State of charge of Lithium Ion batteries at low discharge current.

[joseph.anand]

Hi I am working on a project that is powered from a 3cell (11.1v, 2000mAh) lithium ion battery pack. The max discharge current is less than 80mA, and typically around 50mA. I am unsure of the voltage levels to use for measuring the state of charge at this discharge rate. I have seen a 3.5-3.58v for 0% at 1C discharge rate. The only option for me seems to be to use a constant current source and discharge the battery to get a measurement of the discharge profile.

ps: My ADC has a usable resolution of 8bits which is not very good.

[samgab]

They'd be fully discharged at about 2.7V per cell, so 8.1V. But for longevity of the cells, it's better not to discharge below 3V per cell, so 9V.
There isn't much capacity left once you get to 3V per cell anyway, so you aren't losing much in the way of cycle duration.

[IanB]

In most cases, brand name lithium ion batteries (Panasonic, Sanyo, Sony, etc) have data sheets from the manufacturer that give actual discharge curves at different rates. All though discharge voltages are generally similar and can be estimated as samgab indicated, they do vary a bit and there is no substitute for actual manufacturer data.