<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<meta content="text/html;charset=ISO-8859-1" http-equiv="Content-Type">
<title></title>
</head>
<body bgcolor="#ffffff" text="#000000">
Drake,<br>
<br>
I'll re-word this in an attempt at precision.<br>
<br>
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. That is equivalent to
saying: The amphours that a battery can deliver depends on the
discharge current, and slightly on prior discharge current. There is
some amphour capacity loss associated with high discharge current that
isn't recovered by lowing the discharge current. In the tests
performed by Dennis Doerffel and Suleiman Abu Sharkh it is was in the
five to ten percent range. Five to ten percent is not insignificant,
but for most RE applications where we are trying to avoid deep
discharges and high discharge currents are only a fraction of the time,
it isn't much to be concerned with either. Also it isn't clear from
that study if the capacity loss is related purely to the high discharge
current or if it is caused by high discharge current to a deep
discharge level.<br>
<br>
At high discharge current the terminal voltage is lower. That means
more energy is being dissipated internally, the battery gets hotter,
the energy efficiency is lower. And the battery life is shortened by
both high discharge rates and by deep discharges. I certainly wouldn't
want to treat a set of batteries the way that Doerffel and Sharkh did.<br>
<br>
Kent Osterberg<br>
Blue Mountain Solar, Inc.<br>
<br>
<br>
<br>
Drake Chamberlin wrote:
<blockquote
cite="mid:20100119154257.C3415121CB6@friskymail-a4.g.dreamhost.com"
type="cite">This is a very interesting thread. <br>
<br>
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?<br>
<br>
Thanks,<br>
<br>
Drake <br>
<br>
<br>
At 08:00 AM 1/19/2010, you wrote:<br>
<blockquote type="cite" class="cite" cite="">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.
<br>
<br>
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. <br>
Darryl<br>
<br>
--- On Tue, 1/19/10, Hugh <a class="moz-txt-link-rfc2396E" href="mailto:hugh@scoraigwind.co.uk"><hugh@scoraigwind.co.uk></a> wrote:<br>
<br>
> From: Hugh <a class="moz-txt-link-rfc2396E" href="mailto:hugh@scoraigwind.co.uk"><hugh@scoraigwind.co.uk></a><br>
> Subject: Re: [RE-wrenches] Capacity loss due to rapid discharge<br>
> To: "RE-wrenches"
<a class="moz-txt-link-rfc2396E" href="mailto:re-wrenches@lists.re-wrenches.org"><re-wrenches@lists.re-wrenches.org></a><br>
> Date: Tuesday, January 19, 2010, 3:41 AM<br>
> <br>
> Re: [RE-wrenches] Capacity loss due to<br>
> rapid<br>
> discharge <br>
> Thanks, Kent,<br>
> <br>
> <br>
> It is clear then that high discharge rates have a big<br>
> effect on<br>
> the voltage, but only a small effect on the actual<br>
> battery<br>
> capacity. That was my instinct but it's good to<br>
> have it<br>
> confirmed. And this will be important information for<br>
> those who<br>
> use Peukert's Law 'naively' to conclude that a<br>
> battery has<br>
> dramatically lower capacity when discharged at higher<br>
> rates. (In<br>
> fact if it were true that capacity is reduced to this<br>
> extent it would<br>
> also have very serious implications for battery<br>
> efficiency.)<br>
> <br>
> <br>
> Battery voltage during discharge does depend heavily<br>
> on discharge<br>
> rate, and hence it is hard to assign a voltage that will<br>
> accurately<br>
> represent a 'good' end of discharge where the<br>
> battery will not be<br>
> damaged/worn out/ sulphated. <br>
> <br>
> <br>
> I have worked with Australian inverters that use<br>
> amphour<br>
> measurements to calculate the SOC as well as using voltage<br>
> set-points<br>
> as a back up. This seems laudable on the face of it,<br>
> but the<br>
> added complexity makes it very difficult to interpret why<br>
> the<br>
> generator is still running. (Is it low voltage, low<br>
> amphours,<br>
> high load, favourite time of day for generator, favourite<br>
> time of<br>
> month for a boost charge, failure to synch, manual start,<br>
> etc..... <br>
> ) <br>
> <br>
> <br>
> It's not easy to get an accurate calibration for<br>
> SOC based on<br>
> amphour logging, especially if the battery is wandering<br>
> between 50%<br>
> and 80% SOC over a period. This is the most efficient<br>
> zone of<br>
> operation, so it's not a bad state of affairs per<br>
> se. While it<br>
> is good to get up to full charge periodically, this will<br>
> involve a lot<br>
> of gassing and hence lost amphours/watthours in the<br>
> system. I<br>
> prefer not to do this with fossil fuels unless it seems<br>
> essential to<br>
> the health of the battery.<br>
> <br>
> <br>
> Anyway meantime I do have to assign genstart voltages<br>
> for a<br>
> couple of systems that function automatically, and I will<br>
> probably<br>
> continue to use values around 23.5 volts (11.7, 47 volts)<br>
> for this<br>
> purpose. There's a bit of guesswork involved<br>
> because of the<br>
> unknown current and temperature, but that's a value<br>
> that has worked<br>
> well enough in the past. I am running a poll on my<br>
> blog to see<br>
> what other people choose.<br>
>
<a moz-do-not-send="true" href="http://scoraigwind.blogspot.com/%A0"
eudora="autourl">http://scoraigwind.blogspot.com/ </a> It's<br>
> not a very well defined question, but so far the answers do<br>
> peak at 47<br>
> volts so I am not alone in my choice.<br>
> <br>
> <br>
> Using a voltage rather than SOC for such purposes<br>
> (user guidance,<br>
> genstart, etc) does have the advantage that it combines SOC<br>
> and energy<br>
> use into one parameter. If the user is hitting the<br>
> battery too<br>
> hard for the present conditions then the voltage will tend<br>
> to hit the<br>
> chosen 'danger level' and this can be a warning to<br>
> back off or to<br>
> start the backup.<br>
> <br>
> <br>
> Thanks again for all the input to this thread.<br>
> <br>
> <br>
> best<br>
> <br>
> <br>
> Hugh<br>
> <br>
> <br>
> At 11:47 -0800 18/1/10, Kent Osterberg wrote:<br>
> Hugh,<br>
> <br>
> <br>
> You may be interested in this<br>
> article<br>
> "A critical review of using the Peukert equation for<br>
> determining<br>
> the remaining capacity of lead-acid and lithium-ion<br>
> batteries" by<br>
> Dennis Doerffel and Suleiman Abu Sharkh from the School of<br>
> Engineering<br>
> Sciences, University of Southampton. It is available<br>
> from
<a moz-do-not-send="true"
href="http://www.sciencedirect.com/science" eudora="autourl">http://www.sciencedirect.com/science</a>
for a<br>
> small fee. <br>
> If the link does not work, just search for the lead<br>
> author's last<br>
> name.<br>
> <br>
> <br>
> <br>
> The authors describe testing lead acid batteries at high<br>
> rates of<br>
> discharge from fully discharged down to the point that the<br>
> terminal<br>
> voltage is 10.0 volts. After letting the batteries<br>
> rest, they<br>
> continued to discharge further at a lower rate until the<br>
> terminal<br>
> voltage was again 10.0 volts. Results were compared<br>
> to<br>
> discharging at the slow rate only. The total amphours<br>
> delivered<br>
> when a low discharge rate follows a high discharge were<br>
> less by 5 to<br>
> 10%. With 10% associated with a C2 and C/20 discharge<br>
> of a 17 AH<br>
> battery and 5% associated with a C/1.2 and C/13 discharge<br>
> of a 65 ah<br>
> battery.<br>
> <br>
> <br>
> In short, the capacity loss indicated by Peukert only<br>
> applies to a<br>
> continuous discharge rate. When a slow discharge<br>
> follows a rapid<br>
> discharge, the total number of amphours delivered is almost<br>
> the same<br>
> (just 5 to 10% less) as if the discharge happened at<br>
> entirely at the<br>
> slow rate. If you were estimating how far your<br>
> electric car<br>
> would travel, that 5 or 10% may be critical. For the<br>
> rates of<br>
> discharge and depths of discharge normally used for<br>
> off-grid homes the<br>
> "lost" capacity is probably even less.<br>
> <br>
> <br>
> <br>
> Kent Osterberg<br>
> <br>
> Blue Mountain Solar, Inc.<br>
> <br>
> <br>
> <br>
> <br>
> _______________________________________________<br>
> <br>
> List sponsored by Home Power magazine<br>
> <br>
> <br>
> <br>
> List Address: <a class="moz-txt-link-abbreviated" href="mailto:RE-wrenches@lists.re-wrenches.org">RE-wrenches@lists.re-wrenches.org</a><br>
> <br>
> <br>
> <br>
> Options & settings:<br>
> <br>
>
<a moz-do-not-send="true"
href="http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org"
eudora="autourl">http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org</a>
<br>
> <br>
> <br>
> <br>
> List-Archive:<br>
>
<a moz-do-not-send="true"
href="http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org"
eudora="autourl">http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org</a><br>
> <br>
> <br>
> <br>
> List rules & etiquette:<br>
> <br>
>
<a moz-do-not-send="true"
href="http://www.re-wrenches.org/etiquette.htm" eudora="autourl">www.re-wrenches.org/etiquette.htm</a><br>
> <br>
> <br>
> <br>
> Check out participant bios:<br>
> <br>
>
<a moz-do-not-send="true" href="http://www.members.re-wrenches.org/"
eudora="autourl">www.members.re-wrenches.org</a><br>
> <br>
> <br>
> <br>
> <br>
> -- <br>
> <br>
> Hugh Piggott<br>
> <br>
> <br>
> <br>
> Scoraig Wind Electric<br>
> <br>
> Scotland<br>
> <br>
>
<a moz-do-not-send="true" href="http://www.scoraigwind.co.uk/"
eudora="autourl">http://www.scoraigwind.co.uk</a><br>
> <br>
> <br>
> -----Inline Attachment Follows-----<br>
> <br>
> _______________________________________________<br>
> List sponsored by Home Power magazine<br>
> <br>
> List Address: <a class="moz-txt-link-abbreviated" href="mailto:RE-wrenches@lists.re-wrenches.org">RE-wrenches@lists.re-wrenches.org</a><br>
> <br>
> Options & settings:<br>
>
<a moz-do-not-send="true"
href="http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org"
eudora="autourl">http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org</a>
<br>
> <br>
> List-Archive:
<a moz-do-not-send="true"
href="http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org"
eudora="autourl">http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org</a><br>
> <br>
> List rules & etiquette:<br>
>
<a moz-do-not-send="true"
href="http://www.re-wrenches.org/etiquette.htm" eudora="autourl">www.re-wrenches.org/etiquette.htm</a><br>
> <br>
> Check out participant bios:<br>
>
<a moz-do-not-send="true" href="http://www.members.re-wrenches.org/"
eudora="autourl">www.members.re-wrenches.org</a><br>
> <br>
> <br>
<br>
<br>
<br>
_______________________________________________<br>
List sponsored by Home Power magazine<br>
<br>
List Address: <a class="moz-txt-link-abbreviated" href="mailto:RE-wrenches@lists.re-wrenches.org">RE-wrenches@lists.re-wrenches.org</a><br>
<br>
Options & settings:<br>
<a moz-do-not-send="true"
href="http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org"
eudora="autourl">
http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org</a>
<br>
<br>
List-Archive:
<a moz-do-not-send="true"
href="http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org"
eudora="autourl">http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org</a><br>
<br>
List rules & etiquette:<br>
<a moz-do-not-send="true"
href="http://www.re-wrenches.org/etiquette.htm" eudora="autourl">
www.re-wrenches.org/etiquette.htm</a><br>
<br>
Check out participant bios:<br>
<a moz-do-not-send="true" href="http://www.members.re-wrenches.org/"
eudora="autourl">
www.members.re-wrenches.org</a></blockquote>
<x-sigsep></x-sigsep>
<p><font color="#0000ff">Drake Chamberlin<br>
Athens Electric<br>
OH License 44810<br>
CO License 3773<br>
NABCEP </font><font color="#0000ff" size="1">TM
</font><font color="#0000ff">Certified PV Installer <br>
Office - 740-448-7328<br>
Mobile - 740-856-9648 </font></p>
<pre wrap="">
<hr size="4" width="90%">
_______________________________________________
List sponsored by Home Power magazine
List Address: <a class="moz-txt-link-abbreviated" href="mailto:RE-wrenches@lists.re-wrenches.org">RE-wrenches@lists.re-wrenches.org</a>
Options & settings:
<a class="moz-txt-link-freetext" href="http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org">http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org</a>
List-Archive: <a class="moz-txt-link-freetext" href="http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org">http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org</a>
List rules & etiquette:
<a class="moz-txt-link-abbreviated" href="http://www.re-wrenches.org/etiquette.htm">www.re-wrenches.org/etiquette.htm</a>
Check out participant bios:
<a class="moz-txt-link-abbreviated" href="http://www.members.re-wrenches.org">www.members.re-wrenches.org</a>
</pre>
<pre wrap="">
<hr size="4" width="90%">
No virus found in this incoming message.
Checked by AVG - <a class="moz-txt-link-abbreviated" href="http://www.avg.com">www.avg.com</a>
Version: 9.0.730 / Virus Database: 270.14.149/2630 - Release Date: 01/17/10 23:35:00
</pre>
</blockquote>
</body>
</html>