The 'split services' approach is much better than running
everything off 1 battery, and then switching to another. The
standard 7AH gel cell is rated at the 20hour discharge current -
about 300mA. Any higher current and the total energy available
decreases. (and that 7AH is a new battery at room temperature,
not a 2 year old one freezing at 18,000ft).

If you have room, paralleling batteries works well, so long as
you have another independent battery to run essential services
if the main one drops below 11V (for me vario/flight director
only, and radio when 5 minutes out on final glide).

At 16:27 05 February 2012, kirk.stant wrote:
On Feb 5, 7:43=A0am, rk wrote:
On 3 helmi, 18:10, JohnDeRosa wrote:

At the panel I use three switches to run the batteries in
parallel.
Battery 1 on/off, battery 2 on/off and then the output of
both to a
master switch and then to the avionics. =A0The thinking of
separate
battery switches is that you can disconnect a "bad"
battery. =A0I
alway=
s
have all three switches turned on during flight - the need
to be able
to turn off a bad battery hasn't arisen for me and I am
unsure how I
would even know (everything dies? =A0smoke?).

You would know that by total power loss of all your batteries.
You
have essentially parallel connection with all your batteries
during
flight. When one fails, it drains your good batteries empty.
When your
radio or computer starts blinking it's too late. Best option is
to use
simple selector switch to choose one battery at time, and
keep other
batteries disconnected.

rk

I've been using a somewhat different approach. Two batteries,
each
powering half the avionics. So batt 1 has the radio and nav
computer
bus; batt 2 has the logger/PDA and backup vario bus.
Instrument
"busses" hooked up via individual 3 position switches so that
either
buss can be powered by either battery or disconnected.
Normally, run
the batts/busses independent, but have had occasions when
one batt
went low early, and just switched the bus it was on to the other
battery for all instruments for rest of flight.

I also use the built-in voltmeters in various instruments (AR-
4201,
SN10) to monitor the health of the batteries during flight.

Seems to work OK for the past 12 years...

Kirk
66