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ADC Pi Plus and battery power monitoring

954 Views - Created 16/03/2017

16/03/2017

Posted by:
wazabi

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Hello,

I just acquired an ADC Pi Plus for air quality measurement and was wondering whether I can use the same ADC Pi Plus to monitor the voltage coming out from the battery powering the Raspberry (my goal being to shut down the pi properly when the battery level is getting low).

Is that possible?

Given that my battery provides power up to 5.6V, I assume I need a voltage reducer between the battery and the ADC Pi Plus?  Would you know of a reducer that I could use?  Ideally, I would like to avoid using a breadboard as I am trying to build an integrated solution.

The battery output is USB.

Thanks!

 

16/03/2017

Posted by:
andrew

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Location:
United Kingdom

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Hello

You can use the ADC Pi Plus to measure the battery voltage.  The inputs on the ADC channels have a voltage divider to convert the 5V input down to 2.048V which is the highest value the ADC chip can measure.  To measure higher voltages you just need to add an extra resistor in series with the input which will increase the resistance on the input side of the divider and therefore increase the voltage it can measure.

We have a calculator for using the ADC Pi Plus with higher voltages.  You can enter the voltage you want to measure and it will tell you what size resistor you need and the multiplier value you will need when measuring the voltage using our software libraries.

In your case to measure a 5.6V input you will want an maximum input voltage of 6V to give you a bit of overhead.  You will need to add a 3.16K resistor in series with the input which will give you a maximum voltage of 6.011V.  When you read the voltage from the ADC you will need to multiply the value that is returned by 1.1881 to give you the correct voltage.

AB Electronics UK Tech Support

16/03/2017

Posted by:
wazabi

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Thanks, Andrew.  Excellent.  Two questions:

1. Will this result in some power loss on the battery?

2. Do I need to connect the battery ground somewhere on the ADC Pi Plus -- or do I only need to connect the + to one of the ADC Pi Plus pins?

 

16/03/2017

Posted by:
andrew

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Location:
United Kingdom

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There will be a small amount of power lost through the resistor divider circuit but it will only be about 0.25mA so almost nothing compared to the amount of power used by the Raspberry Pi.

The battery ground should be connected to the ADC Pi Plus through the Raspberry Pi but you may get a more accurate reading by adding an extra ground wire between the battery and the GND pin on the ADC Pi Plus.

AB Electronics UK Tech Support

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