6) How I made a hollow-decked twin wheeler

 This was a lot of work but I have been meaning to do this for some tme. I didn't like the bulky battery "brick" hanging under one end of the previous version of the twin wheeler. It was a LithiumIronPhosphate e-bike battery made in China and available on the internet via Hong Kong. They come wrapped in Duct tape and are often known as "duct tape batteries."
I wanted to dismantle the battery to see how it was made and whether the smaller cells within could be rearranged somehow to a more suitable shape. The battery cell balancer circuit board that comes with the battery has about 8 wires leading into the battery pack so I figured there must be 8 cells of some sort within the pack.
When I took it apart I was amazed to find it actually had 8 flat strips of 11 cells inside it, each one a bit like an oversized AA cell (see further down page).
This means I could do what I had always wanted to do: make a hollow deck as a sort of composite sandwich arrangement and put all the batteries in the deck so you end up with an electric skateboard with no visible battery pack.
I still have the wireless Wii nunchuck control system. The receiver for this plugs in (top left) with its blue light glowing so you can see you have
achieved a wireless connection with the chuck.
Come on now, that is pretty slimline for an electric skateboard, let alone a self-balancing one with 2 motors.
The entire skateboard deck is filled with LiFePO4 battery cells. Undersaide of deck is now in polycarbonate so you can see them.
 Another side view with electronics tidied up under a polycarbonate cover.
Close up of control electronics.
 One end has "feet" that rest on ground at startup while other end has small wheels which only touch ground if you are moving forwards and you cut the power or hit an obstacle and you tip forwards - at least it lets you keep rolling forward rather than crashing to a sudden stop.
  70's logo and view of polycarbonate impact shielding for the wireless receiver and the on/off switch.
The receiver for the wireless Wii Nunchuck is the white thing that is plugged into a socket in a protected recess at end of the board. 
Battery level indicator on other end.
The combined power unit / chassis containing the 2 drive motors, 2 wheels, reduction gearing via chains/sprockets and the support metalwork.
This was the smallest neatest most compact way I could devise for doing all these functions. Board deck just bolts to the top. Chain tension is fine tuned
using spacer washers under the motor mounting bolts as the motors are bolted up from each side rather than upwards onto underside of the deck. This has to be
done as chain provides drive in both forwards and reverse directions, so a tensioning pulley for example cannot be used.
The dreaded e-bike battery once about 10 layers of duct tape and a layer of cardboard had been very carefully peeled off.
BEWARE: This is very tricky to do. Not for a beginner. POTENTIALLY DANGEROUS.
- VERY easy to short out banks of cells with sparks and rapid heating/melting/fire. This is a very powerful battery. Wear eye protection.
You have 8 strips of 11 cells. Each strip is wired in parallel via metal strips running acoss top and bottom of each bank of 11. Some are cross linked
from one stip to the next and some are not. Short these out with scissors or whatever and you will get a big spark!
Each strip is glued together with rather weak glue. The duct tape is very strong and very sticky, therefore the problem with the teardown is that you can easily
break the glue binding the strips of 11 cells together as you try to remove all the duct tape. The metal strips top and bottom are very fragile and they
will not alone hold the strips of batteries together very well. All this means the duct tape removal has to be a slow and very careful process.
 The metal strips are very thin, not much more than thick metal foil, yet the red/black wires emerging from battery pack are very thick. I don't
see why as if you really tried to draw a high current from the pack it looks as if some of the metal strips would just melt. Maybe this is deliberate,
 an intentional "fuse" of last resort.
 Banks of cells separated here into pairs.
Strip of metal linking each bank are being cut (again incredibly difficult to do without shorting things out, I used fine pointed end self locking
surgical forceps to hold/lift the metal strip and pointed scissors that cut properly right to the tips). Link metal strips then replaced with
longer copper insulated wires so later on each strip of 11 cells can be laid flat.
Again, be very careful, I cannot stress how easy it is to short things out with sparks. I know because I did it even though I was being super careful.
 9 banks of 11 cells eventually separated out. Skateboard deck is made of 2cm square box section alloy rails each side. 2cm x 1cm
wooden cross spacers then used to prevent the top/bottom layer of thin marine ply from bowing inwards and pressing the batteries. These
spacers are also used to separate the metal strips running across each end of each bank of 11 cells. If these made contact there would be
much shorting out and flying sparks. Wood is a good insulator! At this stage I didn't know how long it would be so made side rails over-long for
cutting down later. Reminds me of packs of sausages. You can see all the duct tape all over the place!
 This circuit board comes with the e-bike battery and is involved in charging but also "balances" each bank of cells after charging so they are all at same voltage.
This apparently prolongs life of whole battery.
Upper deck is thin marine ply. Fixed to alloy side rails with self tapping srews. Also dozens of self-tappers used to screw it to the cross braces. This
requires very great care - if you miss the thin wooden cross brace you will end up puttng your self tapper straight into a battery cell.
All these self tappers mean you end up wth a really rigid composite stressed skin structure that does not bend when you stand on it and more importantly does not put any mechanical pressure on the rows of cells inside when you stand on it.
Underside of this composite deck is marine ply one end and I used sheet polycarbonate for the other end so you can see some of the battery cells through it.
Polycarbonate is the incredibly tough stuff they use for boat windows, bullet proof glass etc. Not the cheaper perspex for greenhouses that is too brittle and cracks easily.
Mine was sold as "Makrolon" but there are others.
View of one end showing from left to right: Plug in wireless Wii Nunchuck receiver, on/off switch protected by polycarbonate bulges,
recharging socket.
 Control system:
Top left: Arduino board that "reads" data from wireless Wii Nunchuck receiver.
Bottom left: 24V to 12V reducer (small additional voltage regulators taking this 12V down again to 5V are on the microcontroller boards themselves).
Top right: "Sabertooth" robot motor controller (i.e. 2 channels one for each of 2 motors).
Bottom right: Box containing a microcontroller from a robot shop based on the ATmega32 chip (see links on left for control).
Tiny box behind motor (i.e. near to centre of the board) not visible and contains (i) the balance gyro/accelerometer combination and (ii) the "headlock" gyro that is used to detect sudden direction changes (one wheel hits a small pebble for example) allowing the microcontroller to boost power to that motor to counteract the effect before you even notice it.