Sometimes this betrayal is merely the result of a slack and slipping alternator drive belt, but his problem is especially common in the fall. On the first few cold mornings, batteries that apparently had no problems all summer suddenly can't hack it anymore. The trouble is usually age. Batteries only last about five years, or even less if the roads are very bad where you live. Internally, batteries have plates which have an active element in little honeycomb cells. After years of vibration and heat, much of this active element shakes loose and falls into the bottom of the battery case. What remains has enough power to turn the engine over once or twice in summer weather, but when the engine is cold and hard to turn, the battery has no stamina. That's when you hear that most dreaded of automotive sounds, the protesting moan of a starter that can't turn the engine.

Remember also that modern cars have a constant load on the battery even when parked. The ECU is a computer with state of the art integrated circuits and doesn't draw much current, but it does want some, and it wants it all the time. Your car's clock and your alarm system want their few milliamps too. And don't forget the accessory items that can sneakily steal power. Is the light in your glovebox on all the time because the switch is maladjusted? How about the courtesy light in the trunk? When reaching for one of the heater controls on my Integra I sometimes hit the rear window defrost button by accident, and in daylight the little yellow LED indicating that it's on is hard to see.

In addition to the above loads, batteries sometimes develop internal shorts. I bought a brand-new battery from Sears once that would go flat overnight. After a futile search for sneaky loads, I disconnected it entirely one evening and the next morning it had still gone flat. Sears exchanged it for a more trustworthy version without a quibble.

Then again there's always the possibility you have bad battery connections or dirty solenoid contacts. Here's how to check. First, loosen the battery terminals a little and move them a bit, then retighten. Check where the negative cable is bolted to the body or engine. If the engine isn't turning and you just hear a clicking sound from under the hood, turn the headlights on and have someone observe them while you try to start the car. If they go out, the battery is being loaded down and can't hack the load. If they stay lit, the starter solenoid contacts or the starter's comutator is dirty and the starter isn't drawing its full current. The latter fault is often intermittent.

For more about batteries, see the famous Bill Darden's  http://www.batteryfaq.org
And even more from the famous Sam Goldwasser's battery page: http://www.repairfaq.org/ELE/F_Car_Battery.html