Question: You can see in the photo I sent that on my boat the builder decided to use a metal bar for my battery hold-down. I’ve also seen people use a wooden block or even nylon straps. What is really the best choice for this application? I’m a little worried about the metal being exposed to corrosive battery fumes.
Answer: ABYC Standard E-10 is pretty clear on this one. It reads: “Battery mounting materials shall withstand electrolyte attack."
What isn’t clear in the above statement is for how long. The piece of angle stock you show on your boat looks to be stainless steel stock. Certainly it has the strength required to do the job as long as the fasteners, which are not visible, are appropriate and threaded into something solid. As for the issue of electrolyte attack, you have a maintenance-free battery shown in the photo. Even though you see caps at the top of the battery, these are not to be removed and in fact may be glued shut. This battery is not at all prone to gassing or electrolyte leakage due to its internal chemistry and case design.
Today your hold-down material choices are wood, aluminum angle stock, mild steel, stainless steel, nylon strap, carbon angle stock, and fiberglass angle stock. I prefer the non-conductive choices, combined with electrolyte exposure immunity. So, for me the top choice is an appropriately sized piece of fiberglass angle stock, minimum 1/8" thick. It won’t corrode and it doesn’t conduct electricity.
I’m OK here with the stainless because I know that the exposure to corrosive gasses is absolutely minimal in this case. That piece of stock will last for years. Aluminum bar stock is a bit more reactive to both battery cleaners and acid exposure, so I do not consider it the next best choice. Mild steel will rust. Just remember, if you do use metal, be very careful not to create a short-circuit across the battery terminals when working on the battery.
Wood could rot pretty quickly if exposed to acid, and the carbon is too expensive as well as conductive once you expose the ends of the carbon fibers. As for the nylon straps you often see, they will never corrode. However, if they're exposed to UV for a few years they will lose their suppleness and strength.