Posted by blue_led

Suppose if you have an battery charged to 3.7 V and this battery have Rint = 1 ohm. trying
to charge with 500 mA will fail because voltage drop on internal resistance will be 0.5V and
across batt conections will be 4.2 V and charger will say "enough"

Actually it will reduce current to less than 500mA, in order to not exceed 4.2Volts. bme programs the charger to stop when the current has been reduced to 50mA. This never happens after PR1.3, as the USB subsystem consumes 60-80mA itself and is active whenever a charger is connected.

The script is not finished! I can't find any info about r_sense resistor influence for
bq27200 voltage readings.

bq27200 measures battery voltage through a 10kOhm resistor connected to battery bus.

In addition to battery internal resistance, there's also about 120-130 mOhm resistance between the charging chip and the battery, which adds resistance when charging. The charging chip measures voltage before this resistance, so the voltage that arrives at the battery is lower than 4.2Volt. It makes charging a bit slower, and the final charge voltage will depend on system load.

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