Something simple like an iDuino with USB support is a good plan, but i would recommend sourcing an ADC that can handle higher voltage. Voltage dividers = leakage current, and referencing to ground is not really an option, you want to reference across the battery. Granted, you can reference each node of the network of battereis to ground and do the math, but you're better off with two wires per battery leading to an adc that can handle it. They, too would be cheap. Check out
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