[RE-wrenches] Capacity loss due to rapid discharge
Darryl Thayer
daryl_solar at yahoo.com
Tue Jan 19 05:00:02 PST 2010
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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