I'm setting up a way to switch between each battery independently versus two or three batteries slaved together via the use of a simple circuit of diodes (rectifiers) and capacitors. The switches we all use are break before make so you don't want to lose power to instruments when switching between batteries. I found a suitable rotary mini-switch with up to 6 positions so you can do any combination of batteries you like (individually or slaved together). I realize the diodes cost about 0.15 volts on the output side so being able to get a little extra juice might be better but I also discovered that batteries last longer if you don't regularly deep discharge them. Since I have two main batteries in the baggage area plus one in the tail I figure my main scenario will be to slave the two mains together via diodes so theres no cross-charging and each draws down at half the rate. On a typical day they only get halfway drained and keep the tail battery as a fallback. I don't want to take the tail off to recharge the tail battery every day. This means I need a switch to go between Batt 1, Batt 2, Batt 1+2 and Batt 3. A position for Batt 1+2+3 is an option, but not super attractive if you wan an emergency reserve.

I've been using around 2500 micro farads of capacitor with a 1 ohm power resistor across the leads for several years. Works great.In v2 I'm upping the capacitance.

9B

On Tuesday, April 23, 2013 1:06:58 AM UTC-7, Jim White wrote:
If your batteries are dissimilarly discharged during flight (which is

possibly the reason you want to switch batteries) then the instantaneous

current flowing through your circuit when you join them could be quite high

and cause you more problems.



There are two better ways to solve this problem:



1) Insert a capacitor across the power as has been suggested. A further

enhancement would be a choke across the switch.



2) Power your loggers etc off a separate bus which is connected through a

rectifier bridge to both batteries. The logger will draw power from the

higher voltage battery and neither battery can charge the other. There will

be a voltage drop of between 0.5 and 1 volt but most loggers will work down

to 9v so no problem.



Jim



ps: avoid thermal trips if voltage loss is an issue.