Lately I've received quite few letters wanting to know about the new batteries being sold as maintenance-free.

If you have a conventional battery in your car now, as most do, it contains six cells with six holes. When it was activated a solution of sulfric acid and water (electrolyte) was poured into each cell. When you close an external circuit in your car, as in turning on the headlights or the ignition switch, a chemical reaction in each cell, between the plates and the electrolyte, produces the needed electricity.

But the discharge/recharge process also causes the water in the electrolyte to become gaseous. The gaseous mixture exits out the top of the battery in what the pros call "gassing," the main reason you have to maintain your battery. It causes battery terminals to corrode, preventing good electrical contact. To clean the terminals you must remove them from the battery posts and clean the connections until they are bright and shiny.

The gassing also means that the electrolyte level in the battery cells is dropping, so you must inspect the electrolyte periodically (every couple weeks or so) and add water as necessary to keep the level just barely touching the split ring at the bottom of each cell's fill hole.

If you do these two things, a conventional battery the right size for your car should give three to five years' service.

So why a "maintenance-free" battery? Because a lot of people don't want to check fluid levels and clean battery terminals, or forget to, or don't know they have to.

What is a no-maintenance battery? According to Delco-Remy (supplier of all GM car batteries) and Gould, both pioneers in no-maintenance batteries, it uses calcium in the lead battery plates instead of the antimony conventional batteries use. In tests, batteries with lead-calcuim plates gas much less than ones with lead-antimony plates. Less gassing, less terminal corrosion and need to add water. Gould, Delco and other makers say.

But other batteries are being sold as maintenance-free that don't have lead-calcium plates. They have lead-antimony plates, but using less antimony than usual. Called low-antimony batteries, they gss less than the batteris with conventional amounts of antimony. Sellers sometimes claimed they will last a normal life without adding water. Makers of batteries with lead-calcium plates say that this is not so, but you have to add water if you want a low-antimony battery to last four or five years.

Who's right? It's really too early to say? - the low-antimony and lead-calcium batteries haven't been out long enough to form hard conclusions. GM., however, is so confident about batteries with lead-calcium pates that all their cars rolling to assembly line are scheduled to have Delco's lead-calcium battery, called the Freedom Battery.

No-fill batteries are usually more expensive-sometimes much more - than conventional batteries. If you want a no-fill battery, your choose its capacity just as you would an ordinary battery, and it's installed the same way as a conventional battery.