Battery bag for my E-Bike

Last weekend we borrowed a sewing machine to make new curtains for the apartment. When I was sewing the curtains I realized that It wouldn’t be very difficult to make a custom battery bag for my e-bike. I would like a bag that I could mount in the frame triangle and have room for 4 Turnigy 6S 5Ah batteries. Doing some measurements, calculations and drawing I came up with this design

Battery bag drawing
An outline of the battery bag, the red triangle is the frame, the blue rectangles are batteries and the gray rectangle is the controller mounted on the frame.

I went to the fabric store and found a black nylon fabric that had a PVC layer on one side. This should be fairly water resistant and I plan to spray it with some textile waterproofing spray as well. I was recommended to use a thread for furniture which is much stronger than ordinary thread as well. A couple of hours thinking, cutting, and sewing later:

Battery bag
I use some Depron in the bottom, and some foam in the two corners to protect the batteries and fill out the bag.
The zipper has a bit of fabric folding over it for water protection
Hole for cable
The hole for the cable is also waterproof since the side of the bag overlaps ~10 cm where the cable comes out.
I'm using the cable I made for the E-Puch. This cable is for two turnigy batteries in series and two in paralell with a 60A fuse. The LEGO part is just for size reference.
Everything fits nicely inside the bag

The bag fits perfect on the bike!

Battery bag on bike
Battery bag on bike
Battery bag in frame
Battery bag in frame

Pleas write a comment if you think I should make some kind of drawing and description on how to make a bag like this.

Replace phase wires on Nine Continent 2809 hub motor

The original phase wires (wires between the motor and controller) on my E-bike are way to thin for the currents they are handling. When I got the motor was equipped with 2 m long 1,5 mm² cables. In low-speed-high-torque situations for example steep hills or starts from a standstill the currents in the phase cable can be several times the battery current. My controller limits the battery current to 27 A but the phase current could sometimes approach 100 A. Doing some calculations with the resistivity ρ=1.68•10-8 Ωm giving the original cables a resistance of

\frac{1.68\cdot 10^{-8}}{1.5\cdot 10^{-6}} = 0.0112 \: \Omega/m

Judging by the color I’m not really sure that the original cables are made of copper. They could just as well be made of some other metal which would result in even higher resistance. With the 2 m cables the current path to/from the motor is 4 m and the resistance (excluding the motor resistance) is about

0.0112 \cdot 4 = 0.0448 \: \Omega

This may seem pretty low but when constantly running 100 A through this cable the voltage drop on the cables will be ~4,5 V and the losses will be about 450W. This would of course instantly cook the cables and luckily enough the current to the motor is not constant and this current levels will only appear for short periods of time at very low speeds.

Anyways, I decided to replace these cables for the thickest I could fit into the axle where the cables are fed into the motor, as well as shortening these cables as much as possible. What I’ve read is that 12 AWG cables (3.31 mm²) is the thickest you could get through the axle without stripping the insulation and adding something thinner. The original cables have a PTFE insulation which I think is a good idea since PTFE cables generally have thin insulation and is very resistant to heat and mechanical wear. I ordered a couple of meters of 12 AWG PTFE cable from Apex Jr which was tho only place I could find that sold this dimension of wires online in small quantities. This cables are definitely made of tin plated copper which can be seen on the cut surfaces.

Comparison of original and new phase wires
Comparison of original and new phase wires

Doing the same calculations with the new shorter and thicker cables

\frac{1.68 \cdot 10^{-8}}{3.31 \cdot 10^{-6}} \cdot 1 = 0.005 \: \Omega

This will result in a voltage drop of 0.5 V and 50 W losses at 100A which is much more manageable. I guess this will give me some additional efficiency but its probably unnecessary with the current performance of my controller. Later this summer there will probably be a post about re-programming current limits and eventually upgrading the MOSFETs of my controller.

Taking the motor apart was easy, I first removed the side cover on the cable-side by removing the nine hex screws and then used a knife to cut the glue and pry the cover off. Before I did this I made a mark on both the cover and the hub so I can put it back exactly the same way. I’m not sure what tolerances are used when manufacturing these but I don’t want to risk a wobbly wheel.

Inside the 9C 2807
This is what the motor looked like when one of the covers was removed.

Getting the wires through the axle was hard work and took me more than an hour, the method that worked for me was to put a thin wire through and the used it to pull through the phase and hall wires all at once. It helped a lot to grease the wires with soap to get them through. I kept the original hall sensor wires.

Wires through the axle
Wires through the axle

Electric MTB

My electric MTB. I forgot to mount the battery pack for the photoshoot.

I’ve got a 6 km bike ride to work, the route is mostly flat but there is a high bridge I need to pass and the wind is not your friend when biking westward in Gothenburg. Despite this it is quicker for me to go by bike than public transport. The problem is that I do not want to be all sweaty when i arrive to work.

The solution: Put a motor on the bike

My bike is an Crescent Balder 24 speed mountain bike with front suspension and aluminum frame. Here in Sweden there are some regulations limiting an E-Bike to 250 W of power and a maximum speed of 25 km/h. That is not enough for me but I need a drive train that is possible to limit when driving on public roads.

After several hours of reading at the Endless Sphere forum I settled for a Nine Continents conversion kit including a 2809 rear wheel motor, 48 V 27 A motor controller, twist throttle, regeneration break handles and some other parts needed for the conversion. This kit has what is called an Infineon controller (based on the Infineon XC846 chip) and is programmable and possible to limit within legal limits.

E-Bike controller
E-Bike controller mounted behind the saddle

This is a 12 MOSFET version of the controller with a somewhat splash proof aluminum case that i mounted on a luggage rack from Biltema. One of the reasons that i choose the Nine Continents kit was that someone wrote on the web that it was one least bad china made E-bike kits regarding weather resistance.

E-Bike kit delivered
E-Bike kit delivered

The motor comes already laced in a 26″ rim and has mounts for disc brakes and a 7-speed freewheel, the only thing missing is a tire.

The 2809 means that the stator and magnets are 28 mm wide and each stator tooth have 9 turns of wire wound around it. There are different configurations of this motor, for example 2807 which have 7 turns wind resulting in a faster motor. The 2809 has a top speed of 35 km/h, on flat ground with 48 V battery voltage, which is enough for me. The slower speed means that the motor will be more efficient than the faster one at slow speed, for example uphill or with strong headwinds.

To power the motor I use Lithium Polymer (LiPo) batteries made for electric RC airplanes. At the moment I have 4 Turnigy 5000 mAh 6S 20C LiPo Pack batteries with 6 cells @ 5.0 Ah each. These are connected together as two 12s1p (12 cells in series and one paralell) packs like the one below.

Battery Module
One of my two 12S1P battery packs. These can be connected in parallel as well for a 12S2P pack. The serial wiring contains a 30A automotive fuse and a Anderson SB50 connector.

But the two packs can also be paralleled for a 12S2P pack with twice the range. The serial/paralell harness is made out of HXT 4mm Gold Connector for the battery, 10 AWG (~5,27 mm²) wire, a 30 A automotive fuse and a Andersson SB50 connector.

12 cells, at 3,7 V nominal voltage, in series results in a battery voltage of 44,4 V. 2 cells, of 5,0 Ah in parallel results in a total 44,4 V 10 Ah battery or 444 Wh of energy. My plan is to, at a later stage, extend this to 3 packs in parallel to 666 Wh of energy. Commuting to and from work with the bike has shown a energy usage of ~12-14 Wh/km. Mostly dependent on the amount of pedaling from me and wind speed/direction.

The battery specs say that they will deliver 20C continusly which means 20 times the capacity. My current battery with 10 Ah capacity will hence deliver a maximum of 200 A @ 44.4 V which is almost 9 kW of power.

There are a couple of different types of batteries used for electric bikes LiPo (which I use) has the highest power- and energy-density. The downside with LiPo is that they can be quite dangerous if you mistreat them, for example overcharge or puncture them in which case they will explode in a ball of fire. There are other lithium chemistrys for example LiFePo4 which are more stable but have lower energy density and much lower power density. There exists LiPo batteries that can deliver up to 90C while I haven’t seen LiFePo4 batteries capable of more than 10C. There are nickel and lead based batteries as well but they belong to the last century IMHO. Maybe I’ll write a more in-depth post about batteries, battery management and chargers in the future.

If you have any sort of experience repairing bicycles the install is easily made in a couple of hours, the most difficult thing was that the dropout has to be filed a little to suite the large axle on the hub motor. Since all torque from the motor is transfered through the dropouts the axle is 14 mm instead of the ordinary 10 mm bike wheel axle. The hub motor axle is flat on two sides making it fit in the 10 mm dropouts. Since the bend radius on the non-flat sides is 2 mm larger than on a regular bike wheel the frame dropouts has to be filed to a bend radius of 7 mm for a good fit.

Original dropout and axle to the left, hub motor dropout and axle to the right.

Since an aluminum frame is not as strong as a steel fram I decided to use what is called a torque arm to strengthen the dropouts. When using a hub motor that delivers a considerable amount of torque to the small flat sides of the wheel axle this is a must if the dropouts should survive. For example a powerful hub motor on an aluminum front fork without torque arm could end bad if the motor manage to twist itself out of the dropouts and the wheel comes loose. The rear wheel is probably less dangerous but I still don’t want the bike to break.

Torque arm
Torque arm. Note that the rear derailleur isn't used.

In the picture above you can see that there is no wire to the rear derailleur. The bike is originally a 24-speed bike but the hub motor only has room for 7 sprockets. Since the gear lever was mounted on the handbrake lever on this bike the rear 7 gears had to leave room for the regenerative brake lever supplied with the conversion kit making this a 3-speed bike. The left gear and handbrake lever is still intact controlling the front derailleur and brake.

I have used the bike to commute to work for a couple of months now and it works perfect. I have some future upgrades I want to make:

  • Move controller to water bottle mount
  • Replace phase wires to 12 gauge
  • Create fiberglass battery box
  • Tidy up cabling

This time of the year the weather in Gothenburg usually not invite to biking so I’ll have the winter to perform these upgrades on the bike.