What are the issues if you want to use 6s batteries with the voltage down regulated and use them in a 3s system?
Daryl

That's a very general question in need of some further details.
First, if you are talking about the RX & Servo voltage, SBEC's can be used.
Motor power is another issue. The current involved makes it difficult to
lower the 6S voltage to about the same as a 3S for motor power.

I was thinking of motor power.

First thing....how do you plan to reduce 22.2 volts to 11.1 volts?
Second........why would you want to carry around all that extra weight even if you could fit the pack in?

If you could fit it in and the craft can carry the weight, a better choice would be (2) 3-cell packs of the same mah rating plugged in parallel to double the mah rating and provide some extra run time.

I have some 6s batteries I wanted to use on a smaller quad copter but I've decided not to. I'll try the 3s in parallel.

Weight and heat. You can probably find a regulator to knock the voltage down but it will either be a big demand for a small component, or a heavy regulator. And the extra battery is a lot of extra weight for a plane, uh, vehicle designed for 3S.

Weight and heat. You can probably find a regulator to knock the voltage down but it will either be a big demand for a small component, or a heavy regulator. And the extra battery is a lot of extra weight for a plane, uh, vehicle designed for 3S.

If you didn't regulate the voltage, you can easily fry any electronics not designed for the 6s voltage, the prop will spin 2x the speed and likely fry the motor.

You can adjust how much power the motor delivers by changing prop - a bigger prop will draw more current and use more power. Or, you could try 4s with a smaller prop.