A car battery is expensive. Yet many people neglect it and, as a result, end up buying a replacement battery long before they should.

With proper care, a battery can generally be expected to last three to five years; but neglect can drastically shorten its life.

Now, of course, keeping the battery filled with water (preferably distilled water) and keeping the battery cables clean and tight help assure maximum battery life.

But another thing that's very important, and is often overlooked, even by those who take the time to check their battery's fluid level, is the battery hold-down. That's the device that holds the battery firmly against its tray. It's important.

A battery doesn't like exessive vibration, which can shorten its life. And it doesn't like to fall out of its support tray against the motor and possibly into the fan. This can shorten its life, too.

That's why the battery hold-down is there, to keep the battery in place. Sure the battery takes a certain amount of bouncing around, just as the rest of the car (and passengers) do; but if the hold-down is loose, the battery can bounce and vibrate much more than it normally would.

And if the battery hold-down is missing, as it sometimes is in older cars (sometimes because corrosion has eaten it away, or the last person to install a new battery just didn't bother to replace the hold-down), the battery can fall out of its tray and be damaged.

Checking to see if the battery hold-down is loose, or if it's even there, is easy. Not all battery hold-downs look the same, but each serves the same purpose.

Simply by raising the hood and looking at the top of the battery you'll see the hold-down. To see if it is holding the battery tightly in the battery tray, grasp the battery by a corner and try to move it up and down or side-to-side.

The battery should not move at all. If it does, tighten the hold-down until the battery can't move. Before tightening, be sure that part of the battery isn't hanging over the edge of the tray.

If the hold-down is missing, replace it as soon as possible -- dealers' parts departments have them. If you're considering buying a used car, checking the hold-down may reveal the previous owner's devotion to routine maintenance. A missing hold-down may hint at careless maintenance in other areas, too.

A couple of safety tips: when checking the battery hold-down, the engine should be off, the transmission in park, and the parking brake on. Don't wear any jewelry on your fingers or wrists, or loose-fitting clothing.