Need Help with OLJK two battery switch

Yep, I have flown with two switches for years. Simply switch on the
new battery just before switching off the old battery. Please, don't
tell me some horror story about "exploding" batteries! I have taken
readings with a 10v 'dead' battery connevted in parallel to a 12.8v
'fresh' battery and all I read was 300 m/a as the fresh battery tried
to charge the dead battery. And don't tell me about voltage spikes
either, *because all those red-necks jumping their car batteries seem
to be surviving quite well, don't they?
JJ

My panel is wired according to JJ's plan. Two batteries. Two
switches. One buss. The avionics shop owner/technician and I
discussed the "make before break" type of switch, but he said his two
switches would work just as well. And as JJ said, switch the second
battery "on" before switching the first battery "off". So far, I
haven't had any probems.

And, I resemble JJ's remark about red-necks jumping car batteries!

Ray Lovinggood
Born and raised in SC, currently employed and living in NC.
And if you want to see some red-necks, just come to my family
reunion...

OLJK = DPDT? Make before break? The only web reference to OLJK
points back to us! The idea of a capacitor to keep power going during
a battery switch cycle is an interesting idea. How big of a cap would
we need to survive a 1sec loss of power with a 500ma load allowing for
a 1VDC drop?

I personally use the two (fused) batteries in parallel approach and
haven't had any issues at all. But am toying with the two batteries
and two switches (three counting a master) approach. A late winter
project. That and replacing all my connectors with powerpoles. I
REALLY like these modular connectors which come in a myriad of
colors.

One thing to note. Before a speech I gave at the 2007 SSA convention
on "Soaring Avionics Wiring" I wanted to know if flight recorders
would survive a temporary power outage while switching batteries. So
I wrote to every manufacturer and found that all except the
Volkslogger will easily survive several seconds of lost power. PLUS
they each will keep the same log file intact. A "small" gap in a log
file is legal as long as it is obvious that it is the same flight.
This has more to do with a lost GPS signal than a power outage but the
effect is the same.

My $0.02.

- John DeRosa

On Mar 2, 5:53*pm, ContestID67 wrote:
*How big of a cap would
we need to survive a 1sec loss of power with a 500ma load allowing for
a 1VDC drop?

Constant current discharge of a capacitor (will be close enough)...

C = t . i / deltaV
= 1 . 0.5 / 1
= 0.5 F

Easily doable with today's supercapacitors. But as I pointed out
earlier, mechanical switch issues usually happen over time frames of
10's of ms. Not 1 sec. And why do anything if it is not really needed.

Darryl