What is the indicated maximum charge?

It will get to 80%-86% charged. It never gets all the way to 100%. I leave it 24/7 on the Sonos brand Wireless charger and it never gets up to 100%. I even tried turning it off by holding the button in back down for around 7 seconds until I hear the the three notes indicating it is now turned off. Then eventually, the Roam speaker always turns itself back on, but never after getting to 100% charge. I know not getting to 100% is not the end of the world, but it is very annoying it never gets 100% charged and that it keeps turning itself on. So far, there are no solutions to solve this.

Can you test charge using a USB connection?

I have done that too. It keeps turning on even after I have manually turned it off and never gets to 100% charge. My main point is I bought the Sonos Roam with a matching Wireless charger at the same time. So far, I have NEVER had my Sonos Roam charged to 100%. My reasonable theory is, it should charge to 100% with the wireless charger it was originally purchased with. Also, a Sonos Roam speaker should REMAIN OFF when manually turned off until they are manually and purposefully turned on. My experience with this Sonos Roam is even after I purposely manually turn it off while sitting on the wireless charger, it always turns itself back on and never charges to 100%. If I manually turn it off while off the charging platform and then set it on, the same events occur. I am basically ready to give up on this particular Sonos device. All of my other one have been flawless in their performance. I have a Media room with Sonos Arc+2 Era 300’s+Sub setup for 5.1 Atmos. Also, a  Master Bedroom with Soundbar+SL100’s+Sub setup for 5.1 audio. 

My ROAM’s charge to 100%. There can be minor measurement and calculation rounding errors that result in topping off at 98% or so. While this is annoying, it’s not a major issue. If the unit tops out at only 80% I think that there is a fault. I suggest that you take this up with SONOS support. If within warranty, I’d expect a replacement.