Every model can accommodate up to two (2) of either capacity battery, the 10 Ah or 15 Ah size in the battery storage compartment under the seat.  Capacities can be intermixed, i.e. you can carry one (1) 10 Ah and one (1) 15 Ah.  However, they can not be connected in parallel with the Dual Battery Connector accessory.  The batteries need to be the same capacity to be able to connect together.  If you wish to have one 10 Ah and one 15 Ah capacity battery, you can carry both and manually switch between them as you see fit.