OK No luck. Let me explain in case someone else wants to try. So the Aventure charger has three terminals, positive, negative, and a sense line. I was hopping it was easy to debug, but no luck. Unfortunately this is a trend I've seen, making the charger paired to the battery.
So if you attach the sense line to the negative terminal, then you can measure the 54V charge voltage on the + and - but ONLY if you plug the charger in the wall FIRST, THEN ground the sense wire. If you ground the sense wire first, THEN plug it in to 120V, then NO charge voltage.
OK I thought, maybe the sense pin in the battery is grounded so when you plug the charger connector in, it then starts.
So In spliced the Aventure connector to the Cycle Satiator. Plugged it in and no cigar. The Cycle Satiator has to "see" battery voltage or it won't start. There are two reasons why that might be. First, maybe Aventon has a diode in the battery so the charger can't see the battery voltage. Second, maybe something else with that sense line turns on the battery. I don't know. I could figure it out if I spent more time, but didn't. So now it looks like the Aventure charger or nothing. One good thing, it looks like the battery light turns blue when the battery is around 90%, so if you unplug it then, it will last longer, but you must watch it constantly, so not very practical.