Hi there, 

I have a 2012 ford focus zetec and the battery light has been coming on and off again. I have had a new altenator fitted and it still came on. The garage replaced the alternator but it is still happening. I have changed the battery now as well and again, it has happened. 

Any ideas?

ETIS (Olde Worlde) says this for S-MAX:

The charging system for this vehicle is composed of a generator combined with a generator control module. 
Charging is controlled by the PCM and the GEM.

The PCM communicates with the generator control module using a LIN connection. The PCM will instruct the generator to increase or decrease charging voltages in accordance with the requirements of the power supply systems. In turn the generator control module will inform the PCM of the current output of the generator.

The generator control module monitors the generator output. If a fault develops in the generator control module, control circuits or the generator, the PCM will be informed and a diagnostic trouble code (DTC) will be set.

If warning light stays on when engine speed rises then it could be:
*Generator.
*Generator control module.
*Generator LIN communication system. 
*PCM.
*GEM.

Generator On-Vehicle Tests - No-Load Test

Turn off all electrical loads and the ignition switch.

Switch the multimeter to the voltage function.

Connect the leads of the multimeter across the battery terminals.

Read the voltage (base voltage).

Start the engine.

Run the engine at 1500 rpm with no electrical load.

Read the voltage. The voltage should be in the range of 14.1 volts to 15.1 volts. If the voltage increase is less than 2.5 volts above the base voltage, carry out the Load Test. If the voltage increase is greater than 2.5 volts, REFER to the Ford diagnostic equipment.

Generator On-Vehicle Tests - Load Test

With the engine running, turn on the air conditioning (if equipped), turn the blower motor to high speed and the headlamps to high beam.

Increase the engine speed to 2000 rpm. The voltage should increase a minimum of 0.5 volts above the base voltage. If the voltage does not increase as specified, REFER to the Ford approved diagnostic tool. If the voltage increases as specified, the charging system is charging correctly.

Try that but get any fault codes.

I suspect LIN wiring as a first look?