How Close do Batteries Need to Be in Voltage for a Parallel Connection?

Question

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?

Answer 1

That small difference should be fine.

I don't think I'd want to charge a gel-cell or AGM in parallel with a flooded battery, but if yours are same make and type there should be no problem.

Answer 2

When you connect two batteries with different voltages, you'll have a current \$I=\frac{\Delta V}{2*R_{int}}\$, assuming both batteries have the same internal resistance. If this current value is under the maximum charge/discharge current, it's OK to connect the batteries.

As a rule of thumb, \${\Delta V}\$ should be less than the voltage drop (\$V_{unloaded}-V_{loaded}\$) under nominal load, minus ~50% safety margin.

Answer 3

Consider using big powerful diodes at the output of each battery. This way you drop the output voltage of each battery by about 0.7V, but only the diode leakage current (usually in microamps) will flow from one battery to another...