Electronic – Non-rechargeable Battery Gauge circuit / Coulomb counter

I'll put the links in an answer so I don't run out of room again.

I have used this one.

There is also this one

Like I said before in the comments, the first one does more than just measure voltage and then approximate the capacity. When first connected it charged the battery to 4.2 volts and shut down, but read the capacity as only 85%. On the second discharge, it read the capacity as 0% at 3.5 volts, and still allowed discharge until about 3.1 volts before it stopped the discharge. After that it got more and more accurate with each charge/discharge cycle. It is far from perfect, but more accurate now. It has problems with accuracy under about 3.5 volts as the batteries tend to be closer to empty than the pcb thinks, so it always reports abouts 30% remaining when it is closer to 20%. Other than that it seems to be OK. I doubt it is coloumb counting or I think that it would get more accurate. However, I have only done full charge/recharges maybe 5 or 6 times and after that it has been 30 or 40 partial charges/recharges.

The second link is to an interesting looking fuel guage. It measures the impedance and current of your battery pack. It differs from the first link as the first one is a charger as well and protects from over charge and over discharge and protects from short circuit. I have not yet used the fuel guage from the second link so I can't say whether it actually works as advertised.

Hope this helps.

If the ebay link is dead, I will try to find another.

In regards to the questions asked, you need the capacitor C14 to provide a low impedance input to the ADC during sampling. If you look at how an ADC module works, there's a capacitor that holds a charge while the ADC conversion is taking place. Before that can happen, this capacitor needs to charge to the value supplied by the pin. This is called the sample time. In order to do this, current must flow into this capacitor. This is where the external capacitor (C14 in your case) comes into play. When C14 is present, the current, or charge, primarily transfers from C14 to the internal capacitor while the voltage across C14 remains stable. When C14 is removed, the current must flow through R1, which is a high impedance of 10k, which causes a voltage drop across R1 during the sample time of the ADC. This will cause the sporadic measurements you are seeing.

Although you will require a capacitor at C14, the value you list of 0.1uF is quite large for the purpose of transferring charge. You could probably get away with 100 to 1000pF. This would allow you to turn on and off the circuit much quicker.

Here's a link to an app note that helps clarify this. See section 7. ADC App Note