<!doctype html public "-//W3C//DTD W3 HTML//EN">
<html><head><style type="text/css"><!--
blockquote, dl, ul, ol, li { padding-top: 0 ; padding-bottom: 0 }
--></style><title>Re: [RE-wrenches] Capacity loss due to rapid
discharge</title></head><body>
<div>Thanks, Kent,</div>
<div><br></div>
<div>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.)</div>
<div><br></div>
<div>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. </div>
<div><br></div>
<div>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.....
) </div>
<div><br></div>
<div>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.</div>
<div><br></div>
<div>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.</div>
<div><br></div>
<div>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.</div>
<div><br></div>
<div>Thanks again for all the input to this thread.</div>
<div><br></div>
<div>best</div>
<div><br></div>
<div>Hugh</div>
<div><br></div>
<div>At 11:47 -0800 18/1/10, Kent Osterberg wrote:</div>
<blockquote type="cite" cite>Hugh,<br>
</blockquote>
<blockquote type="cite" cite>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 <a
href=
"http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6TH1-4GG2J4C-8&_user=10&_coverDate=04%2F21%2F2006&_rdoc=1&_fmt=high&_orig=browse&_sort=d&view=c&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=2b8498cbb0415fe6dda0e003c6a23598"><span
></span>http://www.sciencedirect.com/science</a> for a small fee.
If the link does not work, just search for the lead author's last
name.<br>
<br>
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.</blockquote>
<blockquote type="cite" cite><br>
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.<br>
<br>
Kent Osterberg<br>
Blue Mountain Solar, Inc.<br>
</blockquote>
<blockquote type="cite" cite><br>
_______________________________________________<br>
List sponsored by Home Power magazine<br>
<br>
List Address: RE-wrenches@lists.re-wrenches.org<br>
<br>
Options & settings:<br>
http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org<br
>
<br>
List-Archive:
http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org<br>
<br>
List rules & etiquette:<br>
www.re-wrenches.org/etiquette.htm<br>
<br>
Check out participant bios:<br>
www.members.re-wrenches.org</blockquote>
<div><br></div>
<div><br></div>
<x-sigsep><pre>--
</pre></x-sigsep>
<div>Hugh Piggott<br>
<br>
Scoraig Wind Electric<br>
Scotland<br>
http://www.scoraigwind.co.uk</div>
</body>
</html>