[RE-wrenches] Capacity loss due to rapid discharge

[Drake Chamberlin]

This is a very interesting thread.

A lot has been said.  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.  If the discharge rate 
is high at times and low at others, the amp hour loss will not be 
very much.  Does this seem correct?

Thanks,

Drake


At 08:00 AM 1/19/2010, you wrote:
>Thanks Hugh and all who contributed to this thread.  It has been 
>most educational.  Since a battery plate will dissolve in water it 
>seems that there should be a min voltage under fast discharge that 
>should be avoided.
>
>The loss of SOC accuracy comes from charging and watching the 
>voltage while charging.  And if a person charges slowly the best 
>accuracy is maintained.
>Darryl
>
>--- On Tue, 1/19/10, Hugh <hugh at scoraigwind.co.uk> wrote:
>
> > From: Hugh <hugh at scoraigwind.co.uk>
> > Subject: Re: [RE-wrenches] Capacity loss due to rapid discharge
> > To: "RE-wrenches" <re-wrenches at lists.re-wrenches.org>
> > Date: Tuesday, January 19, 2010, 3:41 AM
> >
> > Re: [RE-wrenches] Capacity loss due to
> > rapid
> > discharge
> > Thanks, Kent,
> >
> >
> > It is clear then that high discharge rates have a big
> > effect on
> > the voltage, but only  a small effect on the actual
> > battery
> > capacity.  That was my instinct but it's good to
> > have it
> > confirmed.  And this will be important information for
> > those who
> > use Peukert's Law 'naively' to conclude that a
> > battery has
> > dramatically lower capacity when discharged at higher
> > rates.  (In
> > fact if it were true that capacity is reduced to this
> > extent it would
> > also have very serious implications for battery
> > efficiency.)
> >
> >
> > Battery voltage during discharge does depend heavily
> > on discharge
> > rate, and hence it is hard to assign a voltage that will
> > accurately
> > represent a 'good' end of discharge where the
> > battery will not be
> > damaged/worn out/ sulphated.
> >
> >
> > I have worked with Australian inverters that use
> > amphour
> > measurements to calculate the SOC as well as using voltage
> > set-points
> > as a back up.  This seems laudable on the face of it,
> > but the
> > added complexity makes it very difficult to interpret why
> > the
> > generator is still running.  (Is it low voltage, low
> > amphours,
> > high load, favourite time of day for generator, favourite
> > time of
> > month for a boost charge, failure to synch, manual start,
> > etc.....
> > )
> >
> >
> > It's not easy to get an accurate calibration for
> > SOC based on
> > amphour logging, especially if the battery is wandering
> > between 50%
> > and 80% SOC over a period.  This is the most efficient
> > zone of
> > operation, so it's not a bad state of affairs per
> > se.  While it
> > is good to get up to full charge periodically, this will
> > involve a lot
> > of gassing and hence lost amphours/watthours in the
> > system.  I
> > prefer not to do this with fossil fuels unless it seems
> > essential to
> > the health of the battery.
> >
> >
> > Anyway meantime I do have to assign genstart voltages
> > for a
> > couple of systems that function automatically, and I will
> > probably
> > continue to use values around 23.5 volts (11.7, 47 volts)
> > for this
> > purpose.  There's a bit of guesswork involved
> > because of the
> > unknown current and temperature, but that's a value
> > that has worked
> > well enough in the past.  I am running a poll on my
> > blog to see
> > what other people choose.
> > http://scoraigwind.blogspot.com/  It's
> > not a very well defined question, but so far the answers do
> > peak at 47
> > volts so I am not alone in my choice.
> >
> >
> > Using a voltage rather than SOC for such purposes
> > (user guidance,
> > genstart, etc) does have the advantage that it combines SOC
> > and energy
> > use into one parameter.  If the user is hitting the
> > battery too
> > hard for the present conditions then the voltage will tend
> > to hit the
> > chosen 'danger level' and this can be a warning to
> > back off or to
> > start the backup.
> >
> >
> > Thanks again for all the input to this thread.
> >
> >
> > best
> >
> >
> > Hugh
> >
> >
> > At 11:47 -0800 18/1/10, Kent Osterberg wrote:
> > Hugh,
> >
> >
> > You may be interested in this
> > article
> > "A critical review of using the Peukert equation for
> > determining
> > the remaining capacity of lead-acid and lithium-ion
> > batteries" by
> > Dennis Doerffel and Suleiman Abu Sharkh from the School of
> > Engineering
> > Sciences, University of Southampton.  It is available
> > from http://www.sciencedirect.com/science for a
> > small fee.
> > If the link does not work, just search for the lead
> > author's last
> > name.
> >
> >
> >
> > The authors describe testing lead acid batteries at high
> > rates of
> > discharge from fully discharged down to the point that the
> > terminal
> > voltage is 10.0 volts.  After letting the batteries
> > rest, they
> > continued to discharge further at a lower rate until the
> > terminal
> > voltage was again 10.0 volts.  Results were compared
> > to
> > discharging at the slow rate only.  The total amphours
> > delivered
> > when a low discharge rate follows a high discharge were
> > less by 5 to
> > 10%.  With 10% associated with a C2 and C/20 discharge
> > of a 17 AH
> > battery and 5% associated with a C/1.2 and C/13 discharge
> > of a 65 ah
> > battery.
> >
> >
> > In short, the capacity loss indicated by Peukert only
> > applies to a
> > continuous discharge rate.  When a slow discharge
> > follows a rapid
> > discharge, the total number of amphours delivered is almost
> > the same
> > (just 5 to 10% less) as if the discharge happened at
> > entirely at the
> > slow rate.  If you were estimating how far your
> > electric car
> > would travel, that 5 or 10% may be critical.  For the
> > rates of
> > discharge and depths of discharge normally used for
> > off-grid homes the
> > "lost" capacity is probably even less.
> >
> >
> >
> > Kent Osterberg
> >
> > Blue Mountain Solar, Inc.
> >
> >
> >
> >
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> >
> > --
> >
> > Hugh Piggott
> >
> >
> >
> > Scoraig Wind Electric
> >
> > Scotland
> >
> > http://www.scoraigwind.co.uk
> >
> >
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Drake Chamberlin
Athens Electric
OH License 44810
CO License 3773
NABCEP TM  Certified PV Installer
Office - 740-448-7328
Mobile - 740-856-9648  
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