18650 ebike battery pack
 Building an 18650 battery pack 
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 some 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 builders just heat shrink the batteries together which leaves them vulnerable to damages from dropping or crashes.  Initially I thought that building a battery pack in a Hammond aluminum box would be a good idea, but I ran into issues and abandoned the build. Here's my build blog on that attempt.

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

November 2015
Now I will be using these plastic battery holders. They allow for series or parallel connections and have decent sized tabs that should handle the current required. I ordered them through Amazon and they were shipped on the slow boat from China.

To build this battery I will need 12 of these holders. Each battery holder will have 4 groups of cells in parallel, and the 12 cell groups will be arranged in a flat 3 x 4 array. I will end up with a pack that is about 12" x 9", and will be about 1.5" thick. This will fit well on the the rack of my ebike. The whole pack will be inside a box built of Lexan (polycarbonate). It will be nearly bulletproof.

I initially thought I would connected them using tabs made from copper sheet, but I decided that thick copper wire would be better. This is 14 gauge wire from a house wiring project. I bent clips to go between the tabs on adjacent battery packs.
Next I soldered the clips to the tabs, and to a "bus bar" connecting all the cells groups in parallel. Not perfect but I think the solder connections are solid.

I was happy to see that these plastic packs are fairly heat resistant, and that the soldering does not make them melt down. They do melt but it takes a lot of high heat to do it.

This technique is much easier than the method I tried previously and abandoned. It will probably take about 10-12 hours to get the pack all soldered together.

Here's a test fit of the batteries. They still fit (whew!). They are very tight, and will require a ribbon or a strip of cloth under each group of cells to make it easy to pop the cells out of the holder.

There will be three rows of four packs in series.


The ends of each row use some thick copper plate as a bus bar to connect to the next row, and as the main battery pack positive and negative terminal. It's pretty hard to solder to the thick copper.

The battery holders have nice slots to allow them to be fastened together with plastic wire ties. I'm about half done.

It took me about 8 hours to build the basic pack. This picture shows 12 batteries in series (one in each holder), producing 43.7 volts. The ribbon of cloth is in place to help remove the batteries as they are very tight.

I took it to the garage to see how it fits on the bike and its a bit larger than the shelf but will work fine.

Added wires to the pack and removed the motor controller (ESC and wiring from my old LiPo pack. I cut out and bent up the bottom half of the polycarbonate plastic case. As shown it has room to mount the ESC and stash the wires.

The plastic case is made from re-used plastic.

Here's the top cover. The jumble of wires will be hidden in one compartment of the case. The controller will be screwed to the bottom of the case and be exposed for cooling. The case is 1 1/8" thick, 13" wide and 14" deep with a notch to clear the set stays.
On my previous LiPo pack I had used a loopback cable with large Anderson connectors as my on/off switch. This worked great. I thought this time I would use a large DPDT switch as a power switch for my battery pack. I added a battery in the last row of my pack which completed the circuit to the controller as apparently the switch was on. I smelled a whiff of that electronic ozone smell and immediately removed the battery. Hmm, the switch was stuck. Apparently it welded itself together. Looks like I will not not using that switch.

Along with the larger fuse holder I obtained one of those big twist switches to switch a car battery as I had good luck with using that as an on/off switch on another e-bike build.

The pack wiring is completed and the batteries are loaded. Next I will do the first balance charge and test the pack with the e-bike.

Here's the battery pack mounted on the bike. The bike bag will go on top of the battery pack so it won't be too noticeable. I blasted up and down the block to test the pack out and it worked fine.
One thing I had noticed during my initial test rides was that the power seemed way down in this 10Ah 18650 pack compared to my old 48V, 10Ah LiPo battery pack. This was very disappointing to me as the high acceleration and fast speeds of this system made it very fun to ride. Recently the system died when I was starting a ride. I found that massive corrosion of the fuse holder wires had caused it to fail. After making my own fuse holder using spade lugs and putting it back together I did some testing and it seems much faster and accelerates more strongly. Stupid cheap fuse holder! I am now a happy e-biker.

I hadn't used the ebike to commute in several years. Partly due to life circumstances, partly due to the wonky weather we have been having, and partly due to range anxiety. Well, the weather I can't do anything about, but the life circumstances got fixed so it was time to fix my range anxiety issues. The existing pack works fine, but is only 10Ah, which would get me to work and about half way home.  Also I learned that high power (about 80A max) with just 4 of these Samsung 25R cells in parallel makes them sag in voltage significantly. Sure each cell can deliver 20A (20A * 4 = 80) but they don't like it. Because of this I decided to double the pack size. Same voltage but 20Ah. This would allow me to get to work and back with plenty of power to spare, and also make the bike a bit peppier.

Also in the past few years, other options have materialized to allow 18650 batteries to be built into battery packs without soldering wires to the battery, which can damage them. Pictured are the battery holders (white boxes) I bought from AMTech. I bought 12 battery holders that hold 8 batteries in parallel each. Also shown are the Samsung 25R batteries and the pile of boxes they came in.
Here are the batteries in one of the packs. The packs are 3D printed from ABS plastic. The cover (not shown) is screwed on.

Though you can get connectors to go between the packs from AMTech,I decided to make my own out of thick copper sheet. The ends of all the batteries and the copper connectors were coated in conductive dielectric grease to prevent corrosion.

Here's the pack all put together with the connectors. It's really compact. I like it better than the flat pack I made previously.
I decided to use a battery management system (BMS) this time so I don't need to manually check the cell array voltages manually. This one is good for up to about 80 amps. 80A x 50V is 4000 watts, which is a bit more than this system can deliver
I built a polycarbonate (Lexan) box for the battery and mounted the BMS and ESC to it, then added the wires to the individual cell arrays.
I put the battery in the Otivia box that I had sitting in my basement for the past 10 years. It's a nice locking plastic aero shaped box that's made to be clamped to a bike rack. I bolted the battery box to a piece of plywood, and clamped the plywood through the Otivia box into my integrated rack.

After some fiddling I got it all mounted up and solid.

Here's the Otivia box all mounted up with the lid on it during a test ride. The new battery pack performs well. It's much better than the pack with 4 cells per cell group and now feels very peppy. I think the LiPo still has faster accelleration and a higher top speed but this pack is close to the same performance, is safer and should allow me to get to and from work without recharging.
Here's the Cuda-e with the new battery pack and box. It's looking very black. I'm going to add some florescent yellow tape.
Because it's been several years since I rode this bike any distance, and I did a complete rebuild in the meantime, I had to do several rides to shake out any issues. It's now all ready for the commute. I was ready to ride the bike to work for the first time in 5 years or so and realized that I now have a laptop bag that I have to bring to and from work that I didn't have back then. The Otivia box would be fine for shoes and pants and jacket but I need to find a way to transport the laptop bag too.
I decided that I would need to build a large-ish Coroplast tailbox to fit the computer bag, lock, shoes, lunch and other stuff needed for e-bike commuting.

The Otivia box and the battery and wiring were removed, and a bit of extra aluminum support for the extended tailbox was added. The battery will still be supported by the main rack and I'm not going to be carrying anything too heavy.

I cut the bottom of the tailbox and re-added the battery mount bottom plate.
I re-added the battery box and some tailbox side panels. The computer bag fits!

The side panels didn't want to bend to match the curve of the bottom of the box, so later I used a heat gun to heat the inside of the tailbox sides to make them bent to match the curve of the bottom of the box.

I also used the heatgun to bend another panel for the tailbox lid.



The battery is all mounted under the seat back and the wiring is completed. The on/off switch is mounted through the side of the fairing.
A coroplast cover was made to protect the battery and wiring from the elements and from prying eyes.
Here's the tailbox with completed lid. I reinforced the seams on the inside with Gorilla tape.
Here's the completed tailbox. I used a furniture lock to hold the lid closed and keep it safe.

I rode to work today for the first time in about 5 years. It was wonderful. A lot was changed on the bike in those 5 years. The paint, seat cushion, battery, tires and tailbox are all different but it felt the same. The tailbox was rock solid with no rattles. The bike made it to work and back on a single charge.

Here are the stats from the CycleAnalyst:
  • It took 38 minutes to go 16 miles and use 10.95Ah (464.21 Watt/hr).
  • Trip efficiency was 28Wh/mi.
  • The battery pack started out at 48.8V and ended at 42.9V.
  • The 92 max amps were probably delivered during a few jackrabbit starts at the beginning of my ride this morning.
  • The average speed was 25.4MPH, but the max speed was 45.6MPH in an area with a bad intersection and a narrow road where I need to go fast to avoid being passed.
  • At the end of the ride it would only deliver about 1500W max. Min voltage was 36.8V.



Back to the electric bike projects page