From: Lee Hart
Just be aware that this is not a true resistor, and not constant. It is
the combined net effect of all the internal processes; mechanical,
chemical, state of charge, temperature, aging, etc.
Jan Steinman via EV wrote:
Which is where Mr. Peukert came in the door, right?
Pretty much. :-) All he did was measure what batteries actually did,
without regard to *why* they did them. When he graphed the data on log
paper, he got a (more or less) straight line. Peukert's equation just
describes this line:
Pc = I^Pe x T
where: Pc is the Peukert capacity (amphours at a 1-amp discharge rate)
I is the desired discharge current (in amps)
Pe is the Peukert exponent (the number we are arguing about)
T is the discharge time (in hours)
Pc is roughly a battery's 20-hour amphour capacity (because capacity
hardly changes if you discharge it at a 1 amp rate). Pe is close to 1.1
for any practical EV battery. Once you know Pc and Pe, the equation lets
you predict the apparent capacity at other currents.
Example: You have a 100ah battery. Testing comes up with Pe = 1.1 and
Pc=110ah (the capacity you'd get if discharged at 1 amp). What is its
capacity if you discharge it at 100 amps? Plug in the numbers and solve
for T:
Pc = I^Pe x T
110 = (100)^1.1 x T
110 = 158.5 x T
T = 110/158.5 = 0.694 hours
100a x 0.694h = 69.4ah <-- this is its apparent capacity at 100a
You can curve-fit this equation to *any* battery, and it will predict
over at least a 100:1 range with reasonable accuracy.
--
The storage battery is one of those peculiar things which appeals to
the imagination, and no more perfect thing could be desired by stock
swindlers. Just as soon as a man gets working on the secondary battery,
it brings out his latent capacity for lying. -- Thomas A. Edison
--
Lee Hart's EV projects are at http://www.sunrise-ev.com/LeesEVs.htm
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