Keep in mind that all protrusions are going to generate significant drag. So if you can find ways to incorporate the batteries
within the airframe, you'd be better off.
There's also the problem of mass-transfer. In general, aircraft should keep the center of mass behind the center of lift, and ... bad things happen when this isn't maintained. With liquid fuel, tanks are actively pumped to retain both forward-aft and left-right balance, something difficult to achieve with solid battery packs.
There's the problem of both energy consumption profiles and battery delivery/drain cycles. An aircraft generally needs maximum power to get off the ground (hence: all batteries delivering at or near their maximum output), but only partial power once airborne. What you'd like to do is to drain a few batteries completely in the take-off and ascent stages, then jettison them, but this doesn't match the batteries' own power delivery capabilities. You might be able to switch to jettisonable packs after TOaA, to completely drain those.
If fit between wing spars, you might end up with a roughly rectangular package which could be ejected aft of the aircraft from the wing, with a door sealing off the cavity. The battery itself would require some sort of deployable wing itself, as well as guidance and control systems and surfaces, possibly a small propulsion unit. A guided descent stage might actually be one of the more viable options.
It's also possible that jettisoning additional batteries on final approach would make landing dyanamics for the aircraft itself simpler.
On whole, though, I'm questioning the usefulness of this, particularly given coplexities, a likely low airspeed, and competition with ground-based alternatives (high-speed rail, Chunnel) which would bypass the power storage requirements entirely, and would likely operate at equivalent or greater speeds, direct to city centres.