If i put two batteries in parallel one battery 10v and the other 5v how do i calculate the resulting voltage when they are in parallel ?

 

If you do that, ur 5V will start to charge, and even if the 5V one is rechargeable, it will probably blow up in ur face.
So basically u won't need to worry about voltage measurements.

 

You must not connect batteries of different voltages in parallel, as you risk melting wires, fire, and explosion.

 

If you do that, ur 5V will start to charge

When I was a small child, I think that so but never tried. So now, I asked it for confirming.

 

If you run 2 batteries or v sources, like a solar panel & backup battery, provide each with a blocking diode[ OR GATE]. Running 2 12 V batteries in parallel is common in RVs, but preffer 2 6s in series.