Seems like what's really needed is a way to swap batteries at home, so that you can leave one pack at home to charge from your solar panels while you drive to work with the other one, then swap in the evening when you get home. Otherwise you'll have to charge a stationary battery during the day, then charge the car pack from the stationary battery at night, with all the attendant losses.
I'd be concerned about receiving a battery that has had more charge cycles than mine. Kind of a good way to unwittingly shorten your vehicle's life span.