A Home-Built Ebike Battery Pack from 18650 Cells

April 3, 2015
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Editor Note: Affordable and powerful 18650 battery cells are the hottest thing in the DIY electric bike revolution (most well-known from their use in cordless tools).  If you are looking for ready to buy 18650 packs with BMS go to Luna Cycle website. Bike builders everywhere are discovering that clustering many 18650s together are an effective way to build not only an affordable pack, but one with unbeatable performance and safety.

Note that even though they are safer than soft pouch LiPos (popular with RC enthusiasts), it is still a lithium chemistry, which is always dangerous when mis-handled (fire risk). Lithium pack-building is for advanced users only and should not be tried at home. Plenty of people quote that the Tesla EV uses 18650s, which is true…but the original ebike builders to use 18650s is the $14,000 Optibike which has been using 18650 packs before any other ebike builder.  

The Following was submitted by Damián Rene, a DIY builder in Madrid Spain…thanks especially for the beautiful pics and video taken by Damians talented girlfriend Tania Netsvetaylova. It was the video that really caught my eye and caused me to reach out to Damian to get him to write this story. I knew before seeing them, that the pictures were going to be beautiful as well, and they were.

 

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“When I started into building an EV, I realized soon that there was an accentuated performance gap between the 3 main components: Motors, controllers, and batteries. Motors and Controllers are pretty easy to source good ones that perform well at a decent price. However, when looking for the third component, the battery pack…you run into problems. It is very hard to find a decent ebike battery pack that will put out good performance at a reasonable price. All Cell packs for example puts out decent performance, but are too expensive. (read our article on commercialy available ebike battery packs).

Even if you save up the money and buy something like an All Cell pack (because it is using just ‘decent’ quality 18650 cells) the pack itself does not have good energy density. Meaning it is large and heavy for the power it contains, and it only puts out 30 amps, which for me is not enough. With ebikes we often say you can have it fast, light weight, or cheap…but you can’t have all three. With lithium battery packs you can have cheap, high energy-density (small and light), or high amperage (fast) but you definitely can’t have all three…and sometimes you are lucky to find just one. So like with ebikes,  same with ebike battery packs, if you want the performance, light weight, and low cost…you have to resort to building yourself.

So I started on my mission of building my own battery pack, which begins with tons of research on endless-sphere.com. Building a battery pack is serious business, not for newbies, and it must be taken seriously. Using a DIY lithum battery pack also takes a lot of common sense and knowlege. If a lithium ebike pack is not handled correctly, they can burst into flames. Of course catching your house or garage on fire is a topic that should not be taken lightly. Of course, I wanted to build my pack to be as safe as possible.

So I started figuring how different battery packs were made. I spent tons of time on Endless-Sphere. There is where I found the most valuable information about EV technical info that I could find anywhere on the Internet. About EV battery packs I found that they are made basically into groups of different types as cylindrical, prismatic (or pouch) cells, in a variety of serial and parallel configurations. (series gives higher voltage, parallel give more amp hours)

Soon I focused on packs made with the 18650 cell format. I found they were the safest, most manageable, and with a high energy density. These cells would make it possible to build the pack I desired. Also, it was possible for me to find high-quality 18650 cells for cheap.

So there was time for a deep search on how the major suppliers made their own packs. Cells are usually attached together with plastic holders that group them, and then spot welded with nickel strips to make the different parallel and serial configurations. Then I find that there was some home builds that had the cells glued together. This was the key I was looking that must let a battery being powerful at the minimum volume.

 

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I discussed advantages and disadvantages of joining the cells with hot glue on the forums.

Some builders said to me that the space holders let air between the cells, and is necessary for cooling purposes. But I realized it was the opposite. When the cells are in contact, the heat is transmitted faster between their plastic covers than from cover to cover through air. So the heat travels faster to the exterior.

Also high heat would thaw the glue, but even in the summer, if the cells are attached together (making fast heat transition) and if the EV requirements and battery performance is counted, the heat will never be enough to affect the pack.

Another issue is how do you exchange a single cell if one fails? One failed cell can ruin an entire pack. Exchanging an 18650 cell is always difficult in any 18650 battery pack, because the cells need to be spot-welded with metal strips to secure the connections. So apart from removing the spot-welded strips it is still needed to also remove the glue.

The last huge issue to consider with any battery pack is the Battery Management System (BMS). The BMS is a cell manager which controls what the pack is doing and shuts down in case it senses anything is wrong. For example, a BMS protects the battery from overcharging or undercharging…both of which can cause the battery pack to fail or worse…burst into flames. The problem with a BMS is its very hard to find a good one available that fits the size of the pack I plan to build, it is expensive, and it is hard to assemble. Most commercially available packs have a BMS and it is the recommended way to go for ultimate safety and reliability.

As an experienced DIY, I decided I can safely go without a BMS because of my knowledge. Basically I use a Cycle Analyst and a high quality smart charger for my BMS. Meaning the Cycle Analyst lets me know the voltage of the pack when I am riding and shuts down the power if I forget and lets me know if my packs voltage is dropping too low…Similarly, the smart charger makes sure my pack does not get charged over the allowed voltage. Charging cannot safely be done unsupervised on a DIY pack such as mine without a BMS…and if it is, it should be done somewhere in an outdoor barbecue for safety. Sounds ridiculous…but that is the downside to DIY. Especially for newcomers to electric bikes, a good BMS is essential for fire safety!!

Ok so now you know some of the disadvantages and problems associated with a DIY pack. The advantages are that the energy density and specific energy reaches unbeatable values for a very affordable price…especially in my case using recycled cells.

So I get the equipment (spot welder, nickel strips, BMS, battery shrink wrap, hot glue, some instruments and tools) and the 18650 cells I could bought at a good reasonable price (they came from Bosch rejected welded packs), and I started to make a 13S / 15P, 48V 42-Ah pack that later would give the very interesting performance of a max range of 300 Kilometers (186 miles) at 25-MPH on an EV made for the tests (that’s another Story). All in a pack that is below 10Kg (22.7 lbs) with this dimensions: 420mm x 144mm x 67mm.

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Please watch the video that my wonderful girlfriend Tania (a filmmaker) made documenting my battery pack build…it is a good illustration of what it takes to make a 18650 DIY pack.  Again, do not try this at home without serious research and knowledge. This article is not meant as a how-to guide…but more to show you what is possible if you do your homework”

 

 

Damián René & Tania Netsvetaylova From Madrid, Spain.

And his latest video:

More pictures for your viewing pleasure:

 

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Spot-welding the first parallel group. This parallel module provides 4.1V and a range of 42-Ah. By connecting 13 of these together in series, the pack will provide 53.3V when fully charged to 4.1V per cell, and per parallel group.

 

I’d like to add a note here about “nominal” voltage. Due to the odd-shaped discharge curve of these lithium chemistry cells, they have a fairly flat voltage across the broad middle of the voltage during the discharge of most of their capacity. The 4.1V per cell will drop off fairly rapidly on a ride to around 3.7V per cell, and then…near the end of the ride, the battery voltage will start to drop off rapidly to about 3.0V per cell, at which time the Low-Voltage-Cutoff (LVC) in the controller will cut the battery off to protect it.

This means these cells have a nominal “average” voltage rating of 3.7V per cell, sooo…a 13S pack (13 paralleled modules connected in series) would have an average “nominal” voltage of (13 X 3.7V =) 48.1V, and this allows this “custom” pack to use commonly available chargers and controllers.

 

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The spot-welding connected many nickel strips. A thin connection would get hot from the current, so a thicker connection between the cells would run cooler.

 

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Preparing to connect the first parallel group to the second group. Notice the hot-glue between the cells to hold them together. The parallel cell group on the right will be flipped over so the positive ends of the group on the left will be connected to the negative ends of the group on the right.

 

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A close-up of the two spot-welding probes.

 

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Two rows of positive ends on the left…connected to two rows of negative ends, then positive, then negative again. If he stopped building it here, it would be a 4S / 15P pack

 

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Here, all 13 parallel groups are finally connected in series. Each parallel group has had a thin wire connected to each positive and negative end of it. Doing this will allow for the option of balance-charging each parallel group individually (if desired)

 

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This BMS has a digital read-out, and it is showing a 40V. Since this pack has 13 P-groups in series, each cell is at 3.0V per cell.

 

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Very large diameter heat shrink sleeves can be ordered cheaply in a variety of sizes.

 

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After testing a few times, the pack voltage is down to 36V.

 

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If charging is done at a common 5A rate, the large gray plug shown (the one that is un-plugged) is much larger than necessary, but if the pack can provide a current level of  60A (like this one), the connector (the one to the right, which is plugged in right now, between the battery and controller) must be large enough to ensure it doesn’t overheat. If the cells are charged up to the recommended 4.1V per cell, this 13S pack would be at 53.3V when it is fully charged.

 

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When I read that Damian was using 60A peaks on a hub motor, I knew it had to be a direct-drive. Here, he is showing the colored LEDs that he installed for night-riding.


Why 4.1V?

You may have noticed that every place in this article where the charging voltage is mentioned, we state it should be charged to 4.1V per cell. It has been widely published that these cells “can be” charged as high as 4.2V per cell, so many ebikers do that to try and make sure that they get every possible mile of range out of their pack. The truth is that due to the odd curve in the discharge of these cells, there is almost NO extra range between 4.1V and 4.2V.

But that isn’t why we do NOT recommend charging to 4.2V per cell. Charging to 4.2V per cell is when some BMS’s fail, and then the battery catches on fire. Also…charging to only 4.1V can actually double the life of your expensive battery pack. If this interests you, click here to this article we wrote about how Tesla cars have an eight-year warranty on their 18650 packs (with five full cycles a week), and how they were able to get their packs to last that long.

Ride safe, and have fun…


If you liked this article, you might also like…

What’s inside an 18650 cell, and why it’s important

Introduction to battery pack design and building, part-1

BMS’s, what the hell do they do?


Written by Eric, April 2015

Grew up in Los Angeles California, US Navy submarine mechanic from 1977-81/SanDiego. Hydraulic mechanic in the 1980's/Los Angeles. Heavy equipment operator in the 1990's/traveled to various locations. Dump truck driver in the 2000's/SW Utah. Currently a water plant operator since 2010/NW Kansas


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