Battery lore for motorists.

 

A fully charged car battery will read 13.2 volts.  Draw just a very small amount of juice from the battery, say turn on the headlights for a few minutes, and you will find the battery voltage has dropped down to 12.8 volts.  So most of us call a 12.8 volt or better battery fully charged.  Starting the engine, especially in winter, takes a lot of juice and as the battery charge is used up the battery voltage will drop off.  Once the battery voltage drops below 12.0 volts the battery may not have enough charge in it to start the engine. 

   When the engine is running, it turns an alternator under the hood to charge the battery.  The alternator has to supply current at a slightly higher voltage than the battery in order the charge it.  A properly working alternator will keep the car’s electrical system at 14 to 15 volts.  Cars have a built in voltmeter which you can see.  You may have to read the car’s manual to find it and turn it on.  On new cars (anything less than twenty years old) the voltmeter is digital and you can easily see your system voltage accurate to a tenth of a volt.  With the engine running the voltmeter ought to show 14-15 volts from the alternator.  With the engine off, the voltmeter will read battery voltage.  If battery voltage is above 12 .0 volts it’s good. 

   If the battery is below 12 volts either the alternator isn’t putting out enough voltage, or the battery is old and shot.  Winter is hard on car batteries and 4 or 5 winters is all they are good for.  A new battery is only fifty bucks at the auto parts store and you put it in yourself.  Assuming the alternator belt is nice and tight and not too worn, you have to replace the alternator.  Rebuilt alternators are every bit as good as brand new alternators and a lot cheaper.