Using Vbat as ADC for battery level monitoring

[jacinta sartor]

Posted on July 27, 2017 at 05:18

I'm updating an old PCB design that used the STM32L432KB. The new design is using the STM32L452CE which has the Vbat pin.

In the old design I used a voltage divider across the battery and connected it to an ADC pin to periodically check the battery level.

In the new design instead of using a voltage divider to an ADC pin can I use a connection from the positive terminal of the battery straight to the Vbat pin?

I don't wish to use Vbat as a backup power supply or any of the other feature, just as an ADC input since the board is to be powered with the battery only. 

#adc #vbat #battery-life #battery

[pksn]

Posted on July 27, 2017 at 06:05

In the new design instead of using a voltage divider to an ADC pin can I use a connection from the positive terminal of the battery straight to the Vbat pin?

Yes. Possible.

Below is the image snapshot from

http://www.st.com/content/ccc/resource/technical/document/reference_manual/group0/b0/ac/3e/8f/6d/21/47/af/DM00151940/files/DM00151940.pdf/jcr:content/translations/en.DM00151940.pdf

of the MCU.

Couple of points:

1. Enable the bridge (CH18SEL) only during measurement. else it will drain battery continuously.
2. The read analog value represents will be one third of the actual battery voltage.

[S.Ma]

Posted on July 27, 2017 at 08:33

Check the normal operating input voltage of vbat to make sure weather the battery can be direct connect to vbat pin or not. As i recall, the adc will have an inyernal divider to scale the voltage within mid range of the adc. It has a power switch to reduce leakage. Using vref adc channel, the accurate vbat voltage can be calculated properly.

If the vbat is 5v, it would probably require an external divider...

[S.Ma]

Posted on July 28, 2017 at 08:33

If the battery is 4.6V, the vbat gpio maynot be 5v tolerant, hence make sure not to beleive 5/3 will solve it...