A solar charge controller allows solar panels to charge specific types of batteries, especially lithium. MPPT stands for Maximum Power Point Tracking, and most people building solar e-bikes are using special voltage boosting MPPT's in order to take maximum advantage of the solar PV system. We currently offer two types of voltage boosting MPPTs: the Genasun GVB as well as a lower cost option from Grin. Genasun is a premium brand, and their waterproof controllers carry a 5 year warranty.The Grin MPPT is less expensive, and likely less waterproof, but it allows the user to reprogram the max charge voltage of the MPPT so you can adjust to different voltage batteries.