My FRST has a problem with charging. I have had my alternator tested which has proved that it is working as it should be.

The alternator charges no problem when there is no load on the battery ie lights etc turned off but at night when the lights are on and say i've got the indicators flashing the battery light on the dash board lights up faintly and say I touch the brake pedal which switches the brake lights on the battery light gets stronger and goes bright red should I put the main beam on or the wipers on. I bought a tester from Halfords today and it does show that the alternator does charge with no load but there is no charge getting to the battery when the above are switched on.

I think the above makes some sense but does anyone know what the problem could be and how to put it right? Any help will be appreciated.