Battery makers try their hardest to make lead-acid batteries truly 'maintenance free' but they should actually label them 'low maintenance' or 'reduced maintenance' batteries - even with maintenance free types they have to get the electrolyte in somehow when they make them, and they don't do it before the lid is sealed on. You can always peel back the stickers that cover the filling plugs and top them up, normally to 10mm above the plate tops - don't put too much distilled water in because you will dilute the acid too much. They haven't started to fit truly 'zero maintenance' 'gelled electrolyte' batteries to cars yet, these are normally reserved for motorbikes, golf carts, electric invalid scooters etc where there is a danger that they can become tipped over or inverted and they don't want the acid mix to leak out. The electrolyte in these batteries is like the jelly in a pork pie and won't evaporate like normal battery acid.
Because batteries are a small chemical factory and their final destination and conditions of use are out of the manufacturers hands they can never be truly free of maintenance. Many people think that it is cold weather that 'kills' batteries, but all the cold weather does is find out that the battery can no longer supply the cranking power (amps) needed to start a cold engine. This is partly due to batteries becoming less efficient in cold weather but mostly because they have previously been damaged by hot weather and / or high under-bonnet temperatures - which is why some batteries are located in the cooler boot area of the car, the longer power cables and associated volt drop in the cables negates some of that advantage, but the battery will last longer.....
The lead acid battery is the Cinderella of car equipment, very often neglected, loaded up with all kinds of power guzzling ICE equipment, heated seats, heated windows and mirrors etc. etc. etc. and expected to go about its tasks without complaining. As with most modern things, they are so good that we forget about them, and while we will happily pay a fortune for alloy wheels, fancy gear knobs and expensive body kits, over £600 for the latest smart phone or £40 to £50 for a Blu-Ray disc or game but can't see why we should pay £50 to £75 for a good quality battery that will serve us every day for 4 to 5 years and ask for very little care in return!
PS, if you do a lot of short journeys or leave the car for long periods without running the engine the battery can suffer from sulphation, this is when the battery is kept below 50% charge state and the plates develop a white coating (you won't be able to see this on modern batteries, but it will still happen). The sulphation reduces the power the battery is able to supply and is pretty well irreversible ----- so, as with most equipment the best thing to do is use the car very day and go for a long run (over 30 miles) at least once a week to keep everything in good order.
----- Double Post Prevention - new post below: 1302852296 -----Maintenance free usually means you can't put water in because the caps are sealed. I suppose you could break the seals on the caps.
Note this: According to PREVENT BLINDNESS AMERICA, in 2003 nearly 6,000 motorists suffered serious eye injuries from working around car batteries
I myself got a splash once changing a battery. Said car had a wee plastic vent tube that went onto the battery. I pulled it off... and splashed my face/eye. I ran straight into the house and stuck my head under the kitchen tap. Seemed OK so went up to the shower. Fortunately no injury, but quite a fright. I always use glasses now.
I bet there are also 30 million people in america who poke themselves in the eye with a cocktail stick, and 50 million who scald themselves with McCoffee every year! I bet they also tried to claim damages from the battery maker even though they had probably taken the plugs out of the battery and turned it upside down to look inside it.
