I wondered if it would be okay to come back with an update on this old thread? I figured something out that may possibly help other people.
I first bought my bike a year ago, and was having epic problems with it lurching every time I pressed the pedal. It is a Gazelle Ultimate C5 step over.
In the end I bought a powermore range extender battery to use in place of the big battery, and that instantly solved the problem. I've been riding the bike almost daily every since and it's been an utter Godsend and is a pleasure to ride.
Then this week a penny dropped in my mind and I had that idea of plugging the big battery into the powermore range extender socket to see if that also makes the bike work fine. To my astonishment, it does.
So the bike works with either the big battery or the range extender battery, as long as they are plugged into the range extender socket.
It's only when the big battery is plugged into the big battery's own socket that it causes the motor problem.
I figure that this means that there is a problem with the socket. I am going to take the bike back for a service and ask the people in the shop if they can help with that.
I just wondered if I could update here in case it helps anyone else because this was a really stressful bug to deal with when I first had the bike and I would be really glad to if I could help other avoid having to deal with that.
Thanks!