I have the same problem and I am certain it is your steering rack itself that is the problem, if you park against a kerb and turn into it after you've started her up the belt will stop turning as the rack jams and stops the steering pump ans belt from turning, the crank and alternator keep spinning but the belt remains still so the battery light comes in, it will continue to drive until either the belt slips off or the battery dies, also you have no power steering and the brakes are also terrible as there's no vacuum to the servo. no fun when it happens and there's no hard shoulder on the motorway I can tell you, new steering rack, changing it isnt too bad, just take the track rod ends off and the mounts and pull it out the drivers side.
my rack use to be stiff on start up then was fine for the rest of the day, but over time it gradually got worse until the belt stopped and must have got trapped and sheared the rubber off the bottom pulley, nightmare