Ok, so wiring batteries in parallel of different voltage is bad because the batteries try to equalize voltage with high currents essentially by discharging. The thing is, even among batteries of the same type, the voltage is slightly different. How far apart do the voltages have to be that I should consider not paralleling them?
So, an example: I have 3 lead acid batteries (fully charged) that are from the same place, same make, model, and voltage (kind of, they're supposed to be 12V each). They have: 12.75V, 12.78V, and 12.85V. This is clearly a range of voltages, but does the .03 drop from one or the .1 drop in the most extreme case matter much?