Car batteries have come a long way.
In the old days there were only lead-acid types. Now there are also Gell-Cell and Absorbed glass mat.
The gell-cell and absorbed glass mat are very expensive. Lab tests suggest these are best for heavy duty applications.
The basic driver has a choice betwwen two types:
The "serviceabe" types have removeable caps and you have to check them once a month with a hydrometer.
Thus, is is mandatory to purchase an inexpensive hydrometer. Directions are on the package.
The "maintenance-free" varieties are sealed at the top. You have NO access to the acid
So why would one want a "Serviceable" battery instead of a maintenance free?
A serviceable battery can take an emergency charge far faster than a maintence free. A maintenance free battery may have to be trickle charged for 24 hours to bring it back to maximum rating, while a "serviceable" battery can be "quick-charged" in a very short time. For emergency vehicles and situations where you MUST have a dead battery fully recharged in a short period of time "serviceable" wins hand's down.
Batteries last between six months and two years.