Long story: The battery connectors slipped loose during a ride and the power was on and off. So I fixed the connectors and tightened them.
The the connector to the controller broke a piece so I got one from a car parts store and clipped one end and spliced it to the batteries.

Alas. I crossed the wires! When I plugged the battery to the bike it went backwards and the controller started smoking. I pulled the wires apart and it stopped.

The I carefully re-spliced the wires correctly and turned off the key at the controller and joined the battery to the controllerand rode around on pedal power for a little while.

Then I turned on the key. It went forward correctly and things seemed very powerful - maybe too powerful because the throttle was unresponsive.

Has anyone else had to replace a controller and/or throttle?

Thanks