At 20:22 25 January 2017, wrote:
I am still using sealed lead-acid (SLA) batteries, because they are
cheap
a=
nd give good service for several years in my experience. The
power needed
=
in my glider is modest so even on a long flight the battery does
not get
fu=
lly discharged, that lengthens its life. I take the battery home
when I
am=
done flying, charge it, and keep it in moderate temperatures.

Last summer I kept a spare battery in the glider trailer, in case I
forget
=
to bring a battery to the airport when I go there to fly. At the end
of
th=
e season I tested the charge capacity of that spare battery along
with my
o=
ther ones and some owned by our club. I found that the battery
that was
st=
ored in the trailer was pretty much useless, even though it was
good
before=
that summer, and not that old. It held much less charge than
some much
ol=
der batteries that I have. It was never used during the summer,
and its
vo=
ltage held up, but measured capacity (amp-hours) was reduced to
a fraction
=
of 1 AH by the fall.

Also, some club batteries that were perhaps left in the club gliders
for
we=
eks on end lost most of their charge capacity, despite being less
than 2
ye=
ars old. The latter may be due to being left in an uncharged state
for a
w=
hile, I don't know, since the club does not keep battery charging
records.
=
But it could also be due to the hot summer temperatures in the
glider
cock=
pits, whether tied out or in the trailer? Not Arizona-hot, but even
in
Ve=
rmont on sunny summer days the temperature inside a glider on
the ground
ca=
n reach well above 100 degrees F.

So, is it useless to store a spare SLA battery in the trailer?

The Yuasa manual for their NP batteries (probably typical SLA
batteries) has a graph of self-discharge at various temperatures.
At 0 C/ 32 F you lose about 10% of charge in a year. At 40 C/ 104
F you lose 50% charge in 5 months. Leaving a SLA battery part or
fully discharged is always a bad idea, so leaving a battery to cook
unattended over a summer is definitely a bad idea.