>>
So I thought that on the input i use a 78S10 or something: simple 10V
regulator with suitable current limiting.  (Somebody know of a low-drop
2..3A 12V regulator that accept 30V?)

Then i use one simple H-drive for each motor, controlled in sequence by a
PIC.
<<

I don't see any good reason for the voltage regulator.  All its going to do
is dissipate a lot of power and cut the voltage to the motors.  You could
have nearly 5 volts sustained accross the regulator during normal operation
when the engine is running and the batteries are being charged.  That's up
to 15 watts.

One of my current projects includes driving a 4A DC motor bidirectionally
from a standard 12V gasoline engine power system.  We use 4 FETs in an H
bridge controlled with PWM from a 16F876.  Works great.  You do have to
protect the circuit from nasty spikes, which we do with an inductor,
capacitor, and if all else fails a fuse.  The scheme would work fine at
higher voltage.  It only depends on the voltage rating of the FETs.  You can
use PWM to make sure the effective voltage to the motor doesn't exceed its
rating.

>>
I can cheat by connecting one side of all motors together and connect that
to either + or minus by a half H, then i only need another half H per motor.
And then i may afford having that common half H as a relay, gaining less
voltage drop.
<<

That configuration is fine as long as two motors never need to be driven in
opposite directions at the same time.  Frankly, the voltage drop accross a
reasonable FET at 3A won't be enough to worry about.  We don't even heat
sink ours and they just barely get noticeably warm.  However, relays should
also work well here because you can guarantee that the current is always off
when the relay is switched.  That will greatly extend its life, although
FETs will still be more reliable.


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