Battery Power?

With Jack driving SAG, his Pedego battery was now backup to mine.  With no fear of losing power before the end of the ride, I took off and powered up to level 5.  I was able to get up to 18 to 20 MPH. Unfortunately, that lasted only 2 miles!  There were 4 bars on the monitor when the entire system shutdown.  Fortunately, Jack was close by.  We switched batteries and I was able to finish the day's ride.  Still I'm very disappointed in my battery's performance.