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William,<br>
Here I am repeating the RE equivalent of GAAP, Generally Accepted
Accounting Principles, as applied to battery sizing...<br>
<br>
A battery is considered empty at 10.75V (12V), or 1.79 VPC. When you
test a battery's capacity, it's generally down to this point. How
many amp-hours that is, and what actual percentage that represents,
depends on several factors: C/rate (Peukert's Exponent),
temperature, programmed charge/discharge efficiency, etc. It's at
best a fuzzy number.<br>
<br>
The 10.75V is the historic rule of thumb that represents "empty" - I
suppose that would be 100% DoD. I don't know then what 80% DoD would
be - perhaps 11.4V (1.9 VPC) or so?<br>
<br>
I believe that Windy followed these principles in designing his
sizing spreadsheet in the 1980s: the historic rule was, and still
is, "don't ever discharge below 20% SoC (80% DoD) or some permanent
damage will result".<br>
<br>
This rule translates on the spreadsheet to [(calculated amp-hours
needed)/0.80 max DoD = rated capacity of battery bank in amp-hours
(at the C/20 rate)].<br>
<br>
Does that clarify anything?<br>
Allan<br>
<br>
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<font face="Times New Roman, Times, serif"><b>Allan Sindelar</b></font><br>
<small><a href="mailto:Allan@positiveenergysolar.com"><font
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<span style="font-size: 10pt;">NABCEP Certified Photovoltaic
Installer<br>
NABCEP Certified Technical Sales Professional<br>
New Mexico EE98J Journeyman Electrician<br>
<b>Positive Energy, Inc.</b><br>
3201 Calle Marie<br>
Santa Fe, New Mexico 87507<br>
<b>505 424-1112</b><br>
<a href="http://www.positiveenergysolar.com/" target="_blank"><u>www.positiveenergysolar.com</u></a><o:p></o:p></span></font>
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<br>
On 3/25/2011 5:40 PM, William Miller wrote:
<blockquote
cite="mid:6.0.1.1.2.20110325163640.0356cec0@millersolar.com"
type="cite">
<font size="3">Allan:<br>
<br>
Is it possible to discharge a battery to 80%. What is the
battery
voltage one would expect with that DOD?<br>
<br>
William<br>
<br>
<br>
<br>
At 02:28 PM 3/25/2011, you wrote:<br>
</font>
<blockquote type="cite" class="cite" cite=""><font size="3">William,<br>
I use 80%, as the intent is that this is maximum, not typical.
It's
directly used to calculate the battery capacity on the
spreadsheet. 50%
is more of a target occasional DoD, but it's not a maximum.<br>
<br>
In the sunny Southwest where Windy and I are planted, that's
pretty safe,
as multiple days without sun happen, but not frequently. It
may be less
so in Atascadero.<br>
Allan<br>
<br>
</font><font size="3" face="Times New Roman, Times"><b>Allan
Sindelar</b></font><br>
<font size="2" color="#000099" face="Times New Roman, Times"><u><a
moz-do-not-send="true"
href="mailto:Allan@positiveenergysolar.com">Allan@positiveenergysolar.com</a></u></font><font
size="3" face="Times New Roman, Times"><br>
NABCEP Certified Photovoltaic Installer<br>
NABCEP Certified Technical Sales Professional<br>
New Mexico EE98J Journeyman Electrician<br>
<b>Positive Energy, Inc.</b><br>
3201 Calle Marie<br>
Santa Fe, New Mexico 87507<br>
<b>505 424-1112</b><br>
<u><a moz-do-not-send="true"
href="http://www.positiveenergysolar.com/">www.positiveenergysolar.com</a></u></font>
<br>
<br>
<b><br>
</b><br>
<br>
On 3/25/2011 1:48 PM, William Miller wrote: <br>
<blockquote type="cite" class="cite" cite="">Friends:<br>
<br>
I am using a modified version of the Dankoff load chart (with
credit given) to calculate off-grid loads. There is a section
where you enter the maximum depth of discharge (DOD) for a
battery bank. It is my understanding that the maximum DOD for
a lead acid battery bank is 50%.<br>
<br>
Is this correct in concept?<br>
<br>
Is this a correct value to enter in this cell?<br>
<br>
Thanks,<br>
<br>
William Miller<br>
<br>
PS: I called Ronald Parades at Trojan before this e-mail was
sent and here is what he said: He recommends 20%-30% DOD.
Any greater DOD results in shorter battery life. He noted
that in many approaches a design criteria of multi-day
autonomy which is rarely or never used actually calculates
into the final AH result as a virtual decrease in DOD.<br>
</blockquote>
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