18650 ebike battery pack
 Building an 18650 battery pack  - abandoned...
By Warren Beauchamp
An easy to build battery pack
There has been an explosion of choices in new high power batteries for the intrepid e-biker, but the best option as of this writing are the 18650 form factor Lithium ion batteries using the NMC formulation. These batteries are called INR or NMC and are of  Lithium manganese nickel formulation. This chemistry combines the safety of LiFePo4 batteries and the high energy and density of LiPo.A disadvantage is that they do not have the longevity of other formulations. After 250 cycles these cells will be down to about 60% capacity. Is this really a problem? Not for me, it will take me several years of fair weather commuting in the Midwest to get to that many cycles and by then a new, better, cheaper batter will be available.

The biggest issue with building e-bike battery packs out of a bunch of small  batteries is how to connect them together. They can be soldered together but the heat of soldering may damage the battery. They can be spot-welded but that requires specialized equipment that is not available to the average home builder. In addition, many battery pack buildings just heat shrink the batteries together which leaves them vulnerable to damages from dropping or crashes.

This pack will be built inside a Hammond cast aluminum box. To build the pack I'm thinking about thin fiberglass sheets for the conductor substrate. The conductors would be glued to the fiberglass sheets using RTV silicone glue. Dimples on the conductor sheets would contact the cells. The cells could be pre-glued together into a brick. Using silicon glue would make it (relatively) easy to disassemble the pack later if need be. In addition, a fiberglass perimeter shell would help keep them together. The pack would be pressed together using the screw-on lid to keep the battery connections in tension. There is also room in the chassis for wires and connectors.

I am looking into building a 44V x 10Ah pack using one of these cases with Samsung 25R cells . I can't believe the energy density these little cells have. Each cell has 2500Mah (2.5Ah) of capacity. These batteries have a 3.7 V nominal voltage with a 4.2 V maximum while charging. Each cell is 0.72" in diameter.



Using the dimensions for the box and the 18650 cells I whipped up these drawings, which show the layout for a pack of 48 total cells. They are arranged into 12 groups of 4 parallel cells. (12S 4P).

4 parallel cells x 2.5Ah each = 10Ah.
12 groups of 3.7V cells in series = 44.4V nominal, 50.4V max.

This drawing shows the interconnects and battery orientation for the bottom of the pack (top drawing), and the top of the pack (bottom drawing. Here's a PDF of this drawing to allow printing it out to scale on a single sheet of paper.

The main positive power wire connects to the leftmost set of interconnects on the bottom drawing, and the negative power wire connects to the rightmost interconnect.

The interconnects will be made of .021" copper sheet. I'll print these drawings out 1X and glue them to the copper sheet to use as templates to cut out the interconnects accurately.

In addition a thinner wire will attach to each interconnect for use in balancing the pack and testing the pack balance. This pack will not have an internal battery management system. I will use a bulk charger to charge it normally and check it periodically to ensure the cell groups charge and discharge at the same rate.

   
   
9/3/2015
Here are the 48 Samsung 25R batteries in the aluminum box. As calculated there is still space in the case for wires, a fuse, main power and balancing connectors.

The Samsung 25R cells cells are rated at about 8C (20amps) max continuous current, so 4 cells in parallel can crank out 80 amps. Peak current (short duration) is 35 amps per battery, so that means 140 amps peak for the pack. That's a lot of current!

These batteries are packed for max density. Some battery pack manufacturers space the batteries apart to give them space in case they get hot. While e-bike acceleration can use high current and my e-bike can use in excess of 90 amps during a wide open throttle charge from a standstill, 99% of the time that this pack is used it will be at under 20 amps (1000watts), which should not cause the cells to get hot.

 

9/12/2015
It took a bit of trial and error to get the cell conductor layout to print out to actual scale but now that I figured that out I created a PDF so that it will be easier next time I have to print it. I used spray adhesive to fasten the 1X printout to the 6" x 12" copper sheet. Here you can see the conductors ready to be cut out.

I drew in tabs for the main power wires and the balancing wires. Also I added numbers and arrows to make it easier to figure out where the  conductors go after they are all cut out.

To cut the conductors out I used my old trusty aviations snips. I did have to do some filing and flattening afterward to clean up the jagged edges and ensure they were flat. Cutting out the conductors took several hours.

After the cutting and cleanup it was time to add the dimples. the + and - symbols are sort of in the right place but I measured and drew lines on the paper that was still glued to the conductors after they were cut out to make sure. After that, using the dimple pliers to make the dimples was easy.

Next, I wetted out 3 layers of fiberglass with West Systems epoxy and left it to cure. I embedded another copy of the layout between the layers of fiberglass to make it easy to see where to glue the copper conductors down...

9/13/2015
The fiberglass top and bottom substrates for the conductors turned out well. I tried to tin the dimples with silver solder but I don't think my iron is hot enough. Maybe I will try the torch :) 

Next steps will be to solder the balance wires to the tabs and glue the conductors down to the top and bottom substrates.

9/22/2015
The balance wires are soldered to the tabs. I laid up more fiberglass to use for the perimeter box to hold the 48 batteries in place. Once the epoxy cured I cut out the flat panels to make the sides of the box. Here they are held together with tape. I will use more fiberglass on the corners to make it strong.
9/27/2015
I glued the conductors down to the bottom of the fiberglass substrate with automotive grade RTV silicone glue, then fastened the fiberglass box together with more fiberglass. The plastic wrap helps press the wet fiberglass to the sides of the box and keeps the mess contained.

After adding the batteries to the box I could see that it would be difficult to ensure that they all had good connectivity so I abandoned this battery pack idea.

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