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<div class="moz-cite-prefix">Hi William;<br>
<br>
One thing to consider is that you want to cycle the batteries at
least 20% DOD. If you look at the cycle charts for regular Lead
acid batteries, the total KWH you can get from a battery for its
lifetime is fairly equal from around 20% DOD to 80% DOD.
(basically you can get twice the cycles at half the discharge
rate) However this nice linear relationship goes out at the ends
(over 80% DOD, or less than 20% DOD) You just don't get your
money's worth cycling deep cycle batteries at very shallow depths.<br>
Another consideration is that it is very inefficient to charge a
battery completely with generator power. The last half of the
absorption phase is at fairly low current for fairly long time
periods. You should have the genny shut down when its efficiency
gets low (might be less than 25% of rated capacity, depends on
the generator)<br>
My basic design philosophy is to not cycle the bank more than 50%,
but have the generator shut off once the total watts (charging +
loads - PV ) drops below a certain threshold. Also having the
generator kick in when loads exceed a certain discharge rate (say
C20) is important too. If they discharge at higher rates, you
won't have your design amp hours because of the Puekert's
exponent. (basically: batteries have less capacity at high
discharge rates)<br>
Last point, I don' think its reasonable to expect the system to
reach full charge every day, I'd shoot for once a week, and save
a bunch of fuel. Deep cycle batteries can sit at less than full
charge for a couple of weeks before the lead sulfate starts
crystallizing into a non-disolvable form. In this case, you want
a balance of fuel and generator run time vs battery life. <br>
I have a large system like this that the batteries have already
outlived the generator, so that's probably not exactly optimum.
I've since starting using smaller battery banks, also because
batteries have gone up in price.<br>
For these types of designs, I find software like PV Design Pro to
be very useful. It can characterize the total % of energy from
PV, as well as model an hourly basis of the system operation over
an average weather year. You still have to ride herd on software
simulations, as they can turn out some pretty stupid designs if
you're not careful.<br>
<pre class="moz-signature" cols="72">R.Ray Walters
CTO, Solarray, Inc
Nabcep Certified PV Installer,
Licensed Master Electrician
Solar Design Engineer
303 505-8760</pre>
On 8/16/2013 11:53 AM, William Miller wrote:<br>
</div>
<blockquote cite="mid:FA13B38DE3B94D0D896108247346F91C@Bob"
type="cite">
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<div><span class="353174017-16082013"><font face="Arial" size="4">Friends:</font></span></div>
<div><span class="353174017-16082013"></span> </div>
<div><span class="353174017-16082013"><font face="Arial" size="4">We
have been receiving a flurry of requests for design of
off-grid systems where the loads are enormous, relative to
normal off-grid requirements. It is obvious that the size
of PV and battery arrays to power these loads is beyond what
is practical. We have dubbed these systems
Generator/battery with PV assist, or PV assist for short.</font></span></div>
<div><span class="353174017-16082013"></span> </div>
<div><span class="353174017-16082013"><font face="Arial" size="4">These
systems will cycle batteries multiple times per day. It is
my understanding that the extra battery cycels will shorten
battery life. Since generator run is expected, we are
willing to increase generator run time in order to prolong
battery life.</font></span></div>
<div><span class="353174017-16082013"></span> </div>
<div><span class="353174017-16082013"><font face="Arial" size="4">The
parameters I suggest are an aggressive load start and a very
high battery start parameter. This will prevent deep
discharge of the battery bank. I know shallow discharge is
not the norm, but I don't believe shallow cycling is a
problem as long is it as at the top of the voltage window,
i.e. the batteries achieve absorption at least once per day.</font></span></div>
<div><span class="353174017-16082013"></span> </div>
<div><span class="353174017-16082013"><font face="Arial" size="4">We
know a load start ciuld result in an abbreviated run time,
curtailing absorption period, but we are assuming there will
be a battery votage start at least once per day allowing
full absorption.</font></span></div>
<div><span class="353174017-16082013"></span> </div>
<div><span class="353174017-16082013"><font face="Arial" size="4">Has
anyone else considered these issues, and what conclusions
did you make</font></span></div>
<div><span class="353174017-16082013"></span> </div>
<div><span class="353174017-16082013"><font face="Arial" size="4">I
am looking forward to a spirited discussion as usual. I
throuroughlyenjoy and beneift from them, even thought
the advice is usually conflilcting. Thanks in advance.</font></span></div>
<div><span class="353174017-16082013"></span> </div>
<div><span class="353174017-16082013"><font face="Arial" size="4">William
Miller</font></span></div>
<div><span class="353174017-16082013"></span> </div>
<br>
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