[RE-wrenches] Capacity loss due to rapid discharge

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.
> 
> 
> 
> 
> _______________________________________________
> 
> List sponsored by Home Power magazine
> 
> 
> 
> List Address: RE-wrenches at lists.re-wrenches.org
> 
> 
> 
> Options & settings:
> 
> http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org
> 
> 
> 
> List-Archive:
> http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org
> 
> 
> 
> List rules & etiquette:
> 
> www.re-wrenches.org/etiquette.htm
> 
> 
> 
> Check out participant bios:
> 
> www.members.re-wrenches.org
> 
> 
> 
> 
> -- 
>  
> Hugh Piggott
> 
> 
> 
> Scoraig Wind Electric
> 
> Scotland
> 
> http://www.scoraigwind.co.uk
>  
> 
> -----Inline Attachment Follows-----
> 
> _______________________________________________
> List sponsored by Home Power magazine
> 
> List Address: RE-wrenches at lists.re-wrenches.org
> 
> Options & settings:
> http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org
> 
> List-Archive: http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org
> 
> List rules & etiquette:
> www.re-wrenches.org/etiquette.htm
> 
> Check out participant bios:
> www.members.re-wrenches.org
> 
> 