[RE-wrenches] Battery venting issue

[Phil Undercuffler]

Allan,

The International Fire Code
(2006 version quoted below, confirm the version used in your region) 
give us clear guidance on the ventilation requirements for stationary
battery systems, and provides both engineering parameters and prescriptive
solutions clearly spelled out
.  These can be used to support your installation within a structure an
inspector should be able to understand.


2006 International Fire Code

608.6 Ventilation. Ventilation of stationary storage battery systems shall
comply with Sections 608.6.1 and 608.6.2.

608.6.1 Ventilation. Ventilation shall be provided in accordance with the
International Mechanical Code and the following:

   1. For flooded lead acid, flooded nickel-cadmium, and VRLA batteries,
   the ventilation system shall be designed to limit the maximum concentration
   of hydrogen to 1 percent of the total volume of the room; or
   2. Continuous ventilation shall be provided at a rate of not less than 1
   cubic foot per minute per square foot [1ft3 /min/ft2 or 0.0051 m3 /(s .  m2
   )] of floor area of the room.

608.6.2 Cabinet ventilation. When VRLA batteries are installed inside a
cabinet, the cabinet shall be approved for use in occupied spaces and shall
be mechanically or naturally vented by one of the following methods:

   1. The cabinet ventilation shall limit the maximum concentration of
   hydrogen to 1 percent of the total volume of the cabinet during the
   worst-case event of simultaneous "boost" charging of all the batteries in
   the cabinet; or
   2. When calculations are not available to substantiate the ventilation
   rate, continuous ventilation rate, continuous ventilation shall be provided
   at a rate of not less than 1 cubic foot per minute per square foot [1 ft3
   /min/ft2  or 0.0051 m 3 /(s . m 2 )]

Therefore, the installer needs to show that either (1) the maximum hydrogen
concentration that can accumulate in the room is less than 1 percent of the
total volume, or continuous ventilation needs to be provided to the room.
 The theoretical gassing rates of flooded lead acid batteries are:  one
Ampere-hour of charge will generate 427 ml of Hydrogen (H2) and 213 ml of
Oxygen (O2) @77F, 1 atm.  VRLA batteries are an order of magnitude more
efficient in recombination and coulombic efficiency than flooded by their
very nature, and produce a fraction of those numbers.
Also, as you point out, a VRLA battery that is gassing is a VRLA battery
in the process of being destroyed and there's only so much H2 inside.  
However, in accordance with 608.6.1.1 we shall use the absolute maximum
gassing rate of 427ml of Hydrogen, to show a worst-case scenario of a
unregulated charge.  From there the installer can perform a simple
calculation to show whether the design room volume and air exchange rate of
the area in question will ensure the maximum concentration of Hydrogen
remains beneath the required 1% level.



If it helps, t
here are UL1741 Listed enclosures with UR recognized batteries available on
the market.  For instance, OutBack's
In
tegrated Battery Racks are listed
 for use indoors to house batteries for use with a grid-interactive or
standalone inverter for residential applications.


Second, the EnergyCell batteries are a VRLA batteries, so 608.6.2 would
apply.
 Per the installation manual of this Listed product, the product is
naturally vented and meets the requirements of 608.6.2 method 1 when the
EnergyCell or equivalent VRLA battery is used, and no additional
ventilation of the battery rack is required.

Good luck with it,

Phil



On Wed, Feb 5, 2014 at 6:38 PM, Allan Sindelar <
allan at positiveenergysolar.com> wrote:
>
> Wrenches,
> 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.
>
> 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.
>
> 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.
>
> 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.
>
> To me this is a common-sense issue, but common sense doesn't cut it when
needing to prove a procedure. Can anyone help?
>
> 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.
>
> Thank you for any links, reports or other resources you may be able to
send my way.
> Allan
>
>
> -------- Original Message --------
>
> Mr. [AHJ],
> 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:
>
> 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.
>
> 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:
>
> 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.
>
> 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:
>
> 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."
>
> 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.
>
> 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.
>
> Nevertheless, we must accommodate the most hazardous potential outcome,
which would be unregulated overcharge of an already full battery during
periods of high insolation (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, Marc Kurth
of Centex Batteries, LLC in Bastrop, TX, 512 308-9002. He in turn spoke
with the 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.
>
> 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.
>
> 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.
>
> 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.
>
> 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.
>
> 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.
>
> 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.
>
> 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.
>
> 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.
>
> 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.
>
> 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.
>
> Thank you for your consideration of this defense of our installation
practices.
> Allan Sindelar
> --
>
> Allan Sindelar
> Allan at positiveenergysolar.com
> NABCEP Certified PV Installation Professional
> NABCEP Certified Technical Sales Professional
> New Mexico EE98J Journeyman Electrician
> Founder, Positive Energy, Inc.
>
> A Certified B CorporationTM
> 3209 Richards Lane
> Santa Fe, New Mexico 87507
> 505 424-1112 office 780-2738 cell
> www.positiveenergysolar.com
>
>
>
>
>
>
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