Monitoring Battery Level

[GreenEE]

I need to have my micro-controller monitor the battery level. The CPU runs off of a regulated source, but battery is about 1.5 volts above the regulated voltage and can not be fed directly into the ADC. I do not want to use a simple voltage divider because it would always be drawing power. I have only one general purpose pin available, but I might be able to open up one more if really needed. Suggestions?

 

[itsmoked]

A voltage divider fed thru a FET you turn on to read.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[RajeevCell]

Use the "simple voltage divider", but add an analog switch in series. And then close the switch only when you want to measure the battery level with your ADC. Once in every 10 minutes should do.

There are many battery monitor chips available. What type of battery are you using?

 

[GreenEE]

RajeevCell,

3 AA. The uC runs off of a 2.7V regulator.

 

[VEBill]

I wonder if (using diodes and a capacitor) you could use just one pin as an output to control the analog switch or FET, and the same pin as an input to read the analog value.

Another approach would be to use very high value resistors in the voltage divider to minimize leakage, but have a little capacitor in parallel with the sample to give you enough voltage for enough time to measure into your lower impedance input.

Another approach would be a simple serial resistor to divide the voltage with the input Z. And then use software to calibrate it back into reasonable accuracy.

 

[jimkirk]

Depending on your processor you can tie one end of your voltage divider to a GPIO pin, the other end to your battery +, and the center node to your ADC input. Normally set the GPIO to be an input. Make sure the input voltage range is 5 volt compliant. This should draw next to no current from your battery.

If it's not 5 volt compliant, you might end up drawing some small current through the divider resistors and the input protection diode. (It won't cause any damage since the voltage divider resistors will limit the current.) If that's the case you might want to drive an external N channel MOSFET to enable the divider.

Set the GPIO pin to a LOW output to read the divider down battery voltage.

Some processors have this sort of function, with an internal voltage divider built in. Check the documentation.