Venting an AGM battery just in case

[Trailfrog]

I'm building a battery box as part of my storage/fridge mount in the back of my 4dr JK. I have the SEARS Platinum Marine battery and have read that these don't need to be vented because there is little chance they will off gas but could if overcharged. I don't want to take any chances so I'm thinking I will seal the battery enclosure and run a tube from the box to the outside of my jeep. How large a tube do I need to vent the battery enclosure?

 

[dwh]

Enough to let maybe a half a cubic foot of hydrogen per hour (that the battery is being overcharged) percolate up and out.

1.5" or 2" should be plenty.

 

[mhiscox]

It may seem obvious, but if you're going to the trouble to vent the batteries--and we're proud of you for it--you've got to get the hydrogen out the top to a point higher than any chance of a spark or flame. Be sure that you can manage that, and that your setup eliminates any pooling in the battery box. If you can't get everything right, it'd be better to have a highly ventilated battery box that will keep the hydrogen from concentrating.

The whole thing is a little tricky, with concerns about air currents, thermal sinking, and so on. Check here for a brief overview:

http://www.bdbatteries.com/hydrogenventing.php.

 

[Trailfrog]

Wow! Thanks, glad I asked the question.

 

[Trailfrog]

It may seem obvious, but if you're going to the trouble to vent the batteries--and we're proud of you for it--you've got to get the hydrogen out the top to a point higher than any chance of a spark or flame. Be sure that you can manage that, and that your setup eliminates any pooling in the battery box. If you can't get everything right, it'd be better to have a highly ventilated battery box that will keep the hydrogen from concentrating.

The whole thing is a little tricky, with concerns about air currents, thermal sinking, and so on. Check here for a brief overview:

http://www.bdbatteries.com/hydrogenventing.php.

So using the copper pipe as a heat sink would that require the copper to extend outside the vehicle or could I run copper pipe from the battery box to the point I need to exit the vehicle and run rubber hose from there through an exit hole in the vehicle somwhere?

 

[mhiscox]

So using the copper pipe as a heat sink would that require the copper to extend outside the vehicle or could I run copper pipe from the battery box to the point I need to exit the vehicle and run rubber hose from there through an exit hole in the vehicle somwhere?

Better if it extended out the vehicle, I would think, but by like 1%. Let's be honest here; Valve-regulated lead-acid batteries connected to a correctly-functioning charger providing an appropriate charging current do not routinely cause a safety issue. The hydrogen comes about when high overcharge currents cause electrolysis of water. The valve is there to release the hydrogen when there's a problem. So there's not an issue if all is working correctly and the batteries are being charged appropriately. At a certain point, one much higher than you'd ever intentionally charge the battery, the cell can't recombine the hydrogen and oxygen and only then would you have a problem.

So while the heat sink hydrogen escape tube is a sound and proper approach, the biggest contribution to safety would be to make sure your charger supplies the proper output and that there's no chance of short circuits. If all remains well, there won't ever be anything coming out of the valve.

Since you're becoming an expert on this, there's a big thread here:

http://www.cruisersforum.com/forums/f14/ventilation-of-battery-compartment-33757.html

that you might wish to read. If you go through it, keep in mind that there are posts related to both VRLAs and flooded-cell batteries and that you'll care most about the VRLA information.