Electrical Problem

I bought a car about 18 months ago that had a new battery in it and the battery is now dead. I took it to a shop to be recharged and was told that they tried but that it would not hold a charge. Can a bad alternator "ruin" a battery?

As long as the alternator is charging I wouldn't think it is bad, because if it wasn't charging you wouldn't be able to start the car.

By any chance, do you forget to turn off some accessories that are powered by the battery ?

Can a bad alternator "ruin" a battery?

Yes, a charging system that is providing too high and/or unregulated voltage can kill a battery in no time.

Too many charge/discharge cycles can ruin an automotive battery.

A battery with a factory specced heat shield can be killed by high underhood temps if the heat shield is not reinstalled.

A battery that isn't securely mounted will in time (depending on driving habits) short internally as the inside structure breaks down from road shock.

Hope these help you.

In addition to what Dr. Sparks mentioned: If the vehicle isn't used very often and the battery is usually in a very low state of charge, the plates in the battery develop a film of sulfate (similiar to what rust is to steel). As time goes on, that "sulfated" battery film slowly comes off the plates and accumulates at the bottom of the battery. When the accumulated sulfide muck at the bottom of the plates gets thick enough, the plates are "shorted" and the battery becomes worthless. So...all the above mentioned possibilities plus this one apply.