[RE-wrenches] Capacity loss due to rapid discharge

[Hugh]

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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