On Tue, 26 Dec 2006 01:39:44 +0000, Steve Paavola wrote:

Gary Emerson wrote:

wrote:
Better yet is to use diodes so that both batteries will always be "on"
in parallel and you're always pulling from the best battery with no
fiddling required from the pilot. Relatively low voltage drop diodes
are available with 18 Amp forward capacity.

If both batteries are on-line all the time, how do you know when one is
getting weak and needs to be replaced? Or do you replace both batteries
when voltage is marginal at the end of a flight?

I used two small diodes to provide a "fail safe" supply to the
Volkslogger, but the radio, vario, transponder etc get theirs via a 2 way
toggle switch.

Then when one battery dies in flight, I will notice and switch to the next
one manually. But in the meantime the logger always draws current from the
highest voltage battery and hence no interruption to the trace.

The diodes are equivalents of IN 5817 "Shottky rectifying diodes". They
are cheap and tiny. At the low currents drawn by the logger, the forward
voltage drop is very small (less than 0.2V). No cooling is required. I
soldered them directly in line with the wires from the fuse holder to the
toggle switch, without a circuit board.

I test the voltage on each battery during my pre-flight by setting the
Volkslogger on its battery voltage function and disconnecting the
batteries one at a time. (If the electric vario had a battery voltage
function, I could switch it between the batteries and measure their
voltage in flight.)

The weak link in this setup is the toggle switch which has to be sized to
handle a significant DC current.

(The u/c warning buzzer is now also running off a 2 diode "fail safe"
supply - but there is another story behind that ...).


Regards


Ian