I wonder if he knows about the battery drain that all the BS4 bikes have through the Gear Position Sensor?
Also - I wonder if he tried removing the corrosion on his old regulator at the earth bolt/wire? Sometimes that is all it takes to reduce the charge current, and his old RR was certainly a bit manky.
Yep, and dry connections in the connector block are common too, as you might be pushing a good few amps through it .
Feeding the output from the RR into the harness/ fuse block on the way to the battery doesnt help either - good RR replacement kits have a direct lead to battery and a circuit breaker.
And a cool running Mosfet type RR which helps stop the connectors drying out.
Most AGM batteries are NFU by the time they reach 11.9 volts, and most motorcycle charging systems are not designed to charge a flat battery - usual manual instructions are - only attempt to start with a fully charged known to be good battery, and Ignoring this is the usual reason for charging system failures. Like that guy is experiencing!
AGM batteries with 260 CCA instead of the stock 130 are not hard to find, and will still have 12.6V + after standing around 3 months.
So first advice might be get a decent battery , put serrated washers on the terminals and make certain it is wedged in tight.
And if you are going to replace the RR get a big Mosfet with a continuous rating higher than your peak generator output with revised direct to the battery wiring and a circuit breaker.
And the kit to connect the gear sensor to a switched circuit, as the Mosfet probably wont have a extra connector for it.
FWIW the old weak generator and RR on my 2017 bike still work fine and even without revised gear position wiring the bike bursts into life after standing 3 months, with a my AGM battery..
FWIW RRs were one of the few things that failed on Honda's, so most Honda forums/ owners will have similar advice!