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.

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 compared to my 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.

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