One thing that was not really discussed in the "battery" posts was
"reserve capacity". After starting power (CCA - cold cranking amps) ,
reserve capacity is #2 in importance. The engine started fine but then
what happens when the alternator / charging system fails. If you are
running electronic ignition, flat screen panel, radios, GPS, cockpit
lights, charging your cell phone, and more, how low will the battery last?
Reserve capacity explained:
To calculate the reserve capacity of a battery, it must first be fully
charged. Manufacturers draw 25 amps of power from the battery at 80°F.
When the voltage drops below 10.5 volts, they stop drawing power. The
duration of time this process takes is the battery's reserve capacity,
which is measured in minutes.
The PC680 is a very popular battery in experimentals. Here are some of
it's numbers.
OEM Part Number PC680
Voltage 12 Volt
Cold Cranking Amps rating at -18°C (0°F) 170
Reserve Capacity 25Amp Draw 24
Whatever battery you are using or intend to use, find out what the
"reserve capacity" is, determine what systems you will need to make a
safe landing, calculate the total draw in amps of those systems, and now
you can calculate and know in advance how many minutes you have to find
a safe landing site. Allow a safety margin. The more critical
electronic systems you have, i.e. electronic ignition, electric fuel
pumps, the greater your need to identify a charging system failure when
it happens and know immediately what systems you can shut down and how
many minutes you have to find a safe landing site. Suggestion: Have a
warning system (idiot light) to identify charging system failure and a
placard saying "land in X minutes". Also, realize that "reserve
capacity" deteriorates with battery age. Did the starter "hesitate" on
the last start up? Is the battery fully charged or did you have to jump
start on the last startup?
None of this is rocket science. Get the info and know the answers
before it happens so you can avoid the panic and survive to tell us all
about your near death experience. 🙂 "There I was, above a cloud deck at
10K and d#m*, the alternator fail light came on, bright as the sun!!!!!
If you survive you are allowed to embellish the story a bit as most of
us are inclined to do.
Larry Flesner
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