Li Ion battery remaining percentage detection

[tiwari.sachin]

I want to test the remaining percentage (capacity) of battery. How can i check it.

Only reading voltage?? I dont think it gives accurate results.

Checking current drawn at that voltage... Well its tricky as I am also using auxillary power.

Any hints or any tutorials, document links???

I tried to google but couldn't find proper details. Maybe I am just not entering right keywords

 

[chuckey]

First of all you must find a data sheet on your battery. With some luck there might be graphs of voltage versus capacity. As you say not very accurate. If the internal resistance is quoted , you could pulse on a high current load and measure how much the voltage falls. You could actually calibrate this test meter your self. Pulse a current of 1c and see how much the voltage drops. Put a load on at c/10 for an hour, pulse the current again, put the c/10 load on and repeat until you get to the end voltage. This test will take about 10 hours. Now you have a graph relating the dv/Ip or the internal resistance of your sort of battery. So if you applied your test to an unknown battery you can see where on the graph the battery sits.
Frank

 

[tiwari.sachin]

Frank... thanks... Let me check it...

 

[FvM]

Battery management systems in a notebook computer track the actual charge balance by integrating the battery current.

Capacity estimation from battery voltage needs at least current and battery temperature as additional information.

 

[tiwari.sachin]

I do have a shunt that can give me current consumption at battery side.

Also have a thermistor to track temperature and ofcourse voltage

I am not sure about checking the capacity using these 3 parameters.


I tried detecting current for 10ms and then calculating for a hour (assumption) and later substracting by total current values and things like that but I dont see it being accurate as the load consumption will be varying and not constant

 

[FvM]

I am not sure about checking the capacity using these 3 parameters.

Battery datasheets give you an idea how they are related. There are usually discharge voltage curves for different temperatures and currents provided.

Varying current measurements can be filtered by an analog low-pass before being digitally processed.

 

[Warpspeed]

There are all sorts of different ideas about how to go about all this, but the most appealing strategy I have heard of so far, seems to be monitoring current flow in and out of the battery.

The strategy is to call exactly 3.200 measured volts 50% charge by forcing the "charge available" display to 50% at that exact voltage when the system is not working very hard.
Any current flowing into the battery increments from there, and any discharge current decrements. There needs to be a calibration figure which takes into account the rated ampere hour capacity of the battery.

Its not perfect by any means, but by forcing the readout to 50% right in the middle of the charge/discharge curve when there is only very low current flow, it roughly tracks what is going on without cumulative error over long periods and many mixed charge/discharge cycles.