thank you Boubka for your diagram.
A priori, to make it simple and quick I only plan for the moment a speed. It should be sufficient and I will see to the use if a second speed is necessary. At worst adding a second relay or a rheostat should not be too difficult.
Otherwise, since the transmission is by chain, as is currently the case with the cart, I also thought that I could put a bicycle chain with a derailleur. The assembly would be even simpler for more speeds.
There are also other points I hadn't thought of:
1.) The motor assistance will not be necessary all the time and it is therefore necessary to be able to unclog it so as not to block it when it is stopped. A pulley system with a belt that relaxes with a joystick (like on some mowers) is surely the simplest solution,
or put the pinion block of a rear wheel of a bicycle which turns unladen in the opposite direction (when the engine is stopped). Bike belt or sprockets ... What do you think?
2.) Conversely, the inertia of the motor can be advantageous when going downhill as an engine brake. In this case, will the motor recharge the battery or do you need an additional electronic part?
3.) last detail: original the cart works with 2 large batteries mounted in series from 6 V - 220 Ah.
To test the engine I used a small car battery (12 V, 50 Ah) and it seemed to work well but maybe another type of battery is more suitable (weight, volume, autonomy, wear).
What do you recommend and where to find (suppliers) a battery equivalent to the original batteries?
If I put an electric bike battery, will I also need a special charger and what is the price for these materials?
