What's the difference between a "maintenance free" and a "deep cell" battery?
Hi, Jack! Let's see.
"Maintenance free" means it's sealed. Means you don't have to keep adding water to it, and you can install it upside-down if you want to. There's a sub-category of this type that's vented, but also sealed; so when it charges it lets off a little gas but doesn't require the constant checking and filling that "floodies", or flooded lead acid batteries do.
Sounds like you're referring to "deep cycle" batteries to me. That's the type of battery that you use in an application like storage, where you've got solar panels. The true "deep cycle" batteries are too sensitive to jostling for automotive or golf cart or marine applications; they like to sit perfectly still and contemplate electron exchange; )
We use "deep(ish) cycle" golf cart batteries in our electric cars; you can get them in sealed form - AGM is what you'll often see these called. You'll notice that the "maintenance free" AGM golf cart batteries are more expensive, too. Floodies tend to last a bit longer and are cheaper, but the tradeoff is that you have to watch where you install them, because you'll be visiting them a lot.
Golf cart batteries are used to being bounced around, and do a pretty good job of delivering a lot of amps over an extended period of time.