When you turn the ignition switch to the crank (start) position, a tremendous drain is made on the amount of energy stored in the battery.  If that energy used to start the car was not replaced, the battery would soon be dead.  

That is the function of the car's charging system -- of which the alternator is the chief component.  The alternator replaces the electrical energy used by the battery to start the car and to perform other functions.  

Sometimes, though, through no fault of the charging system, the battery becomes weak.  It just can't store the energy that it used to.  

When this happens, the battery may be beginning to fail.  Eventually it may weaken to the point where the engine will only crank slowly, or may not crank at all.

There can be other reasons for a weak battery too -- reasons totally unrelated to the battery's health.  A loose belt, for example, may be slipping.  The result is that the alternator doesn't spin fast enough to charge the battery. 

Or you may drive only extremely short distances, never giving the charging system enough time to replace the electrical energy used in starting the car.

Regardless, you always want to know if your battery is fully charged.  With conventional batteries, you check the state of charge with a hydrometer.

In cold weather, it's not a bad idea to check the battery once a month -- even more often if the battery is old.

The best type of battery hydrometer has a graduated scale on its float, and also contains a thermometer.

Follow the instructions that come with the hydrometer.  It's easy to use.

You withdraw enough fluid from each cell to make the float float.  Once you've taken the reading, squeeze the rubber bulb to replace the fluid in its respective cell.

Be careful not to get battery fluid on your body or on your clothes.  Battery fluid -- or battery acid as it's called -- will eat holes in clothes.

When you remove the the battery caps from each cell, lay the caps on the battery.  Don't lay them on a fender.  The acid will damage the paint.

Failing to keep the battery fluid at its proper level, and neglecting to keep battery cable connections clean and corrosion-free are two of the most common causes of dead batteries.

Check the fluid level regularly.  Use only distilled water to fill the battery.