So my wife and I rode the Going-to-the-Sun Road in Glacier National Part this June on our Rad City 4 ebikes. Most of the way up, my wife's bike just up and died. Thinking that she just drained her battery, I gave her my battery and slogged through the last couple miles on my, now pointlessly heavy and dead, ebike. When we got up to the top, I had the great idea of using the regenerative braking to try to recharge my wife's battery. It seemed to come alive on the ride down, but died again when I got to the flats and wanted a little pedal assist.
The next day, after a 12 hour charge, the battery seemed back to it's old self. But as soon as my wife used a higher pedal assist or throttle, the bike died. We got it back home and the battery pack, even when freshly charged, will only cause the display to blink on and then go right back off.
I have contacted Rad Power and, after two months, they are sending me a replacement battery. Since I still have the old battery, I wanted to know if I could repair it.

Here is my situation:
1. When I turn on the key, and push the battery level check button, the lights all come on for about a second and then turn off. If I press the button again, I only get a faint flicker before they go out again.
2. When I attach a multimeter to the positive and negative terminals, the voltage reads about 12 volts. When I turn the key, the voltage momentarily (approx 1 second) goes to 53 volts, and then drops to 23 or so volts.
3. When I open the case, everything about the BMS looks good.
4. When checking the voltage of each battery cell, they all read 4.2. All together, they read 54.5 volts.
5. The BMS is an RP1304AA model

So what is my next step? Since the battery pack is good, I would hate to see it go to waste. Is there something I am missing?

Thank you all!