Battery Monitoring

Hi all i need help with measuring the capacity of a Ni-Mh battery in a project, i do not know which voltage battery exactly that it will be (They use different sets from a range of (7.4v - 9.6v) the cell voltages are 1.2 volts and discharge to around 1 volt completely discharged this so i have read isn't exactly linear.

If i understand this correctly i could have the voltage range detected during power on as such:

9.6 - 8 = 8 Cell

8.4 - 7 = 7 Cell

7.4 - 6 = 6 Cell

having now known the type of battery fitted i could use a voltage divider using 2 50 ohm resistors my maximum would be 4.8 volts and minimum of 3 volts which i could measure using a analog input and convert to a battery level value.

I realize i may be understanding this all wrong so feel free to educate me i do not need to know the exact capacity more when it is getting low and needs changing to alert the user but il await your comments.

Thanks in advance, Chris.

Simply, you can’t differentiate discharged 8-cells (8V) from fully charged 6-cells (1.4 x 6 = 8.4V). Voltage ranges overlapped.

This is a good point, durpppp i will have to add a dip voltage selector switch into the design for the microcontroller.

keep in mid also that commercial battery testers apply some sort of load to the cells when they are testing them,

Your question is a bit confusing, but if I read you right, you want to provide a "Battery Low" warning to the User, when the batteries are nearly drained.

The way I usually do this is to just monitor the battery voltage when the thing is on ["thing" being, whatever it is you are powering with those batteries]. And, if the battery voltage will be greater than the supply voltage on the MCU you are using to monitor the batteries [you didn't mention what you were going to use to do this monitoring], then, yes, you'll need to use a voltage divider. I usually use resistance values in the hundreds of k, so as to not drain battery with my monitoring circuit. Also, use a 100nF capacitor on the MCU [input] pin, to reduce the impedance, so the sample/hold circuitry [in the MCU] can get a stable sample.

I typically define 1.1V per cell as "drained", but consult the manufacturer's specs for the battery you are using.