Brad Vietje
Barbara,

I use these batteries every day in the solar business. While there are differences between deep-cycle solar storage batteries and those used in starting engines, all the major properties are consistent.

Flooded cells use a liquid electrolyte, which can spill or leak. They are the heavy duty workhorses of the battery world, and offer * by far * the most bang for the buck.

However, they require maintenance, including not only topping up the fluid every now and then, but in applications where multiple batteries are strung together, should be occasionally juiced with an equalization charge to desulfate/clean the plates and bring them into balance with each other. When charging they can release highly explosive hydrogen gas, so they need to be vented in most applications, and can not be used in many commercial or medical settings where that would be problematic.

Sealed batteries are usually either "Gel" cells, in which the electrolyte is gelled, (like Jell-O), and can't leak all over when turned upside down or when the cases break open, or AGM, or "Absorbed Glass Mat) cells. AGM's and other "sealed" batteries are actually VRLA (valve-regulated lead acid) cells. They do have tiny vents, but these are to release pressure and hydrogen gas under extreme conditions, such as a prolonged over-charging. The electrolyte is gelled and has been absorbed into fiberglass mats that wet the plates inside. They don't leak, and in most cases do not need to be vented. This makes them ideal for boating and applications where they will be bounced all over, and might otherwise break and/or leak. They are also far less likely to be damaged by freezing.

Gel cells cost a LOT more per amp-hour of storage, but require no maintenance, and never get equalized. VRLA batteries are a middle ground -- more expensive (per Amp-hour) than flooded, but less than Gel cells, and require no routine maintenance except cleaning the contacts of corrosion. When Sealed batteries do get overcharged, the contact between the electrolyte and the plates can be upset, and the batteries can be ruined. In normal use, you never add fluid.

Hope that helps,

Brad Vietje
Newbury, VT