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Wow, i just got seriously chewed out for some suggestions...
OP didn't say he was incapable of working with electronics and he didn't say he wanted an open source option. I work with this stuff every day and that's what I would have said to someone in a meeting room if they asked the same question. If he wants details and likes where i'm going, then he can ask me another question.
No, i did not select specific part numbers for him because I assumed that he was capable of doing so using the very simple parametric search on digi key. I don't know what his system voltage is, what the processor voltage is, what the space constraints and budget are, if he is capable of using SMT, what busses he may have available, etc.
I don't like voltage dividers because of the leakage current as well as the tuning required for every single one due to resistor tolerance, and I like the idea of 2 wires per battery so that he is always reading in the same range of voltage (which allows selection of an ADC best suited to that voltage range). One wire at each battery would mean that he is measuring 12v at the first one, 24 at the second, 36, 48, etc. This requires a variety of ADC's. There MAY also be complications to connecting the pc ground to the car ground, which are avoided with 2-wire. I currently have two projects on the go that are acting little sketchy because the customers are connecting my digital grounds to their pc and analog grounds.
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