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Wrenches,<br>
I need a bit of help here if you have it. Since 2002 we have
installed somewhere between 30 and 35 systems with sealed batteries
installed in manufactured enclosures, originally Outback enclosures
and in recent years Midnite MNBE enclosures. At least ten of these
have been indoors in one form or another - usually a laundry or
mechanical room. Our battery of choice is Concorde SunXtender. We
have only added mechanical ventilation (Zephyr Power-Vent to
outside) if the battery enclosure itself is sealed. Nearly all of
these have been permitted and inspected systems, and we have never
had a problem with the inspectors. Of course, we always vent flooded
systems to the outside, nearly always using a Power Vent fan.<br>
<br>
Now we have. An AHJ failed a system for lack of ventilation, and our
attempts to resolve it have not been effective. The Chief Electrical
Inspector has weighed in, and we are right at the point of filing a
Request for Code Interpretation with the New Mexico Electrical
Division Technical Advisory Panel. <br>
<br>
I have not wanted to just add ventilation to pass inspection because
of the precedent doing so is likely to set for future installations.
The GC on the job supports my attempts to push back, as do the
homeowners. The Chief Inspector thinks that the 700 square foot
unheated room in which our system is installed is a bedroom; it's
actually a storeroom for the homeowners' collectible book home
business.<br>
<br>
My request: please send me documented work by others establishing
that PV systems with sealed VRLA batteries are used specifically
because they are considered safe without venting to the outside. If
you know of good online links, I could use them too. For example,
the AHJ asked for a document stating that the batteries or the
enclosure were specifically approved for this use in an indoor
location. I can't - Midnite battery enclosures are simply listed to
UL508A, which is "industrial control panels" and there's nothing
specific to this application in the standard.<br>
<br>
To me this is a common-sense issue, but common sense doesn't cut it
when needing to prove a procedure. Can anyone help?<br>
<br>
For what it's worth, or for those Wrenches with too much spare time,
below is the text of the original defense of our installation that I
sent to the AHJ. His response was that he's not an electrical
engineer and this would have to be taken upstairs. For what it's
worth, I'm not an EE either... My frustration is showing, I'm sure.<br>
<br>
Thank you for any links, reports or other resources you may be able
to send my way.<br>
Allan<br>
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<br>
-------- Original Message --------<br>
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Mr. [AHJ],<br>
I have done some research as followup to our discussion last week
about battery venting for the [X] job. Here are several
perspectives on the issue:<br>
<br>
The NEC Section 480.9(A) states only that "Provisions shall be
made for sufficient diffusion and ventilation of the gases from
the battery to prevent the accumulation of an explosive mixture".
At root, you are questioning whether ventilation of the batteries
into the storeroom at the [X] home is sufficient under worst-case
conditions.<br>
<br>
The NEC Handbook entries for Section 480.9(A), which are
considered as explanatory support documentation and are not Code
requirements, include two paragraphs that are fundamentally
contradictory to each other. The two read: <br>
<blockquote> The intent of 480.9(A) is not to mandate mechanical
ventilation. Hydrogen disperses rapidly and requires little air
movement to prevent accumulation. Unrestricted natural air
movement in the vicinity of the battery, together with normal
air changes for occupied spaces or heat removal, normally is
sufficient. If the space is confined, mechanical ventilation may
be required in the vicinity of the battery.<br>
</blockquote>
This paragraph refers to batteries in general, including flooded
batteries which release hydrogen gas as a normal part of the
charging process. The Handbook section goes on to specifically
identify sealed batteries as being unlikely to release explosive
gases:<br>
<blockquote>Although valve-regulated batteries are often referred
to as "sealed," they actually emit very small quantities of
hydrogen gas under normal operation and are capable of
liberating large quantities of explosive gases if overcharged.
These batteries therefore require the same amount of ventilation
as their vented counterparts."<br>
</blockquote>
Well, no, not exactly. Valve-regulated batteries may indeed
require the same amount of ventilation, but not for the same
purpose or under the same conditions. <br>
<br>
Flooded batteries release hydrogen gas as a normal part of every
charge cycle. While it is unlikely that the hydrogen gas could
accumulate to the 4% concentration to become combustible, given
its natural dispersion, the hydrogen sulfide released with the
hydrogen gas is an unpleasant irritant and is potentially toxic
with prolonged exposure at high concentrations. Because of the
normal gassing during the charge cycle, we always provide
ventilation of these gases to the outside. With sealed batteries,
the purpose and intent of ventilation is not to ensure ventilation
during the normal charge cycle, but rather to ensure the safety of
the dwelling and its occupants in the event of a catastrophic
failure resulting in the "worst-case scenario" of unregulated
overcharge. In actual experience, the charge regulator (from the
PV array) and the inverter/charger (from a backup generator in an
off grid home) are the bottlenecks through which all charge
current must pass, and failures invariably occur in an "open
circuit" mode, rather than in a "closed circuit without charge
regulation" mode.<br>
<br>
Nevertheless, we must accommodate the most hazardous potential
outcome, which would be <i>unregulated overcharge</i> of an <i>already
full battery</i> during periods of <i>high insolation</i> (or
the equivalent input from an engine generator). In order to
determine the expected amount of hydrogen gassing under worst-case
conditions, I contacted my Concorde distibutor<font face="Times
New Roman, Times, serif">,<span style="font-size: 11pt; color:
black;"> Marc Kurth of</span><span style="font-size: 11pt;
color: black;"> Centex Batteries, LLC in</span><span
style="font-size: 11pt; color: black;"></span><span
style="font-size: 11pt; color: black;"> Bastrop, TX, </span><span
style="font-size: 11pt; color: black;">512 308-9002. He in
turn spoke with </span>th</font>e engineering department at
Concorde Battery, the manufacturer of the batteries used in the
[X] PV installation. Their analysis of calculated gassing and
airflow rates is in the attached pdf document which they provided
to us. The batteries in the [X] PV system are Concorde SunXtender
PVX-9150T, rated 915 amp-hours at the C/24 rate. There are 12
cells in a single series string of 24 Vnom.<br>
<br>
The storeroom in which the PV system is located has interior
dimensions of 19' by 37' by an average of 10' tall, or
approximately 7,000 cubic feet. It's a large open space. The room
has four Pella double-hung windows, each rated by the manufacturer
at 0.3 cfm fenestration, or 1.2 cfm for all four. Each exterior
door (the third door to the interior living space is excluded as a
conservative calculation but also adds to overall ventilation) is
rated at 0.6 cfm, for a total of 1.2 cfm for the two doors and 2.4
cfm for the building, assuming no other openings of any sort, such
as for wires or for natural convective losses due to any other air
leakage or roof ventilation. <br>
<br>
The 2,000 watt PV array will provide at most about 65 peak amperes
of DC current into the batteries, for the equivalent of a
cumulative daily total of around seven hours in summer. (The
inverter/charger is capable of feeding 105 amperes into the
batteries from a generator, but by the specific stated preference
of the homeowners, the home does not have a backup generator and
does not include the ability to accept generator AC input.)
Assuming the worst case of 75 amperes flowing unregulated into
this 900 ampere-hour battery, this C/12 charge rate is capable of
raising the batteries to 30 V DC, or 2.50 volts per cell (vpc).
The cell voltage will not rise about this level because the
internal resistance of the battery, which increases as the voltage
increases, prevents it. Note also that 75 amperes is a peak
current that could only be maintained at midday during conditions
of cold, dry air when the solar insolation intensity is well above
standard test conditions (STC) of 1,000 watts/square meter, when
the sun is perpendicular to the array. As the sun passes across
the sky, the available output current drops substantially. At a
reduced input current, the maximum vpc drops to around 2.40 vpc
(and continues to drop thereafter) and the maximum temperature
also drops, in which case gassing reduces by a factor of about 20
below the rate at 2.50 vpc.<br>
<br>
As an additional factor in our calculations, note that all modern
charge controllers are designed to receive PV input at a higher
voltage and lower current than the nominal battery voltage,
converting this to higher current at the lower actual battery
voltage. The Midnite Classic charge controller in this application
works this way. In a closed-circuit failure of the charge
controller's functions, the higher array voltage and lower current
would pass through to the batteries. As long as the input voltage
is higher than the battery voltage, the batteries will accept
current, but additional voltage does not increase the current into
the batteries or the amount of hydrogen released. Rather, in this
case the PV modules, which are wired as four strings of two
modules each, will not exceed the rated short-circuit of the
modules x 1.25 (per NEC for PV source circuits. With four strings,
this is (8.61 x 4 x 1.25 =) 43.05 amperes. This is less than half
of the maximum input current used to calculate worst-case input
(as shown in the following paragraphs), and as such is unlikely to
be sufficient to raise the cell voltage to even the level
calculated.<br>
<br>
Per the attached engineering analysis by Concorde, assuming that
at a sustained 2.50 vpc the temperature of the batteries rises to
50ºC (122ºF), the amount of hydrogen released at a constant
current at 30V DC, or 2.50 vpc, at 50ºC is 5.6 cc/hour/Ah/cell.
This converts to (5.6 x 915 x 12 =) 61,488 cc of hydrogen released
per hour. Converting cubic centimeters to the more useful cubic
feet, 61,488/21,317 cc/cuft = 2.17 cubic feet per hour of gas
released. This amount is less than the total fenestration of that
room (not including the door to the living space) of 2.4 cubic
feet per minute, or (2.4 x 60 =) 144 cubic feet per hour of
natural leakage to the outside through closed windows and doors. <br>
<br>
To take this one step further, 2.17 cubic feet is 0.031% of the
volume of the storeroom. It would take 30 times this concentration
to exceed 1% by volume in an airtight container. 4.1%
concentration is the threshold at which hydrogen gas becomes
combustible.<br>
<br>
Also at 2.50 vpc, at 50ºC, the airflow required to keep hydrogen
accumulation below 1% is 0.0093 liter/minute/Ah/cell, or [(0.0093
x 915 x 12)/28.32 liters/cubic foot =] 3.6 cfm, or 216 cubic
feet/hour. While this exceeds the default window and door
fenestration of 144 cubic feet per hour, it is sufficient to
disperse hydrogen. Note that these batteries are not in a confined
space; the batteries are located in a space of 7,000 cubic feet.
Note also that is the threshold for staying below 1% hydrogen
concentration; 4.1% is the threshold at which hydrogen becomes
explosive.<br>
<br>
I reviewed our records pertaining to the use of sealed batteries
in residential off grid PV systems and in grid-tied PV systems
with battery backup. We have installed more than thirty such
systems, although the great majority have been installed since
2005. Of those, I have identified at least nine permitted and
inspected systems in which the batteries have been located in what
may be considered enclosed spaces without ventilation between the
interior space and the outside air. Indeed, several of these are
in spaces much smaller that the Shutt storeroom. This is the first
time in which an AHJ has expressed concern about adequate
ventilation of sealed batteries.<br>
<br>
In two of these thirty-plus confined interior installations, the
sealed batteries were installed in custom-made sealed enclosures
which were wrapped in sheet plywood with controlled intake
ventilation. In both of these we purposely installed Power Vent
battery fans (as we install in all of our systems with flooded
lead-acid batteries) ducted to the outside as a safety feature to
prevent the possibility of accumulation of gases within the
battery enclosure itself. However, in all of the remaining systems
we have used manufactured steel battery enclosures Listed to
UL508A. Ventilation from the cabinet into the room where it can
dissipate has always been considered to be adequate in these
applications.<br>
<br>
I believe that I have conclusively established that in a
worst-case scenario, the batteries cannot release enough hydrogen
to come even close to dangerous levels. In practical terms, if a
failure were to occur when the residents were away, the batteries
would be permanently damaged by a failed controller, but no danger
exists to the home. If the residents are present when the failure
occurs, they would in short order smell the "rotten egg" smell of
hydrogen sulfide. Following their noses, they'd find a much
stronger smell in the storeroom, suspect that the batteries were
the source, turn off the circuit breakers on the system (which are
readily accessible per NEC) and open the doors or windows. <br>
<br>
The 2011 NEC Hanbook states, as noted above: "If the space is
confined, mechanical ventilation may be required in the vicinity
of the battery." The storeroom at the [X] residence is simply not
a "confined space" as built.<br>
<br>
Thank you for your consideration of this defense of our
installation practices.<br>
Allan Sindelar<br>
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<p class="MsoNormal"><b>Allan <span class="SpellE">Sindelar</span></b><br>
<span style="font-size:10.0pt"><a
moz-do-not-send="true"
href="mailto:Allan@positiveenergysolar.com"><u><span
style="color:#000099">Allan@positiveenergysolar.com</span></u></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, <b>Positive Energy, Inc.<o:p></o:p></b></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt">A
Certified B <span class="SpellE">Corporation<sup><span
style="font-size:7.5pt">TM</span></sup></span><br>
<st1:address w:st="on"><st1:street w:st="on">3209
Richards Lane</st1:street><br>
<st1:city w:st="on">Santa Fe</st1:city>, <st1:state
w:st="on">New Mexico</st1:state> <st1:postalcode
w:st="on">87507</st1:postalcode></st1:address><br>
<b>505 424-1112 office 780-2738 cell</b><br>
<a moz-do-not-send="true"
href="http://www.positiveenergysolar.com/"
target="_blank"><u>www.positiveenergysolar.com</u></a><o:p></o:p></span></p>
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