Switching Batteries Advice?

I would like to ask the rec.aviation.soaring community for comments
about their experiences switching between batteries while in flight.

It seems to be common practice to carry two batteries. There is
usually a master switch and then an A/B switch to toggle between the
batteries. The issue is what will a momentary interruption in power,
when you toggle the A/B switch, do to your flight recorder/computer?
Most important is the integrity of your flight log, less importantly is
your task and other flight data.

PDAs are not suseptible to this issue as they come with their own
internal ("backup") battery. Also, let's not discuss "well I have
this-and-that device as a backup" (handheld GPS, machanical vario,
etc). The issue at hand is competition, record or badge flying and
quarantee that all important flight log by maintaining power to your
flight recorder. I have not seen a ship yet that had two redundant
flight recorders (but I will bet they exist).

A simple double throw A/B switch is commonly used. My measurements
show that this type of switch is completely open (no power) for 10-15ms
but this depends greatly on the quality of the switch. There are
make-before-break switches in which for a brief period of time both
batteries are connected to each other and there is no loss of power.
However even briefly connecting two heafty batteries together, with one
possibly completely failed, has its own drawbacks (like an onboard
fire). This of course all depends on the pilot knowing when to switch.
If (s)he first waits until a failure is noticed, then all may be lost.


Another approach is having two ship's batteries connected to one
another via a "device" which allows each to (safely) supply current and
to automatically switch between the two if one fails. A pair of heafty
diodes is an obvious and simple choice as one battery cannot "charge"
the other but both can supply power. However, you will typically loose
0.6v-0.8v across a silicon power diode.

Ultimately it seems to depend on the manufacturer of the recorder and
how their electronics handle a temporary loss of power. Some
manufacturers support permanent backup batteries which charge from the
ship's main battery. This may be the best overall solution.

Any of your ideas and solutions are very welcome.

Thanks, John