Measuring individual cell voltage Ni-Mh

Hello world! I'm currently building an Arduino controlled RC car. I've bought 8x AA 2450 mAh Ni-Mh batteries, which I would like to power the entire RC car. My problem is how I should protect the batteries from undercharging. From what I've heard it's safe to discharge Ni-Mh cells to 1.0-0.9v. I know that the Arduino can't measure its own supply voltage via the analog pins. My idea is that I'll take the last battery in the series of eight and connect it's plus-pole to an analog pin on the Arduino and then read the voltage. Is that a good idea? I'm thinking that maybe the last cell I've connected to the analog pin might get discharged a bit faster, or what?

Thanks in forehand for answers and tips! ;D

I know that the Arduino can't measure its own supply voltage via the analog pins.

[u]Actually you can measure Vcc[/u] (but not the voltage into the regulator). I've never done it and I've never studied the code but I understand the idea and it relies on the internal-optional 1.1V reference. (You could also connect the Vcc through a voltage divider and then use the 1.1V reference.)

I'm thinking that maybe the last cell I've connected to the analog pin might get discharged a bit faster, or what?

Yes and no... The Arduino inputs are about 100 Meghohms (almost "infinite" resistance) so virtually no current flows into the analog input, and it would be less than the internal self-discharge resistance of the battery.

However, the Arduino is limited to 5V (technically Vcc) so to measure above that you need a voltage divider and that will consume some current. If you use resistors in the Megohm range you'll get just a few microamps which is "nothing" compared to running a motor, but it might increase the self-discharge rate while the RC car is sitting on the shelf.

Actually, more like 1 Megohm, so 1uA input current.

You can use a resister divider to bring the regulator input voltage down into the 5V range for measuring.

Hi, Do you want to measure the TOTAL voltage or INDIVIDUAL cell voltages?

Tom... :)

TomGeorge: Hi, Do you want to measure the TOTAL voltage or INDIVIDUAL cell voltages?

Tom... :)

I was just thinking about measuring the individual voltage of one cell ;)

Imim: I was just thinking about measuring the individual voltage of one cell ;)

The problem with doing that is that you will need separate connections to each cell and in a normal series configuration of the cells only one of them will be directly connected to Arduino GND.

You could possibly measure the voltage of cell A (the one connected to GND) and then cells A + B and cells A + B + C etc and calculate the voltage of cells B and C by subtracting the voltage across the lower cell(s).

Personally I doubt if all that trouble is warranted for NiMh cells. I would be content with monitoring the voltage of the complete pack and assuming it is evenly distributed across the cells in the pack.

It is with LiPo cells that you can get significant differences in the charge on intermediate cells and that requires balance charging.

...R