The problem isn't the charging system, it's the battery. The internal resistance of the battery determines how much voltage the generator needs to produce, and a battery that develops a surface charge quickly will cause the current flow to drop very quickly after starting. That's because the battery plates develop a coating on them called "sulfation" which makes it extremely difficult to take an effective charge. Another symptom of sulfation will be a very low reserve capacity- - - - -two or three starts in a row will deplete the battery to the point it can't produce much current.
For a quick test, run the starter for 15 seconds with the ignition shut off, then see if the engine will start. If it does, quickly go to a fast idle, and see how long it takes for the charge rate to start dropping. If the amperage drops quickly, or if the engine won't start after a 15-second cranking effort, you've probably got a sulfated battery. It's possible to reclaim it, but it takes some doing. Turn the headlights on, and run the battery down completely- - - -overnight is a good time frame. Then do a slow-rate charge, about 5-10 amps for a day or so, and check the specific gravity of the electrolyte. When it stops going up, discharge again. Recharge- - - -discharge- - - -etc. until the sulfate crystals on the plates break down. It usually takes 5-10 discharge/recharge cycles. Or, buy a new battery and don't leave it unused for very long, which causes the sulfation in the first place. Do a deep drawdown and recharge about once a month, headlights on for a couple of hours, then start and fast idle for 30 minutes. Good luck!
Jerry
