Sell a storage system that allows the batteries to be safely and individually stored and integrated. It could estimate the capacity of each battery and house 20 or so. Such a system could probably pay for itself after using it for less than a year. There are certainly going to be guaranteed minimum capacities that such a system could promise, it doesn't need to be perfect. If enough people invest in such power storage systems, you don't need to have a single system work 100% of the time, just like you don't need every wind turbine to work 100% of the time.