I need help and advice please

[Wilssteve]

I bought a used Hebb E bike it is in beautiful shape and everything works I have one fairly major problem anytime I go up a hill or put a lot of stress on the motor it stalls out I’m then paddling really hard just keep climbing up the hill I can let the throttle off then put the throttle back on end it goes a few more feet in stalls out. On level ground it runs fine. Hey Mechanic who specializes in electrical problems said that it’s stalled out when when the motor was stressed because the battery was old and needed to be rebuilt. I found a place that will rebuild it for $550. Does anybody know if this is my problem have you had the same one and what have you done. Thank you for your responses