Not on the 500 but on a 150...
I have a “Tender” plug, the kind you hook the rectangular plug to and it’s wired directly to the battery via regular soldered and heat shrunk ring terminals for extended periods of dormant. Just like the factory wiring terminals, use the same screws on the battery that hold the bike’s factory stuff. Plug the “tender” in to the pigtail and all is good. No biggie.
Also have a 12v socket. Same kind in a car you’d plug a USB adapter to. The wires use the same ring type terminal to the battery positive and negative with 10 amp fuse wired inline for safety.
No key, I plug an adapter in and boom, I have a hot constant to charge a phone. I could charge a laptop if needed. Or a back up power supply. Even a GPS unit if needed, although I don’t use them. They come in handy though.
That socket is always hot and it doesn’t require the key to be on or the engine to be running.
And, as long as there’s nothing plugged in then there’s no amperage being drawn from the battery. If you leave USB type with an LED indicator light then there’s a SLIGHT draw with nothing USB plugged to it and it might kill it a day earlier when it sat not tended for like 3 months.
Otherwise, no more discharge than the natural discharge of a standard lead acid battery or glass mat battery.
Think about it like this.... the EX has no draw when not being used.
The battery naturally discharges.
Your expensive computer controlled car with a radio that keeps your stations and time draws more but it has a bigger battery so it takes longer.
Long story short. TL
Wire it all directly to the EX battery, don’t leave anything plugged in to the USB terminal when not in use, keep the battery plugged to the automatic battery maintainer/float charger when not in use.