On my 2001 Ford Focus my battery is sometimes low and won't start the car.Battery has been replaced?

On my 2001 Ford Focus my battery is sometimes low and won't start the car.Battery has been replaced?

My guess is if you ruled out the battery by having it tested for free at Pep Boys or Autozone etc even if it is new, then the alternator is going bad or there's a loose or corroded wire that even if the alternator works, the wire connection is compromised to the point where the charge is not reaching the battery. So if you have green gunk growing on your battery terminals start by making them shiny clean, both the cable and the post on the battery itself.

And be certain to keep up with replacing the cam belt per factory recommendation on both the SMPT base engine and the ZTEC 2.0 liter engine. I believe every 100k miles.

Have the battery test along with the alternator

With the engine running, touch the battery terminals with an inexpensive volt meter set @ 20 volts. If the alternator is charging properly, the volt meter will read between 13.5 - 14.2 volts.

If the alternator is charging over 14.5 volts the alternator will boil the acid out of the battery. You need a new lifetime warrantied alternator from an AC-Delco parts store or distributor.