A Home-Built Ebike Battery Pack from 18650 Cells

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.

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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


58 Comments

  1. Great article Glad you posted this.

    • Hi Damian, great work, I am leaving on sweden, want to buy a lithium battery from you, I need 48v, 65 cells. send to me yuor email. mazena@hotmail.com Thanks.

  2. I love the video. The soundtrack is awesome.

  3. Well written and informative. I am using hobby king now, but in the future, I likely will use 18650 cells. Thanks

  4. Great work Damián eres una máquina.

  5. um… the battery gauge in the pictures shows percentage, not voltage 😀

    • Thanks! correction made.

  6. Im using 18650 cells that I got for free from my local recycle depot. Just goto the depot and ask for the used lipo battery packs used in laptops and portable dvd players, also sometimes used in 7.2v drill packs, they all have 18650 cells in them.. break them open and test each battery with a multimeter, if they are 3.7v or higher (4.1 max) then put those ones aside and once you have enough you can return all the lower voltage cells to the recycle depot. Then buy or build your own balancing circuit to suit your required voltage and bingo, you have a high capacity ebike battery kit for practically free.
    Im using my pack on my electric unicyle which has a 60v 500watt motor. The battery pack that came with my wheel was 16 18650s in series offering 132wh which was only about 12 kms on a full charge, now with my extra pack (using the free laptop batterys with a higher ma rating) i have about a total of almost 200wh which equates to about 28 kms on a full charge which is not bad for a electric unicycle that weighs 14kgs in total including batteries!

    • So if a cell is discharged, you expect it to be damaged?

      • If a cell is discharged, it is damaged. You cannot allow the voltage to drop below a certain point or permanent damage will occur.

        • So each time you discharge your computer to the points where it needs a recharge, the cells are actually damaged?

          Most 18650 sized lithium cells even have safety circuitry embedded so they never allow discharge below cut off voltage. This threshold even leaves enough leeway so you can let it sit for a while after it reaches cut-off and it will not be damaged by its self discharge (for a good amount of time).

          You cannot know the health of your battery unless you charge/discharge it at least one cycle. Of course if the cell reads 3.0v when you pick it up, the protection circuitry most probably won’t allow you to charge it.

          • A cell with a voltage reading below the minimum safe voltage indicates that the cell is bad. That is what I am trying to get at. Once a cell drops below the minimum, permanent damage has already occurred. Not all cells have built-in protection. This is the whole reason why you have to have a BMS.

          • No, a laptop battery is protected by special circuitry. Why do you say that 18650s have built-in circuitry? If that were true, why would we need to use a BMS?

          • Because it’s only partially true. Some have, and some don’t. You can identify the ones that have by their increased length. It’s perfectly normal to have battery protection circuits in both the charging circuits and in the cells themselves.

          • I have Samsung 25r cells in my pack. Anyway, my point was just that you cannot let a cell go below a certain voltage or it will get permanently damaged.

          • 18650’s for flashlights and vapes often have a tiny button-shaped BMS on the tip. The 18650’s that laptops and ebike batteries come with definitely do not.

    • I can’t find anyone that will part ways with their old batteries! I called couple computer repair shops, but most of them already sell them to a recycle depots and and the recycling depots don’t want to sell my any for cheap. Any ideas?

      • FYI, in the UK, the council dumps have skips set aside for electrical devices, and there are dustbins full of discarded batteries next to them, until of course they empty them. I just wander in and take what I need. I’ve had hundreds of 18650s, totally free, and my Powerlab 8 has resuscitated even the ‘dead’ ones, from showing as low as 0.9 volts (which most people say are totally useless), back to fully functioning!

        This is not even illegal, and the council workers take absolutely no notice of you. They probably think, “if he’s scavenging for dead batteries, good luck to him…”

        YMMV, depending on what country you are in, but remember: it’s better to ask for forgiveness afterwards, than permission before… 😉

        Max

  7. I see you have been asked before but I can’t find an answer; what is that BMS with a digital read out and where did you get it? Thanks.

    • It is a standard BMS, the “digital read” is just a battery meter installed besides.

      • Please, may we have links to the battery meter? Or perhaps the name of it, and I’ll find it for myself? Also, excellent works of electric engineering art! Many kudos to you, Sir!!

  8. I’m building out of used recycled laptop batteries18650 cells and making a 48volt battery with bms . Just time consuming testing cells and charging then discharging .confused trying to figure out how to convert Mah to amp hours

    • Mah/1000=Ah

  9. I was wondering like John Brown where you sourced your BMS and capacity readout display?

  10. that’s hot. all of it. spot welding. homemade batteries.night-riding.. hot. hot. hot. lol. btw, this led’s lights are hot. what are they. hot.

    • nothing is getting hot! it’s all coooooooooool.

  11. great
    could you share with us some links for good quality bms?

    • Ali Express ALL-TSHOP

      US $24.96 that with free shipping.

      • i can t arrive to find it
        could you share the link

        thanks for you help

  12. where do you find the lcd for this bms?

  13. Please excuse my ignorance.I was curious how many of the 18650’s it would take to go the 186 miles that was stated in the article?

    And yes this is battery ART at its best.Whats really cool too is Damian will only get better and better!

    Tim.

    • Thanks for your kind words. Super-generally speaking you need around a pair of standard 18650 cells (~2.9Ah) to get one mile of full electric range. In this case the battery was made specifically to get a huge range together with a direct drive system with an extremely high efficiency (20″ rigid frame, no chain nor any train losses except bearings, high pressure tires, max speed under 25mph) and it get almost one cell one mile (~7Wh / Km or 11Wh / mile) around 300km (186 miles) with those almost 200 units of NMC 2.85Ah cells used

  14. If the cells are having built in protection such as Panasonic NCR18650B, is it necessary to have a BMS? Can any body coment

  15. Excellent article, thank you. I read below you may also build for others? please check your email.
    thank you very much in advance Joe

  16. If I have 20pcs of 18650 batteries 10A continius discharge current and connected them 10 in series and 10 in parallel the voltage of the pack is 37 volt and 20A continius discharge current?

    • If you have 20 pieces, you can configure them to be 10 pieces in series to get 37V, and then 2 cells per parallel group, for 10S/2P = 20 pieces

  17. Where did you get your charger??? In the process of building this pack. Thanks for all the info!!!!!!!!

  18. That many batteries would cost a fortune.

    The 10,000mAh 18650’s on ebay for £1 each are closer to 100mAh.
    They are all fake.

    • that is because 18650’s can only have a mAh of 3800 right now. Of course 10,000 mAh is fake when the best brands are making their 18650’s 3800 mAh. Avoid brands like Ultra Fire. If you are paying less than $4 per battery be cautious. I got mine from old laptop batteries containing high quality brands like Samsung and Panasonic.

      • anything over 3000 is a fake claim theres no such thing as a real 3800

        • 3500mh is the max on 18650, like the Sanyo GA.

          • shame that no ga ever tested has managed to hold that much,,and at a 10a rating its pretty much useless

  19. Sorry but “good”18650 cells like the ones that I use, Panasonic NCR18650B are not cheap. They are quality cells and although not outrageously expensive if you know where to look,still my 96 volt 26 ah pack cost me $883.00 Canadian. Thats doesnt include the BMS, shrink wrap, nickle strips, etc, but in the end you know you have a high quality pack that will last a long time and give superior performance.

    • John absolutely g8t vid on the pack, is it possible to share the layout, i’m still a bit confused. I’m trying to build a 96 volt 35 ah pack, i’ve scored 600 Panasonic 18650b’s but don’t have the layout right in my head. also have you seen the vid using larger gauge copper wire with solder connections for the vw bus convert to e-city cruiser, your thoughts on solder verse spotweld

      thx

      Ken

      • I can…email me

        • Hi John you can hit me back at ken@ex10nd.com thx

  20. What BMS is use in the photo ? thanks

  21. Need to build a 56-60v battery that I will be using to convert a bike with 20″ moped rims and a 48v 1500w 46.5 kmh — 28.8mph 13 * 5T winding rotor hub motor. I’m looking more for range than speed (mostly flat where I live), although I would like to top 30mph. If my math is right, in order to accomplish this I need to build a pattern that is 16s6-8p. Which 18650 cells should I choose? I’m also not sure which BMS I should use? And then which controller is best for this battery and motor setup? I’ll post the links to the parts I’m currently sourcing and let me know if you think there is a better set up or parts. Thank you

  22. no real world performance data? longevity? charges?

  23. where can i get a 36v 50ah battery built? …not aliexpress or alibaba

    • we can offer you visar@lonmil.com skype id visar10081

      • i need it 2 fit my sondors

    • Hi Broman , hope you are doing great . I am nicole , from Shenzhen Sairi Battery Co., Ltd , we can help you to build the battery packs .
      Please getting back to us via nicole@cnlbattery.com if possible .

      • im looking into 48-72v now

  24. Hey there, I’m seeking assitance in designing and layout for a 36v 10series 6parrelle 18650 ebike battery pack to fit into one of those alumiunm drawer type rear bike rack. I’ve all the ingrediants but need some kind of drawing to put the batteries laying down siding into that aluminum box. Please help

  25. my name is justin, i have constructed a pack of battery locally and its working well but i need a charger, any idea on how i can charge it please.
    please i need to be using it.

  26. Great job René!
    Is it possible to add a temperature probe in the pack and control the temperature with the BMS? It would be interesting to slow down the charge rate if the temperature rises too much. Or have a temperature alarm if you are discharging too fast.

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