In article Michael Ash writes:
In rec.aviation.soaring Alan wrote:

One doesn't want to run the lead acid battery down past about 1/2 its capacity
to get a reasonable service life from it, so that limits you to 3.5 AH.

What kind of unreasonable service life do you get if you use the full
capacity, and at $20 each do you care if you use it up faster? Unless you
can count the number of cycles on your fingers that may be the simplest
and most cost effective way to go, although of course I may be overlooking
something important.

It depends on the details of the battery. Lead acid batteries come in a
lot of sub-types, with varying ability to handle deeper discharges. The
better ones cost more.

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

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.

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.


Alan