Our bike has had some battery related problems that we suspect are BMS problems. We tried a substitute battery and the bike worked fine.

So I checked the battery voltage on our battery with an analog voltmeter. The needle jumped high and then settled to about 48V. This happened every time I checked it.
I also checked the output voltage of the charger. It reads 120V. I checked several times. This doesn't seem right.

Could we have a faulty charger that then damaged the BMS in our battery ? 

Our RadCity3 step-thru has developed a problem after a few month's use. The battery charges and the display reads full charge, but as soon as motive power is called for, the display data goes blank and there is no motive power. Dealing with RadPower's customer service has been frustrating, as they are slowly walking us through generic troubleshooting steps that aren't relevant to our specific problem.
We've checked all the fuses and connections, etc. This is a well maintained bike and there is no dirt or corrosion.

The display lights up, and so perhaps that means it is not the culprit. Or does it ?
If the problem is the controller, how can we just buy one, put it in and see if the bike works ?

Is RadPower the only source for the controller and the display ?