The loose nicad batteries are a solution that I like.
They are installed into a sealed pvc tube ten batteries deep with a strong spring and a threaded cap . Think flashlight.
When one goes bad you don't need to replace the whole thing just find the bad cell.
Nicads will discharge at 10C Ten times normal they will last a long time in the desert heat 120 f.
Not soldered just using spring tension in a pvc tube with a threaded cap.
I am thinking of 24 ten cell tubes.
This battery is on another channel. search for an article about rewinding brushed motors. 61 pages long it goes on and on.
The Allelectronics 36 volt spring return throttle is $12.75
TWIST GRIP THROTTLE FOR ELECTRIC BIKE/SCOOTER, 36V | AllElectronics.com
The other throttle is what I consider heavy duty. it normally sells for $50.00