I believe if you offered a 12 volt 4 amp li-ion battery that you know works with your controller you would sell enough batteries to be profitable. Your customers would be confident the batteries will work with your controller. Purchasing a battery from you when buying the controller would be convenient for your customers to purchase the controller and battery at the same time. Hundreds of batteries are sold from China on Ebay.com so it is possible to ship batteries to the U.S.
~Tom... Working towards becoming a PORKtologist. Weber Kettle & UDS