[thstone]

First, check the spec sheet from the manufacturer of your multimeter. This will contain the specified accuracy in +/- X.xx%.

If you assume that the multimeter is working properly, this is all you need to calculate the error range.

For example, if the multimeter is rated at +/-3% accuracy, then a 12.00 vdc battery could read anywhere between 11.64 vdc and 12.36 vdc.

This could roughly explain the range of values that you're seeing from different multimeters.