At 09:34 -0800 19/1/10, Kent Osterberg wrote:
The Peukert capacity equation gives a "pretty good" estimate of the
total amphours available at the present discharge current,
regardless of what the discharge current was in the past.
In other words it tells you the 'capacity' at a particular load
current based on a fixed terminal voltage (end of discharge cut-off).
It does not tell you about all of the extra amphours you can still
get using a lower current and the same terminal voltage.
I certainly wouldn't want to treat a set of batteries the way that
Doerffel and Sharkh did.
Nor me and it is a pity that the trials did not focus more on partial
discharge situations. But what's interesting is that Peukert only
works at constant discharge rates.
At 03:39 -0700 19/1/10, Warren Lauzon wrote:
Some of the loss is in heat due to internal resistance, and that
cannot be recovered.
Of course. But in this debate we are discussing amphours and the
effect of rapid discharge on the amphours that a battery can deliver.
I was concerned to find out about missing amphours and to estimate
how many are lost and where they went. According to Peukert (as it
is normally interpreted) you lose big chunks of the amphours capacity
by rapidly discharging the battery. According to the Doerffel and
Sharkh data, only 5-10% of the amphours are lost in a high discharge
use of the battery. This is way less than Peukert's law would
predict.
At 10:45 -0500 19/1/10, Drake Chamberlin wrote:
What I think I understand is that if a battery is discharged
completely at a high discharge rate, a significant number of amp
hours will be lost due to heat and inefficiency, and Peukert's
equation will approximate the loss.
Most of the heat is due to the voltage loss. Some of it may be due
to the amphours loss. But in the end Peukert only tells us how many
amphours you can get at a fixed discharge rate and based on a chosen
terminal voltage. It does not actually tell you how much of the
battery capacity is used nor how much is still available under
conditions of slower discharge.
Naive use of Peukert over-estimates the DOD caused by high discharge
currents. Peukert can be correctly used to predict how many amphours
you can get from a battery at a constant charge rate before it
reaches a certain terminal voltage. It does not tell you the SOC of
a battery. This last is a very hard thing to ascertain.
:-)
--
Hugh Piggott
Scoraig Wind Electric
Scotland
http://www.scoraigwind.co.uk
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