Editor’s Note: One of our most frequently asked questions has to do with new battery installations, either to run things like bow thrusters and inverters, or to add capacity to a house battery bank. It simply can't be emphasized enough, regardless of the specific battery technology employed: batteries MUST be properly secured in place, and the compartment they are located in must provide adequate ventilation in the event of excessive gassing.


This question came from a reader who'd installed these slimline AGMs under the main saloon settee.

Question: I just went for some really nice slim-line AGM batteries for my boat. They’re sealed so there is no gassing. Do I still need to provide some sort of ventilation for them? My mechanic says I do, but I’m not sure why. Is it to keep them cool? If they are sealed what’s the big deal?

Answer: This is a question that I get quite often. The short answer is, your mechanic is giving you the straight story and you do need to vent these batteries. Here’s why.

Even though your batteries are normally sealed, they actually do have a venting system integrated into the case top. In the event of an overcharge condition, it is possible for excess pressure to build up inside your battery. Your new batteries are equipped with check valves that are engineered to open at between 2-3 PSI depending upon the specific brand. What will be expelled is hydrogen gas, and you want to vent that as it is quite explosive.

The good news here is that hydrogen mixes with air very readily and so not too much ventilation is required. On an installation similar to yours I installed a small pre-made chromed louver, available at most chandleries, in the end of the settee. Just locate the louver as high on the side of the cabinet as you can reasonably get it. That will vent the compartment to the main cabin area, and from there it will get vented to the outside of the boat. Easy fix.

Enjoy your new AGMs!

Read more: Sealed Batteries: Are they really Sealed?


Written by: Ed Sherman
Ed Sherman is a regular contributor to boats.com, as well as to Professional Boatbuilder and Cruising World, where he previously was electronics editor. He also is the curriculum director for the American Boat and Yacht Council. Previously, Ed was chairman of the Marine Technology Department at the New England Institute of Technology. Ed’s blog posts appear courtesy of his website, EdsBoatTips.