Thanks so much for your thoughts on this. You clearly have spent some time considering and researching your answers, and I appreciate that.
Is there any reason that I could not use my car battery to recharge the batteries and save the cost of the power supply units?
Also, a friend offered to let me use the batteries from his son's 24V bike as a backup set this coming weekend. Do you see any problems with running them in my son's 36V bike?