Under normal driving conditions of say 35-40 mph, NO lights on, flat roads, up hill/down hill, what should the amp meter be showing in the way of amps? Charging? Discharging? And how many amps?
Under those conditions with lights on you want to see a hair above zero. Lights off maybe +3.
It depends on how much current you're using on average.. If you rarely use the lights and drives a number of miles between each use of the starter, then you don't need much charging - too much charging when the battery is full will only boil away battery water and shorten the life of your battery. Though, you shouldn't have discharge running 40 mph, that's for sure.
Your best bet would be to buy a modern Fun Projects regulator instead of a old style cutout - with the regulator it'll only charge what's needed until the battery is full, just like more modern cars.
But for me who hasn't got a starter and who has the lights on all the time to be seen, I can get by with a cutout since my load on the battery is constant - I just adjust the charge to a couple of amps at running speed with the lights on and hopes the generator will hold up for the load
First check to be sure the ammeter is at zero with the engine turned off. If not at zero Remove one battery terminal and check again. Whatever reading you get with the battery disconnected is zero even if the meter does not read zero. It is not a very accurate instrument. If you have a voltage regulator, it will read a higher charge right after you start the engine, but then after a few minutes of driving, it will run just over zero. That would be the same with the lights off or on. If you are using a cutout, the ammeter would run about 3-5 amps while driving. This would be constant with the lights off. It might read slightly below zero with lights on. If you don't do much night driving, this is a good setting. If you drive a lot with the lights on, you would set it to just show about one amp charge with lights on. However if you set it that high, you need to drive with the lights on most of the time even in daytime or you risk overcharging the battery.
All depends how the generator third brush is set and if you're using a cut out or a voltage regulator.
5A charge is the highest I'd recommend with no lights on, assuming a cutout is used.
With lights on drawing 10A, a discharge of 5A will therefore be shown with the engine at speed. A 100Ah battery will cope with that for nearly 20 hours before it runs out.