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Odyssey % of charge (Correction)

 
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nuckollsr(at)cox.net
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PostPosted: Thu Feb 23, 2006 8:35 am    Post subject: Odyssey % of charge (Correction) Reply with quote

I stubbed my toe in interpretation of the graphs in an earlier
post. The revision is below:

At 08:22 AM 2/14/2006 +0000, you wrote:

Quote:


If the charging system is not working does anyone have an approximate % of
remaining capacity versus voltage for a Odyssey PC680 or similar?

What is a safe lower voltage limit one can discharge a PC680 to and not
cause permanent battery damage?


<snip>

Know that ALL lead-acid technologies are 95% used up
at 11 volts for a 12-volt battery. See:

http://www.aeroelectric.com/Pictures/Curves/Capacity_vs_Voltage.gif

This is a curve for a big biz jet battery where we see
that for "light" loads (1 to 2x of capacity), the discharge
voltage has a sharp "knee" that increases the negative
going slope as it crosses 11v/22v line.

This particular battery is capacity rated at CC levels
of discharge i.e., it's a 37 a.h. battery that will give
100% of rated capacity at 74 amps of discharge (typical
for rating bizjet batteries that have a 30 minute emergency
requirement). If one were to discharge this battery like
most of the rest of the industry does (20 hour rate) then
it would probably exhibit a capacity on the order of
50 ampere-hours.

Check out this curve from the datasheet on a Panasonic
33 a.h. battery:

http://www.aeroelectric.com/Pictures/Curves/LA1233_Panasonic.gif
Again, note the shape knee in the curve as it dives
through 11 volts . . . all used up. In this case, the
battery is not specifically rated for emergency power
service in airplanes - it couldn't be called a
33 a.h. device. If we run up the 30-minute line to
11 volts, we find that the battery would have to be
rated as a 26 a.h. device if intended for that kind
of service.

HERE'S THE POT HOLE - THE 30 MINUTE DISCHARGE CURVE
OF 26.4 AMPS SAYS THE CAPACITY IS 26.4A X 0.5 HOURS
OR 13.2 A.H., NOT 26 A.H.

I SHOULD HAVE CAUGHT THAT . . . A 33 TO 26 REDUCTION
IN APPARENT CAPACITY FOR A 15X LARGER DISCHARGE
RATE DIDN'T RAISE THE FLAGS. FOUND IT WHILE ARCHIVING
THE POST.

Bob . . .


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brian



Joined: 02 Jan 2006
Posts: 643
Location: Sacramento, California, USA

PostPosted: Fri Feb 24, 2006 5:37 am    Post subject: Odyssey % of charge (Correction) Reply with quote

Robert L. Nuckolls, III wrote:

Quote:
HERE'S THE POT HOLE - THE 30 MINUTE DISCHARGE CURVE
OF 26.4 AMPS SAYS THE CAPACITY IS 26.4A X 0.5 HOURS
OR 13.2 A.H., NOT 26 A.H.

I SHOULD HAVE CAUGHT THAT . . . A 33 TO 26 REDUCTION
IN APPARENT CAPACITY FOR A 15X LARGER DISCHARGE
RATE DIDN'T RAISE THE FLAGS. FOUND IT WHILE ARCHIVING
THE POST.

Welcome to Peukert's exponent. If you know the capacity at two different
discharge rates you can interpolate or extrapolate the capacity off the
battery at other discharge rates.

--
Brian Lloyd 361 Catterline Way
brian-yak at lloyd dot com Folsom, CA 95630
+1.916.367.2131 (voice) +1.270.912.0788 (fax)

I fly because it releases my mind from the tyranny of petty things . . .
- Antoine de Saint-Exupery


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Brian Lloyd
brian-yak at lloyd dot com
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I fly because it releases my mind from the tyranny of petty things . . .
- Antoine de Saint-Exupery
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