Check the connections on the alternator are secure. Probably best to do this with the battery disconnected (negative terminal first so you don't short yer spanner to the metalwork). Failing that measure the voltage across the battery when the engine is running. Expect to see around 14.2 volts if it's charging. If it's less than 14v (probably around 11 or 12v) then charge volts isn't getting to the battery. Keep yer paws away from the moving bits when the engine is running or it will rip yer fingers oft.