Alan wrote:

Other factors include the output voltage under load at discharge -- a
lead acid battery is rated to 10.2 to 10.5 volts for a "12 volt" battery
at discharge.

Powersonic uses 10.5 volts for 5 to 20 hour discharge rates.

Unfortunately, most 12 volt radios and devices are designed
for a charging electrical system, with a voltage of about 14 volts.

Most panel mounted radios (definitely for Dittel and Becker radios)
bought new in the last 20 years (and even some older ones) meet the
current requirements to function properly to 10.5 volts, and at even
lower voltages, but with reduced power output. "Very old" radios likely
will have problems at 10.5 volts. I have no idea what the percentage of
"old" and "new" are.

When
the battery is down to 75% of the expected voltage for the radio, not all
of them work. I have had aircraft radios that would not transmit below
about 11.5 volts, at which point the battery would be still above 50% charged.

Powersonic shows it's batteries have only 25% capacity left at 11.5
volts. I haven't checked other brands, but believe they are the same for
the batteries we use in our gliders.

Lead acid batteries are normally rated for capacity at a 20 hour rate of
discharge. A 7 AH battery would deliver 7000 / 20 = 350 mA for 20 hours.
Faster discharge rates result in less capacity being available (look up
Peukerts exponent for more details). Discharging faster than that, reduces
the amp-hour capacity of the battery.

At a current drain of 700 ma (10 hour rate) that you might have with
transponder, radio, etc, Powersonic shows a capacity of 90% of the 20
hour rate. That's not a show-stopper, but worth taking into account.

Lead acid batteries have less capacity when cold. One guide suggests that
for every 10 degrees centigrade below room temperature, you should add 10%
to the battery capacity needed. High altitude flight tends to get up into
cold places.

This corresponds with the Powersonic chart, so at -10C (15F), you have a
30% loss of capacity. For winter flying, and spring or fall flying in
places where you can climb to, say, 10,000' agl, it's an important
factor. Especially so, when you know your encoder will likely be using
it's heater, adding 50-150 milliamps to your current drain.

--
Eric Greenwell - Washington State, USA
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