Tony

You would start by insulating the battery well. It should only take
about an inch of foam insulation (R7) to get the heat leak down to a
watt or so but it needs to be carefully made.Startting with a warm
battery, good insulation and a little self heating from the battery
during discharge should get you a long way. Of course the battery
likely won't fit into its mount with all that insulation. After doing
all that adding a heater may make sense but you need to know what you
are doing with a termperature regulation circuit, and good luck finding
somebody who will sign off the work. And yes the expectation for an
insulated battery should be that the cost of running the heater is
well worth it in terms of getting more capacity out of the battery. It
would take two heater plates or nichrome wire on the battery end (or
sides) that parallel the internal plates. It is those two outward most
facing cells that probably conduct most heat into/out of the battery
(in a well insulated battery the terminals and cables may be a
significant leak as as well).

Before looking a the complexity of a heater I'd look at a solar panel
of the glider. They work great cold.

If you need to go to the effort of adding a heater it may also be time
to investigate alteratives to lead acid batteries (which may also need
heaters).

Lead acid batteries have a pretty large thermal mass and the AGM
bateries cores tend to relatively well insulated because there is
little electrolyte or metal contact with the case so in practice they
probably don't get as cold as ambient on typical flights, especially if
like my batteries they sit up on a parcel shelf packed with stuff
around them. I've been curious about the temperature issues and have
thought about drilling thermocouples into the inside of AGM batteries
for some tests but I've never got around to it (just for ground
measuremnts I'd not want to fly with it).

Darryl Ramm

Tony Verhulst wrote:
wrote:
Nice presentation snipped
... One of the curves shows the
rapid voltage drop of a cell freezing as it discharges (some thing to
worry about on that next really long and cold wave flight :-)

If we had properly designed heater elements surrounding the battery,
powered by the battery itself, could we expect more "useful capacity"
from the battery on those cold flights? Useful capacity being defined as
the amount of current delivered to the avionics.

Tony V.