Ford Focus Electrical Failure?

I have just had my third new battery fitted following a complete electrical failure whilst driving. The Ford garage in Birmingham did a full electrical diagnostic with each replacement and found nothing wrong except for a "failed battery". The last time it happened all the lights on the dashboard flickered for about three seconds and then the engine cut out. The RAC recovery engineer suspected a computer management system problem, for the alternater was "steaming out current". This suspicion was discounted by the Ford Technician. Has anyone any idea of what might be wrong? I am disabled and only use the car (01 reg 23K miles) for every other day shopping (4 miles there and back) and occasional long journeys.