battery box venting flow rate? [RE-wrenches]

[Mike Kocsmiersky]

Joel & Kirk,
If I recall IBE battery put out that venting formula that Joel references
and is a good rule of thumb.  Also, Concorde Battery has some data for their
sealed batteries.  It all boils down to keeping the concentration of
hydrogen to less than 2%.  This will be a function of charging, gassing rate
of particular battery, and volume of the battery container or room; I
typically never rely on passive ventilation especially for a battery
container, because as John Wiles was stating you could get exterior forces
pressuring the air back into the container.  I typically use Joel's formula
and actively vent to the outside.  Its also not uncommon to put in a
secondary active vent as a safety, or a hydrogen concentration sensor when
dealing with larger battery rooms.  As Joel's calculations show, it is
typically a small amount of ventilation.

Mike Kocsmiersky
President
Kosmo Solar
(413) 734-1456

-----Original Message-----
From: Joel Davidson [mailto:joeldavidson at earthlink.net] 
Sent: Tuesday, September 19, 2006 8:44 PM
To: RE-wrenches at topica.com
Subject: Re: battery box venting flow rate? [RE-wrenches]


Kirk,

Battery rooms must be properly vented to prevent the build-up of potentially

explosive hydrogen gases released during charging. The correct charge 
controller and charge settings will limit out-gassing during normal 
charging. However, topping off and equalization charging, requirements for 
long battery life, will produce hydrogen. Concentrations of 4% hydrogen are 
explosive. 2% is the recommended maximum concentration for battery storage 
areas. Use the following formula to determine the size of your battery room:
Q = 0.0135 x I x N
Q is the necessary quantity of air ventilation in cubic feet per minute 
(CFM).
I is the charge rate in amperes.
N is the number of series connected cells.
For example, a PV system with eight 100 watt modules (rated 17.1 V, 5.88 A) 
and sixteen 6-volt batteries is wired for 24 volts DC. The required venting 
is 0.0135 x (5.88 x 4) x 12 = 3.81 CFM or 228.6 cubic feet per hour. Many 
rooms are 8 feet in height with 2 air changes per hour. A room 8' x 6' x 6' 
has 288 cubic feet volume and naturally vents 576 cubic feet per hour, 
almost 2.5 times the required 288.6 cubic feet per hour.

Joel Davidson

----- Original Message ----- 
From: "Kirk Herander" <kirk at vtsolar.com>
To: <RE-wrenches at topica.com>
Sent: Tuesday, September 19, 2006 8:37 AM
Subject: battery box venting flow rate? [RE-wrenches]


>
>
> Hello,
>
> Is there any minimum airflow/cfm recommended to safely  power vent a
> battery box?
>
> Kirk Herander
> Vermont Solar Engineering
> 802.863.1202
> fax 802.863-7908
> NABCEP(tm) Certified Solar PV Installer
> Xantrex Certified Dealer Charter Member
> NYSERDA-eligible installer
> VT Solar and Wind Partner
>
>
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