If I understand what you're saying correctly, it won't work. The battery isn't designed to take a charge from the discharge port. I think you'd just fry your battery or controller, but to be explicit: I'm not really sure I understand exactly what you're proposing, so I may be incorrect.
On the topic of adding batteries, though, there is a way: I bought the Current and I've put a little over 500 miles on mine thus far. I purchased the Priority range extender along with it, and was a little surprised with the way it works. As others here and elsewhere on the internet have mentioned, the bike draws from the downtube battery until it reaches approximately 30% or so, then switches over to the range extender battery and draws from that. This means that the "battery remaining" display will drop to about 30%, then seamlessly shoot back up to almost full. I haven't done enough testing to know how the two batteries behave once the range extender battery hits 30%, but the "battery remaining" display doesn't jump around after shooting back up when it first switches -- so whether it switches back & forth evenly, or just drains one before switching to the dwindling energy supply left in the other, I can't comment on.
This design does allow for one to actually add additional batteries to the bike though. I needed the range to be somewhere closer to 50-100 miles, and realized that the controller that comes with the range extender likely doesn't care what battery is plugged into it. There are two power cables, standard positive + negative, that go from the battery itself to a cell phone sized controller in the housing of the range extender assembly. From there, a number of other wires run out and down to the motor.
So I tried attaching a second, larger battery from a previous ebike I owned. I ran a pair of 12AWG wires, just standard positive + negative, and attached them on to the positive + negative wires that run from the range extender battery to the controller inside the rear rack mount; routed those additional wires out the existing hole, and added an XT60 plug on the exposed end. Then I used the existing top tube screws that came with the bike for a bag or a water bottle or what have you, mounted my old ebike's battery base on to it, and now have the built-in 10AH downtube battery (the standard one that comes with all Currents), the 12AH range extender battery (mounted underneath the rear rack, $800 from Priority for the battery + controller + rack assembly), and a 15AH battery available. It works like a charm.
What I do is always be very, very careful not to have the 15AH battery that I screwed into the water bottle attachment on the top tube plugged in at the same time that I have the range extender battery turned on. That would be very, very bad. I don't know if it would fry the controller, the batteries, or both, but it would definitely not be a good thing. So whenever I have the 3rd battery plugged in to the wires that I spliced in to the two power wires running between the range extender battery and the cell phone sized controller (the one that came as part of the $800 range extender accessory), the one that swaps between the built-in downtube battery and the rear rack one, I make absolutely sure that the range extender battery is turned off. So when my built-in downtube battery hits about 30%, the controller in the rear rack assembly changes to accepting power from whatever 48v battery is available to it. With the range extender battery turned off and an added third (48v) battery spliced into the two power cables that run into the controller, that's my third, 15AH added battery. When that dies, I just stop for a brief moment, turn the bike power off altogether, unplug the XT60 connection from that third (15AH) battery, flip the power switch on the range extender battery underneath my rear rack to the "on" position, power the bike back up, and voila, the little controller in the range extender does what it was designed to do and shows me a full backup battery that it starts using.
Again, if I ever carelessly leave the range extender battery turned on at the same time that I have the third battery I added myself plugged in, it would probably fry something. It's been about 200 miles I've gone with this setup, and I've had no problems. The voltage reading when the range extender is flipped to the "off" position is about 0.4v, as measured with a multimeter. When it's on, it's the standard 54v. So when it's off and the third battery is plugged in, as far as the controller is concerned, it's the same 54v that it expects.
I even tried using a fourth battery that I stuck in a bag on the rear rack yesterday (a 24AH battery I still have from my last ebike), and it does indeed work just fine. I can rotate through any number of batteries this way just by plugging them in to the wires I spliced between the range extender battery and it's control board.
Hope this is useful to anybody who, like me, has a need to use the Current for more than just joyrides.
