With the diode solution, when the system is turned on the battery with
the highest voltage will provide the current. As the battery draws
down the second battery will come on line. From that time on both
batteries will be providing power to the circuits. The strongest
battery will provide the most current but both batteries will be
stabilized at a common voltage. Therefore BOTH batteries will require
re-charging after being used for any substantial period of time.

Regarding the 3 position switch. The simple solution is to put a
capacitor between ground and the output side of the switch. The
capacitor must be large enought to provide the current needed during
the switch switching time. This should not be a very large capacitor
for our purposes.

Brian Utley
Gary Emerson wrote:
COLIN LAMB wrote:
"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?"

Well, that is a problem. You do not know. Everything is automatic. If you
have one battery that has lost most of it's capacity, the good battery will
do all the work. The only way you will know is when the total capacity of
both batteries has been reduced. And, then, you will not know whether one
battery has done 50% of the work and they are both down in storage ability,
or one battery is still 90% and the other one is 10%.

So, you will have to determine the capacity of each battery separately. You
could test the battery capacity by switching either off during discharge, or
even by using 2 ammeters - but it is becoming more complex once we try to
extract more information. You can also determine the capacity during
charge.

Colin



During any one flight, you don't need to know. What is a good idea is
to test the batteries if you think you are losing capacity. During the
week charge them up Monday and Tuesday, and Wednesday put a load on each
one and see how long it lasts. Ideally, you'd like the load to test the
batteries for as long as you'd normally fly. Say 4-6 hours unless you
are typically flying for longer. If either one can't go the 4-6 hour
distance and mainain a suitable voltage, then it's due for replacement.
After the Wednesday test, charge them back up so you're ready for the
weekend assuming they pass. I'd certainly do this in the spring and
before any big contest during the year. 7Ah batteries are cheap in the
grand scheme of things.