The alternator in your Ford Focus is the heart of its charging system. The charging system converts mechanical energy into electrical energy that recharges the battery. Most defects in the charging system are a result of a faulty alternator.
Low Charge in Battery
The battery provides your Focus with its start-up power. If the battery is losing its charge, the first thing to inspect would be its electrolyte level. Next, see if the alternator's drive belt needs an adjustment. Then inspect the terminal cables for any wear or corrosion. If all seems fine, you should proceed with inspecting the alternator.
Ignition Light Indicators
If the ignition light doesn't go out when the ignition is switched off or doesn't light up when it's switched on, then the alternator may be faulty. There may also be a problem with the charging system's circuit.
Testing the Alternator
One way to test the charging system and the alternator is to perform an on-car test. Test the battery first with a voltmeter with the engine off--the reading should be 12 volts. Test it again with the engine on--the reading should be between 13.5 and 14.6 volts. Turn on all electronic accessories. If the voltage reading drops below the minimum, the alternator may be faulty.