AK wrote:
1) Should aircraft use switches for both positive and negative (SPXX
vs. DPXX switches) or just the positive (especially in application
from point 2 below) ? I was planning on switching just the positive
only and have the negative side permanently connected; both batteries
and negative of each circuit joined together.
2) I am about to separate radio and transponder into circuit 1 and
rest of the instruments into circuit 2. I will have two batteries. I
am planning to use two switches; one for battery 1 and one for battery
2. Each of the switches should be able to switch a battery to either
circuit 1 or circuit 2. I would also have another switch that could
connect circuit 1 and circuit 2 (they need to be connected while
flipping the battery switches to ensure constant power supply). I
would also need a couple of diodes to make sure one battery does not
drain the other battery and that I don't get overload in any circuit.
The question is what diodes should I use?
3) Does anyone have another solution for battery switching maybe
simpler one?

Regarding the Diodes, take a peak at this...

http://search.digikey.com/scripts/Dk...=497-2738-5-ND

At 1/2 an amp in the forward direction I've measured the voltage drop to
be right at 0.3V So far this is the lowest I've found. Rated for 25V
and up to 10A so it should be very robust in the standard glider 12V
arrangement.

I personally don't use switches (other than a master), but have both
batteries in parallel and diodes in line with both batteries so I'm
always getting power from the best battery.