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Hi Allan;<br>
<br>
This is an interesting thread, as all of us ( I think) have gone
heavier on the solar arrays and lighter on the battery banks based
on economics. I too have noticed higher gassing and more water
consumption, and I too have adjusted set points down slightly to
account for this. I think that it is really too early to tell what
this will mean for battery life. While the batteries will be
reaching full charge more often as you noted, they also will be
cycled more deeply on a regular basis. I also have concerns with
acid loss, and over heating; both of which lead to shorter battery
life. <br>
Over the years I've actually found that my systems with larger
battery banks lasted a long time because daily cycles were
shallower, but as you noted it depends on whether the user makes
sure they reach full charge regularly. Without proper management,
heavy banks can be stuck in Purgatory: never fully discharged, but
never fully charged either.<br>
Meanwhile small battery banks are being charged and discharged at
much higher rates, which is both less efficient and creates
additional heat.<br>
With this design trend to smaller battery banks, I think we all need
to revisit our thermal management strategies as well. The old days
of super insulating the battery box can actually lead to serious
battery over heating. Midnite Solar is looking at adding charge
disconnect programming based on the temp sensor. Basically, most
batteries need to be completely disconnected from a charge source at
battery temps over 105F. Cycle life deteriorates rapidly at
temperatures over 85F.<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>
<div class="moz-cite-prefix">On 5/21/2015 11:56 AM, Allan Sindelar
wrote:<br>
</div>
<blockquote cite="mid:555E1C3F.40503@sindelarsolar.com" type="cite">
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Wrenches,<br>
In the last few years I have changed my approach to off grid
system sizing, primarily due to higher battery costs and lower
module costs. Rather than shooting for a traditional 80-90% of
winter load profile target, I will size a larger array and smaller
battery bank, figuring (especially here in the sunny Southwest)
that batteries will recharge more quickly, will spend more of
their time full (and thus last longer), and will be at least
partially charged during cloudy weather. This even allows for
smaller battery banks that are cycled more deeply (as long as
there's a backup gennie in the mix); the lower cost of a smaller
size may allow for a set of higher-quality industrial cells.<br>
<br>
I first began to rethink my design approach after reading
Christopher Freitas' Solar Pro article on large battery banks a
few years ago. Being able to refill in a day was part of his
strategic approach to battery bank design. I used and wrote about
this approach in my tiny house article in the current Home Power.
However, I'm just beginning to get owner feedback about greater
water consumption and the need for more frequent watering. This is
especially the case when I add array to an existing system, and
the old watering schedule is suddenly not enough. <br>
<br>
When I design new systems I now encourage larger arrays and
smaller battery banks, as the batteries spend more of their time
happily full and recover more quickly after a discharge period.
But I am thinking that I will have to begin changing charging
setpoints in response. Flooded batteries have a range of
recommended bulk setpoints, from 28.8 to 29.6 volts on a 24V
system. When arrays were smaller we set to the higher end of the
range to take advantage of topping off when available, and a two
hour or longer absorption was considered beneficial, as long as
the CC could transition to float at 2% of capacity or so. With a
larger array a better approach may be to lower the bulk/absorption
setting, as well as reduce absorption time and raise the float
transition current ("done amps"). <br>
<br>
I'm writing to encourage a discussion among off grid Wrenches. Are
you changing the relative relationship between array and battery
capacities? If so, how are you adjusting charge controller
setpoint programming? For example, how would you program a Midnite
Classic, Outback FM or Schneider XW if you could achieve a C/10 or
C/15 charge rate?<br>
<br>
Thanks, Allan<br>
<div class="moz-signature">
<div class="WordSection1">
<p class="MsoNormal"><b>Allan Sindelar</b><br>
<span style="font-size:10.0pt"><a moz-do-not-send="true"
href="mailto:allan@sindelarsolar.com">allan@sindelarsolar.com</a></span><br>
<span style="font-size:10.0pt">NABCEP Certified PV
Installation Professional<br>
NABCEP Certified Technical Sales Professional<br>
New Mexico EE98J Journeyman Electrician<br>
Founder (Retired), <span
style="mso-bidi-font-weight:bold">Positive Energy, Inc.</span><br>
<b>505 780-2738 cell</b><br
style="mso-special-character:line-break">
<br style="mso-special-character:line-break">
<b><o:p></o:p></b></span></p>
<p class="MsoNormal"><o:p> </o:p><br>
</p>
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