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One thing to consider is that flooded batteries almost always have a
slight short to ground. Its in the milliamps, but its there. It
runs through the conductive acid mist that coats the outsides of the
batteries. So connecting the battery bank negatives together could
cause some current flow between the banks even with the switch off.
I found this out the hard way once with an ungrounded (vehicle)
battery bank that shocked me when I touched the chassis and the
battery at the same time. <br>
I went to sealed batteries in that case and cured the problem.<br>
<br>
Generally I'm not a fan of connecting new and old batteries in any
way. I have used a battery isolator on RVs to allow the alternator
to charge both batteries, but it never allows them to be connected
together. The isolator is essentially a pair of large diodes wired
to allow current flow to each battery, but no flow from either
battery. Again I probably would never do that on anything but an
RV.<br>
Consider also that you could have some volt drop at high current
across the switch, too.<br>
If you like your new bank lose the switch, sell the old bank to a
neighbor, or use it on a separate system.<br>
<br>
Ray Walters<br>
<br>
<br>
<div class="moz-cite-prefix">On 10/11/2012 5:51 PM, SunHarvest
wrote:<br>
</div>
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<div><font face="Arial" size="2">Thanks Allan,</font></div>
<div> </div>
<div><font face="Arial" size="2">I always value your insight.
There is a TriMetric on this system but I'm looking at the
FM80. I wrestled with the idea that the systems are connected
as the circuits between the two banks are not complete.
However, as soon as I made the connections the voltage on the
new bank dropped and the old bank "seemed" to be rejuvenated.
Yes, it could simply be an issue of I didn't witness a "drop"
but before and after voltage was off by about 2 volts (down
from 25 to 23) according to the charge controller. Not a lot
but enough to raise an eyebrow.</font></div>
<div> </div>
<div><font face="Arial" size="2">Eric</font></div>
<blockquote style="BORDER-LEFT: #000000 2px solid; PADDING-LEFT:
5px; PADDING-RIGHT: 0px; MARGIN-LEFT: 5px; MARGIN-RIGHT: 0px">
<div style="FONT: 10pt arial">----- Original Message ----- </div>
<div style="FONT: 10pt arial; BACKGROUND: #e4e4e4; font-color:
black"><b>From:</b> <a moz-do-not-send="true"
title="allan@positiveenergysolar.com"
href="mailto:allan@positiveenergysolar.com">Allan Sindelar</a>
</div>
<div style="FONT: 10pt arial"><b>To:</b> <a
moz-do-not-send="true"
title="re-wrenches@lists.re-wrenches.org"
href="mailto:re-wrenches@lists.re-wrenches.org">RE-wrenches</a>
</div>
<div style="FONT: 10pt arial"><b>Sent:</b> Thursday, October 11,
2012 4:00 PM</div>
<div style="FONT: 10pt arial"><b>Subject:</b> Re: [RE-wrenches]
Marine battery bank switch</div>
<div><br>
</div>
<div class="moz-cite-prefix">Eric,<br>
It still doesn't make sense to me. Unless you have a complete
circuit, the banks are not in parallel, and your switch on the
positives prevents a complete circuit of both at the same
time. Common negatives should not be an issue - after all, in
a standard off grid system, the DC - and the AC neutral are
common to the same ground rod and thus equipment bonding. A
Trimetric TM2025 has a second battery voltage sense function
that requires the negatives be common; mine tells me the
voltage of my main 24V system and my 12V generator starting
battery, and all works well.<br>
<br>
So I question your perception that one is stronger and the
other weaker. How do you perceive this new difference, and
could it simply be a error in perception? One issue with a
switch on two banks is that you can't use a single amp-hour
meter, such as a TriMetric. So I would tend to place less
value on subjective perception without the metrics to back it
up. <br>
Allan<br>
<br>
<div class="moz-signature"><font face="Times New Roman, Times,
serif"><b>Allan Sindelar</b></font><br>
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<span style="FONT-SIZE: 10pt">NABCEP Certified
Photovoltaic Installer<br>
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Founder and Chief Technology Officer<br>
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<b>505 424-1112</b><br>
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On 10/11/2012 4:23 PM, SunHarvest wrote:<br>
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<div><font face="Arial" size="2">I recently installed a Blue
Sea 4-way marine battery switch to switch between two
battery banks, one new bank and one old bank. I was
hesitant to connect the negative home-runs from both banks
onto one common bus (connected directly to a ground rod),
which then connects back to the DC load center at the
inverter. But that's how I was advised to wire it by an
electrician friend. </font></div>
<div> </div>
<div><font face="Arial" size="2">It appears now that the newer
bank was performing quite a bit better prior to being
connected to the marine switch (and neg lead to the neg
bus) and the old bank is now performing better than
before. I think my electrician buddy was incorrect and
that even the positive leads being on a switch and neg
lead on a neg bus, the banks are still, to some extent,
effectively paralleled and that the new bank is
being pulled down by being indirectly connected to the old
bank.</font></div>
<div> </div>
<div><font face="Arial" size="2">Thoughts and advice on how to
correct the wiring if needed?</font></div>
<div> </div>
<div><font face="Arial" size="2">Sorry to ask such a simple
question, I'm just a little rusty on off-grid.</font></div>
<div> </div>
<div><font face="Arial" size="2">Thanks guys.</font></div>
<div> </div>
<div><font face="Arial" size="2">Eric Stikes</font></div>
<div><font face="Arial" size="2">SunHarvest Solar</font></div>
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