What would be the best choice for batteries for a backup solar system, a marine deep cycle , or golf cart batteries? The marine deep cycle batteries I have looked at are “maintenance free.” This provides no way to add water. Would this be a problem, or do the batteries have to have a way to add water even if they are maintenance free? Thank you, – HP
JWR Replies: The terms “marine battery ” and “golf cart battery” are used almost interchangeably by some manufacturers,. Both generally refer to deep cycle lead acid batteries with extra thick plates. Technically, a marine battery is designed not to spill, even when a ship pitches and lists to steep angles. But that is hardly a discriminating issue for someone with a fixed site retreat house. Batteries with either designation work fine.
I recommend that you do not purchase semi-sealed “”maintenance free” batteries. That will hamper you when the battery gets older and it needs to have some distilled water added, or when you want to do a hydrometer test. Yes, standard batteries do lose a bit more water vapor than their semi-sealed cousins, but at least you can work on them! By the way, a method to minimize vapor loss is to retrofit your lead acid batteries with replacement cell caps called Hydro Caps. These are specially designed to recover vapor and return it in liquid form back into the cell reservoirs. They can cut vapor loss by half. The last time I checked, Hydro Caps were available through a number of vendors including Ready Made Resources ( http://readymaderesources.com  ), Backwoods Solar Electric Systems (http://www.backwoodssolar.com/ ), and Real Goods (http://www.realgoods.com/ )
OBTW, since lead acid batteries sulfate away to the point of uselessness after 8 to 10 years–even if you just leave them “floating”–if you have a big budget and are concerned about a long term scenario, it would be appropriate to store a complete spare set of batteries for your battery bank. This spare set should be special ordered.”dry”, and you would add acid only after you need to put the battery bank into operation.