Battery light being on is NOT always a sign your battery needs replacing. It means your charging system isn't working correctly. So, it can be the battery, but it can also be the alternator or bad ground or another part in the system.
Gauge cluster - Usually it acting funny is a sign your charging system is not working well (hence your battery light).
Double check your electrical grounds?
Also, to get more helpful advice, please add what year your Focus is, the engine you have, and list any modifications that have been made to it.