This is a video showing a good system to charge a 36V lithium ion battery for an electric bicycle using solar power. It uses a grid solar panel (typically around 250W, 32V/8A power output give or take a bit). This choice of panel is for price, availability and more than enough power output. You need a good regulator to control input into a 12V storage battery (100Ah is good if you want to charge during the dark daily, less if you want a mobile solution). The key is to use an appropriate 12V to 36V lithium ion battery charger/DC converter.