Look at any alternator output curve and you will see that amps increase as
RPM increases up to the point where they approach full output and then level
out. That point is usually between 2500 and 3500 shaft RPM. There are a
very few very expensive alternators, like the Eco-Tech, that are built to
produce close to full output at less than 2000 RPM that have a very steep
curve but most alternators require a minimum 2500 RPM to reach any
significant output.

Look at the LN curve I've posted - it is not as stated above. Full
output is at 5000 alt rpm and it then levels out.

That said, the OP's choice of alternators may not be the best if he wants to
charge at dead idle. The output curve has a particularly slow slope. He
should also consider that it takes a certain amount of power to produce 85
amps. Besides the output, the other item on that chart he should consider
is the Torque curve. When you use a pulley ratio that allows close to full
output at 800 engine RPM the torque increases to the point that invites belt
failure and excessive side loads on bearings.

My engine can deliver up to 20kW at engine idle according to the makers
engine manual - more than enough for the 2 - 3 kW taken by the
alternator. Torque will also not be an issue with dual belts.