Selecting an alternator/generator
670 rpm is about half the minimum an alternator needs.

If you look on a car the alt pulley diameter is about 1/3 of the crank pulley diameter.

This means that when the car engine is idling the alternator can cover the demand.

But at max engine speed the alt must not exceed its max rpm - usually about 10000 rpm so the pulley ratios must be checked.

Have seen an alternator seized due to excess speed (not bearings but rotor welded to stator) - owner did a conversion on a v6 motor and did not check the pulley sizes...