# Measurement of Multiple Series Stacked Battery Cells

An old design idea that I posted in response to some discussion over at the Sparkfun forums, come on in and check it out!

This design idea came from some discussion over on the Sparkfun forums where I proposed a circuit that could measure the voltages in across each battery cell in a series stacked set of battery cells. The circuit level shifts each cell voltage down to a ground reference that the microcontroller or ADC can easily measure.

Forgive my chicken-scratch, I think its drawn pretty nicely if I don’t say myself. This circuit also has easily adjustable gain with a single resistor for each battery cell.

This circuit works by sourcing a current from each battery cell that is proportional to its voltage. This is done using a PNP current mirror. Resistor, Ra, sets the reference current according to the following equation.

$I_{b1}=\frac{V_{b1}-V_{be}}{R_a}$

The current is converted back to a voltage using Rb referenced to ground.

The diagram is missing any protection circuitry but we can expand this in a further article. I will also be doing a follow up article in evaluating the performance of this circuit and doing an error budget. For now, I just wanted to get this idea out there.