Sounds like the alternator may not be recharging the battery properly
Use a voltmeter to check
A fully charged battery shoukld read approx. 12.8 without engine running
Should read 13.8-14.2 with engine running with a good alternator
Ford Focus 2001 - My experience with battery light coming on is the following: Put a volt meter on the battery if you get about 12.7 volts DC (Car Cold) or about 14.5 volts DC with the car running, your alternator should be fine. What you want too check is the fuse. In the fuse panel under the hood is a 10 amp fuse (red color) in position 10 (check your manual), if it is blown, that means your voltage monitor/regulator diagnostic module is suggesting it is not functioning. Replace the fuse should help. This also solved some stalling I had with the car, although I'm not sure why.
Amber warning light on ford focus
It is important to pay attention to the warning lights on the dashboard. The Battery warning light turns on when there the battery of a car is having a problem.
The battery warning light illuminates when the battery isn't being charged , get it checked out to find the problem and then repair.
There are several warning light on the instrument panel. There is a high water temperature light. There is a low battery charge light. There is a low oil warning light.
The light next to the battery warning light is the fuel injection system warning light. If it comes on and goes off after a short while it should be ok but if it stays on for longer than 5-10 minutes then get it checked out.
Reset the battery
The low air warning light is in indication the air compressor is not working or can't keep up with replenishing the air system. The red battery warning light is an indication the battery is not charging properly.
The Ford Freestar several warning lights. The red water warning light indicates the cooling system is overheating. The red battery warning light indicates the charging system is malfunctioning.
The warning light will not come on because you have a separate oil gauge on the Ford Focus. This is the gauge that you should watch and it will show you that you are low on oil; do not rely on the check engine light to alert you of it.
If your battery is not charging properly the charging system warning light ( located just to the right of the " F " on your fuel gauge ) will light up . It is the picture of a battery
When the battery light is on. That means your charging system isn't working. You should have your system check. It will drain your battery.
The red water level warning light is an indication the cooling system is overheating. The red battery warning light is an indication the charging system is malfunctioning.