I want to build an electric bike that has a small lightweight power generator for maximum range. The law here in Norway prohibit gasoline powered bicycles and electric bikes are limited to a maximum effect of 250w. Recharging the battery takes a long time and therefore having a power generator charging the battery constantly while riding is more convenient. I talked to the authorities and the only legal way to do it is to have the generator mounted to a trailer behind the bike. So thats the way i will go. I have talked to the guys at the Endless Sphere forum and they recommend the use of SLA batteries. The ebike system is a 48v system with a 250w geared electric hub motor. Should four 12v 7ah batteries be appropriate? As for the generator and charger assembly, thats where i will need help from someone who is experienced with electronics. My knowledge in that field is close to nothing. I need help on choosing the right dynamo and so on. The engine that will turn the dynamo is a Sachs Stamo 33cc two stroke from the 60s. It has a pulley wheel mounted to it which makes it suitable. Motor specs Power: 1.5 hp / 1.1 kW at 6,500 rev / min 1.7 hp / 1.3 kW at 7,000 r / min Operating speed: (4.000 **) 5000-7500 U / min Torque: 0.22 kpm at 4,500 rev / min So, what should the power output of the dynamo be? It must not be too powerful or the engine may not be able to turn it. Do i need a regulator to keep the voltage stable? What voltage is required to charge the four SLA batteries? Will i need some kind of charger to charge the batteries from or can i direct to power directly from the dynamo to the batteries? Any help would be greatly appreciated!! I must wait until i have all the answers before i can order the parts for the electric system. Today i will start building the generator trailer. It shall be a bob trailer that leans with the bike. I will weld it from steel tubes. It will have a swing arm with suspension to protect the generator from mechanical shocks from the terrain. It will all be enclosed in a metal box with ventilation and sound isolating foam to protect it from water and dampen the noise. I intend to use this for daily commuting to work.