"A fraction on the expensive side, but in the long run worth it. There is an annoying feature in the EV3Mindstorms sets, the ease of changing batteries in built models / robots. In a lot of cases you need to dismantle part of the model to change the batteries and in some cases this can be difficult and includes unplugging sensor cables around parts. The rechargeable battery helps with this problem, but is not the whole solution because in accessing the recharge socket you can still need to pull out sensor cables or move a part to get at the battery socket. Perhaps a right angle power plug on the recharging transformer cable would help in most cases. Alternatively a short adapter cable approx 2 inches long that stays in place on the rechargeable battery, (optional feature ), that way it would be a simple matter to plug in the recharging cable from the transformer without trying to get around the robots cables and parts. Plug and socket extension cable would remain on a robot where it is difficult to access the battery recharging socket due to the robots compact and complex construction."