Well done on the centre stand.
This is where I got my battery
36V 15AH LIFEPO4 Lithium Battery electric bicycle bike on eBay (end time 30-Jan-11 09:18:00 GMT)
He will make them up in any configuration so the choice is yours as to where you want to fit it. I have a pair of front panniers and I have my extra battery in one side, it balances the rear biased weight ratio. The new battery will come with a bare leads on the feed and a two pin plug on the charger line, The charger line and power cables come from the same place so I just fitted a kettle plug that I charge through and also use it to get the power to the controller. I then just ran a 1mtr extension to the rear of the bike (you can purchase a lead with the kettle plugs already on) I went into the controller box and brought a patch lead out from the main feed via a switch that I switch between the original battery and the extra one. The patch lead can be either soldered to the 1mtr lead or you can fit a kettle plug/socket. The thing about the way I did it is that I can disconnect the battery and use it on another bike and/or have even a third battery that can go onto the Aurora. All my batteries now have kettle plugs on patch leads so that I can interchange them. If I were to order another lipo4 I would probably just order a 10Amp flat one that would be a similar size to the original and that way it could easily go on the rear rack or better still be slung under the cross bar where the balance is perfect.
I think that the 2 pin plug that comes on the charger is the male version that should fit into the controller box ( what the original battery is supplied with)and you could make a patch lead up to plug a spare battery into that and prevent having to add wires to the controller.
It`s a bit complicated to get across what I mean but if you send me a private message with your telephone number I`ll ring and chat to you. You would need to be a bit adept at soldering to be able to add the battery into your system but although it might sound complicated it is really quite straight forward.
If you can`t face it then you could always just purchase a second battery from Alien and either keep it on the rear rack or better still just suspend it under the crossbar with three or four velcro straps in readiness. You will probably get your journey their and back on one battery if you run medium and just keep the other battery on board in case. Then maybe once a week swap them over to even out the use. The main thing is that you have a good commuting bike, a bit heavier than a lot of the more expensive bikes but some how the bulk and weight make for a very robust and comfortable ride.
Dave