Bill Freeman wrote:

I think the weakest point of this argument is the unknown actual capacity
of the backup battery.  As the battery ages, it will lose capacity at an

unknown

(probably unknowable) rate.  If a cell starts to go bad, the battery

voltage

will
be apparently fine, but at a significant load the internal losses across

that

bad cell can draw down the battery very quickly. Even a very small

internal

resistance can be quite serious.  For example, if the battery develops

even

0.25 ohms internal resistance with a 15 amp load the voltage drop is 3.75
volts,
dramatically reducing the system voltage.

Al Gietzen wrote:

Typical internal resistance in a good quality battery should be in the 10 to
15 milliohms.  Or check out dry cell Hawker Odyssey  www.hepi.com , or
Apollo www.noramcousa.com batteries that have 5 to 10 milliohm resistance.

Good batteries are very reliable; and you don't have to consider whether one
belt failure takes out both backups.  Of course you would want to do simple
periodic checks on their condition.  With a good linear voltage regulator,
the back up battery would be very well treated since it would not be
subjected to intermittent massive loads, like starting.  I estimate the
"essentials" load with elrictric fuel pump, electric gyros, EFI/IG to be in
the 6-8 amp range.  At this load rate, you shold get 90%+ of rated amp-hrs
above 8.5 volts on a good battery.  If your backup is a 16-18 amp-hr, this
battery should provide for over 2 hours of flying.  Apply a safety of two if
you like; you still ample time to find a runway.

This condition assumes you've lost your alternator and main battery.  With
alternator failure and two batteries; well, you can either get where you're
going, or go the airport with the best restaurant.

One good approach is to design your system with two 16-18 Amphr batteries;
where both are used for starting, one is isolated for backup/essentials.
Change out one battery every two years or so, always with the new battery as
the backup.  The Hawker dry-cell 16 Amphr (rated for 25 amps above 10.5
volts for 28 minutes), weighs 15 lbs in a roughly 7 x 7 x 3 package.

If a little more weight is not a problem, use a 25 - 30 Amphr main which
takes the full starting load, and a 16 - 18 Amphr backup

Al Gietzen

 
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