MLS,
I can check the POH for my club's Super Cub next weekend if you don't get satisfactory answers before then.
The theory books (if I remember correctly - I dumped most of this stuff out of my brain as soon as I walked out of the exam hall) will tell you that, for propellor-driven aircraft, Vx is the lowest controllable speed.
This is because Vx is defined as the speed at which maximum excess power is available. (Vy, as a point of interest, is the speed at which maximum excess thrust is available.) Propellors can supply a (very) roughly-constant amount of power at all low speeds, dropping off very sharply at high speed, so all we are looking for is a speed where power required is a minimum. We know that power = thrust * TAS, so a low TAS requires a low power. Remember that thrust = drag, and low drag is, generally, obtained at low airspeeds too - the increase in drag as alpha increases turns out to not be significant above the stalling speed when compared to the decrease in power as a result of the low TAS. Hence, low speed = low power required = maximum excess power = Vx.
In practice, many operators specify a higher Vx because they don't want you operating so close to the stall, at speed-unstable angles of attack, when very close to the ground. But I'm not all that surprised that Vx is given as just a couple of knots above stall speed. Not so sure about "takeoff speed", though - personally, I've never looked at the ASI when taking off in any tail-dragger.
As for the batteries, I'd guess that each aircraft has a completely different electrical system, and that there are lots of options. I know that the Super Cubs I flew in America, with transponders and GPS units, certainly wouldn't have been able to be powered by the tiny dynamo on my club's Super Cub. Maybe the best way of finding out is to look at the label on the battery???
FFF
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