Lap-top computers have finally come into a place of honor in the micro world.

Last year's strong sales of lap-tops, mostly the MS-DOS compatible variety, have placed these useful little machines in the hands of a lot of computer users. Those same sales also have made evident a problem that can occur when you regularly use your lap-top computer, and it's worth knowing the cure. The problem is one of short battery life, prevalent in any lap-top computer that runs off nickel-cadmium rechargeable batteries (which means most of them).

If your lap-top uses rechargeable batteries, you may be a victim of the disease of decreasing battery life. If you've noticed a trend to get less and less portable use out of your lap-top, this is probably happening to you. Don't panic; there is a simple cure.

The problem that occurs is actually inherent in the design of the rechargeable batteries used in most lap-tops. Those batteries are made to take a full charge and use most or all of that stored power over a period as you use your lap-top while away from the office or on the road. As the batteries supply power to the computer, a chemical reaction takes place inside the batteries which makes them receptive to being charged again before the next portable use. The closer to a full discharge the batteries get, the better they are able to take on a recharge for the next use.

So in theory, it is good to run the batteries to near-exhaustion before you recharge them.

Unfortunately, that is not how many lap-top users use their machines. Many users will plug in the recharger overnight, resulting in a full charge, then take the machine and use it for an hour or so. This discharges the battery, but not entirely. The computer owner then recharges the battery again and repeats this process over and over.

As the batteries repeatedly are brought to full charge, then partially discharged, the result is that the batteries' long-term performance decreases; they are actually rendered unable to operate over the period that they were rated for. The result, in understandable terms: A computer that you thought was fully charged runs out of steam after you've had it on for 40 minutes. And you are left wondering why a computer that is supposed to run for four hours on a full charge can't operate for a fourth of that time span.

The fix, fortunately, is a simple one. Since your lap-top's Ni-cad batteries like to experience what engineers call a "deep discharge," make sure they get to do so occasionally. After a full charge, turn on the computer. And leave it on until the batteries get so weak that the computer shuts itself down automatically.

Give the machine another full charge, and repeat the process. After a couple of such treatments, your lap-top's batteries should be back to normal, and you won't be left embarrassed by battery failure at some critical moment.