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?_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>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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