Unusual Plug Pack for Vaccuum

[mbarszcz]

I got the new Lithium Ion version of the Bissell Pet Hair Eraser (the Purple One) and noticed it came with an charging adapter unlike the old one.  The vacuum contains a 14.4V Lithium Ion pack.  When charging, my kill-a-watt shows that it is charging at 5W.  The unusual part is the output voltage listed on the back of the adapter is 5V-23V DC, 300mA.  A variable output voltage plug pack with only two connections to the vacuum 

Does anyone know why there is a variable output voltage on the adapter and how this might work?  Perhaps more than just a simple DC supply and instead acts as a constant current source working in conjunction with some kind of internal battery charging circuit?  That's fine for the beginning of the charge cycle (CC mode), but to complete the charging of a lipo, at some point it needs to switch to CV charging.  How will that work with this arrangement?

Open circuit the output of the charger measures 17.5V.  When plugged in and charging, I measured 15.05V across the output.

[jpanhalt]

My guess without additional information is that your supposition is correct.  USB-C fast and ultra fast chargers do something similar.  Although not related specifically to your case, I find the DigiKey explanation one of the easiest to understand:
https://www.digikey.com/en/articles/designing-in-usb-type-c-and-using-power-delivery-for-rapid-charging?