36 volt battery charges to 41.8 volts.

[JES2020]

he empty battery will bounce. The full will not — evident of the weight of electrons!

Yes, this is a strange phenomenon, as electrons don't have mass and therefore no weight.
I found this explanation of the bouncing battery phenomenon:

Do dead batteries really bounce?

You’ve probably heard of the bounce test for dead batteries- simply drop a battery, and if it bounces, it’s dead. It turns out that this isn’t completely true. Conventional batteries have an outer layer made of zinc, which once hooked into a circuit, is slowly converted to zinc oxide, while...

www.mcgill.ca

 

[harryS]

Electrons do have mass, 9.1x 10-31 kg, as I recall. A constant I still remember. They love me at parties, but only when it's all EE's.

 

[JES2020]

Electrons do have mass, 9.1x 10-31 kg, as I recall. A constant I still remember. They love me at parties, but only when it's all EE's.

harry is always there to challenge.
If you are an EE than this paper should be of interest to you.

Electrons with no mass acquire a mass in the presence of a high magnetic field​

Electrons with no mass acquire a mass in the presence of a high magnetic field

An international team of researchers has discovered that in a very high magnetic field an electron with no mass can acquire a mass. Understanding why elementary particles have a mass is a fundamental question in physics and an area of intense debate.

www.sciencedaily.com

 

[Gionnirocket]

All you do is leave the charger on for 10 to 12 hours AFTER the full charge is achieved.
As far as NOT having a BMS that balances, read the below extreme safety issues.

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This is absolutely the wrong thing to do if you don't have a balance BMS
Though you might get it to go up in voltage... You are cooking the stronger cells in the process as they reach over volt shutdown and then cycle in this state until you remove the charger in hopes that the weaker cells catch up. And if they do its only temporary as they are still at a different internal resistance.
This is exactly how battery fires start.