Hello fellow electronics enthusiasts,

I am trying to use an arduino to monitor voltages of a 6 cell lithium ion battery in series as it charges. My set up is simple. Each terminal of each cell is connected to an analog input pic via a resistor voltage divider. Here's the thing, when the charger is not connected, the voltages are read fine by the arduino as confirmed by a dmm. When the charger I connected the voltage measurements become inaccurate by about a half volt. The dmm reads about 3.9 volts across each cell whereas the arduino reads something like 4.5 volts across each cell. The way I connect the charger is that the negative terminal connects to the battery and the micro controller ground and power goes to positive terminal of the battery. What could be going wrong here? All help and information is appreciated!

Regards,
Khan

 

This is the charger btw

 

Check that the ADC reference voltage does not change when the charger is connected. To get further help you need to post a FULL schematic .

Les.

 

You should have the charger connected to negative of the battery and a separate wire from the negative of the battery to the arduino ground. This ensures that the arduino is not measuring voltage drop across the charger wire.

There may also be a problem if the the charger output and the arduino power supply are both grounded - it's OK if only only one or neither are grounded.

 

Check that the ADC reference voltage does not change when the charger is connected. To get further help you need to post a FULL schematic .


Les.

Hello Les,
Thanks for the input
I'm using the internal reference and you're right. The value of the reference does change a bit, but not enough to explain the half volt inaccuracies. Any other suggestions?

You should have the charger connected to negative of the battery and a separate wire from the negative of the battery to the arduino ground. This ensures that the arduino is not measuring voltage drop across the charger wire.

There may also be a problem if the the charger output and the arduino power supply are both grounded - it's OK if only only one or neither are grounded.

This maybe true. I'll try it and get back to you.

 

As I have said we need a schematic of how it is all connected together.

Les.