I have my battery switch/controller designed. Now it is in final layout phase. This is an interesting electronic project. The circuit has to be able to switch in one of 4 batteries to feed the aircraft. This will be used for the "Santa Cruz Project". As you may have read before, the idea is to use 4 batteries, and one battery #1 is depleted, battery #2 will be switched in and battery #1 will be dropped to the ground. The battery current on this craft will have to handle at least 60Amps continuously, and will run at 16.8V.
Relays are out of the question, due to size and weight. So I'm using FETs. During the flight, the resistance of the circuit is very important, since any resistance will be turned to waste heat. Even a .001 ohm resistance is a fair amount of waste, since the loss is equal to the current squared (60*60 = 3600) times the resistance (.001). So one MILLI-ohm of resistance will have a loss of 3.6Watts. Consider that a 22 ga wire has more than 1 milliohm resistance per inch!
If you are using a 10,000mAH battery, (10 Amp/Hours) at 16V, the capacity of that battery is 16 X 10 or 160 Watt hours. A loss of 3.6Watts amounts to a loss of over 2% of the battery's capacity. While that might not seem like much, when you are trying to get every last electron out of a battery, it matters. And you must consider that there will be many other losses in the rest of the system wiring as well. The ESC (Electronic Speed Controllers) will have several milliohms, the wiring to the motors will have resistance. In all, you will probably lose 8-10% of your battery capacity in wiring resistance.
But back to the switch/controller:
I'm using N-Channel FETs for their low on-resistance, and I'm using 4 of them per battery. I have to use at least two per channel in a back-to-back arrangement because the switch must block voltage in both directions, and I'm paralleling two of those switches for lower ON resistance. N-Channel FETs are hard to drive because their gates must be brought above the source in order to turn them ON. That means I need a voltage about 10V higher than the supply voltage (16V). In order to provide this voltage, I'm using a charge pump made from a 555 timer.
The timing and control is handled by a PIC18F2321, which has plenty of outputs and an A/D for voltage sensing. It will sequence the FETs on and off and control the resistance wires that allow the batteries to fall to the ground.
If someone is interested in the circuit, I have an LTSPICE model that I'll send upon request.