I have an Arduino based project that will be powered from a 12V SLA battery. One of the features of the project is to show the battery charge status. Assuming that this batteries have an output voltage between 10 and 14V (below is not important, because the battery is dead), I was wandering which alternatives i have to measure the charge as accurate as possible using the Arduino ADC and some analog components.
The first and obvious answer is to use a voltage divider....but, that would leave me with a lot of the reference scope unused. For Example: If i divide the voltage by 3, I'll only be interested in the upper 1 voltage (say between 4 and 5), not using almost 80% of the ADC resolution.
Doing some wikipedia research I came to the differential amplifier, so I thought that mabe using one of this circuits with an OpAmp (powered with 5V) and a Zener diode to compare, giving a reference voltage of 10V.http://en.wikipedia.org/wiki/File:Op-Amp_Differential_Amplifier.svg
In my beginners mind this sounds like a much better solution, because I could have an input between (ideally) 0 and 5V that represent usable range of the battery. But I would like some experienced advice on this type of solution. What kind of problems may I face with such a solution?