A Modular System For Building Heavy Duty 18650 Battery Packs

With 18650 cells as cheap and plentiful as they are, you’d think building your own custom battery packs would be simple. Unfortunately, soldering the cells is tricky, and not everyone is willing to invest in a spot welding setup just to put the tabs on them. Of course that’s only half the battle, you’ll still want some battery protection and management onboard to protect the cells.

The lack of a good open source system for pulling all this together is why [Timothy Economu] created DKblock. Developed over the last three years, his open source system allows users to assemble large 18650 battery packs for electric vehicles or home energy storage, complete with integrated intelligent management and protection systems. Perhaps best of all there’s no welding required, the packs simply get bolted together.

Each block of batteries is assembled using screws and standoffs in conjunction with ABS plastic cell holders. A PCB is placed on each side of the stack, and with tabs not unlike what you’d see in a traditional battery compartment, all the cells get connected without having to solder or weld anything to them. This allows for the rapid assembly of battery packs from 7.2 VDC all the way up to 150 VDC , and means individual cells can easily be checked and replaced in the future should the need arise.

For monitoring the cells, a “Block Manager” board is installed on each block, which communicates wirelessly to a “Pack Supervisor” board that monitors the overall health of the system. Obviously, such a robust system is probably a bit overkill if you’re just looking to build a pack for your quadcopter, but if you’re looking to build a DIY Powerwall or juice up a custom electric vehicle, this could be the battery management system you’ve been looking for.

36 thoughts on “A Modular System For Building Heavy Duty 18650 Battery Packs

  1. i have 2 issues with these as there are a lot of them, and to be honest the issues are minor for the most part. My first is that they use a bms. I understand that with large numbers of 18650’s it is much easier but to get better imho to balance charge them. But for my larger issue is that these systems are almost always designed around a concept/design/whatever that they have and it almost never fits the design i need so i end up making some modular custom pack.

        1. Were you using ver. 2? They now have bolts holding them together. And it is also recommended that you shrink wrap the battery. Didi you do that? I ask because I am about to go that route.

          1. Not sure what version it is, but I got it from the first shipment of kick starter, so most likely it is not V2 with bolts. also I didn’t shrink wrap them and that would most likely solve the problem. I just made a large battery 91 cells 48V 20ah for my ebike. used a spot welder it was fun to learn and not bad to use. Made two of those large battery packs.

    1. Regarding your second point, it’s impossible to envision and account for every possible design constraint, so they obviously have to take a best guess as to what will work for most people and go with that. Can’t really hold it against a solution if you have a particularly constraining use case.

      As to your first point, I’m honestly not sure what you’re saying. You have an issue that they use a bms, which you understand with a large number of cells it’s easier but to get better balance charge them? I can’t figure out what that means.

  2. I usually advise that a 18650 pack use welded bus bar construction. But this pack’s purposely designed spring finger contact arrangement looks like a good DiY Power Wall builder’s solution. The spring metal contacts are beryllium copper that’s rated for 4A continuous, 12A burst. Everything is open source, including the injected molded parts STP files. Wow!

  3. This would be great for a small vehicle/device or an immobile system, really great for that so I don’t mean to take a dump on this idea, but it won’t work well in a proper roadgoing EV for two reasons, the maximum overall pack voltage (150V really won’t cut it these days, you want at least 400V) and the cumulative weight of this nice convenient pack housing. Spot-welding is a PITA and requires equipment a DIY’er is unlikely to have, but it is the lightest and most compact way to assemble a battery pack.

    1. It appears that Tim’s modular design was intended for EV applications (do a search on the author, he is a serious EV guy).

      The 150V limit only applies to the as-published code. Customization of the firmware would allow higher voltages.

      I prefer welded construction too, but the extra weight in this weld-free solution does not seem objectionable to me. Plus, it’s open source so the mechanical and/or electrical design can be changed to suit different requirements.

      I am curious about the back-story to the DKblock design. A huge amount of effort went into creating it. I’d say that a lot of money was spent on the Injection molding tooling too (see tooling photos at Github). I could be wrong, but it looks like it started off as a commercial product (back in 2016) that never made it to market. Whatever the story, it’s great all the hard work is shared as a open source community project.

      1. Yeah it’s a long story and over a beer someday…the DK part of the name comes from the design coming out of meetings at the Diamond Knot brewery in Mukilteo, Wa. And yeah the tooling is still viable, the mold is in China and can be used to make parts at any time. I think welded is always preferred for very high performance applications, but the DK is meant for those applications that can have lower current levels, and still have industrial strength hardware quality.

        And yeah the 160VDC max is really dependent on the DC-DC used on the Pack Supervisor PCB. I do have a Analog Devices/LTC design good for 600VDC, which will go on the next version of the Pack Supervisor if someone does not beat me to it!

        So yeah please feel free to use whatever you want from the DKblock!
        Tim

          1. Hey Thomas, it’s actually a happy one, involves building a new passive house, and losing my top software guy, which made me have to work a little harder. But hey I’m a positive guy so one door closes and another one …
            I consider the open sourcing of the DKblock a step in the evolution of the product, not the end game. I’m truly interested in speeding the transition to an electrified world.

            I’m working on a new Block Manager hardware that will be cheaper and much, much improved, using a sub-Ghz TI multicore ARM chip. Maybe someone from the embedded C community can give me a hand on software, to speed things along. After that is done, I’ll turn my attention to the isolated power supply on the Pack Supervisor, so we can get 30-600VDC inputs using a new LT chip.
            Onward!!
            Tim

    2. Most of the weight of the DK system is not packaging, it’s the cells. So while you won’t quite get to the density of the welded cell design, it’s pretty close. DK won’t have the small wires you will need for temp sensing and voltage sensing needed in the welded configuration.
      Tim E

  4. I have reservations about using spring clips to connect to the cells. Every product I’ve ever had that used spring clips had reliability headaches. Sooner or later they corrode, or a cell leaks, or simple shock and vibration causes intermittents.

    Think about what people do to make a proper connector. Both contacts are generally made of specific metals chosen for their performance; gold, silver, or (bottom of the line) tin. Then they have specified contact pressures, to exclude contaminants and break through any surface films. Spring clips only apply low pressures. Gold and silver work at relatively low pressures; but tin needs high pressure (other metals even more). Maybe Tim could gold plate his springs and the ends of the cells; but that is likely to cost more than spot welding.

    In contrast, every commercial module I know of uses welded interconnects. Failures are very rare. Yes, a spot welded takes a little more tooling; but he put a lot of tooling into these battery modules, too. Wouldn’t it be better and less work to spot weld them?

    1. Hi Lee, the springs are made of BeCu and they are gold plated. The cell ends are tin plated, but that is still considered to be an industry accepted practice, and have over 3 years of development show to not corrode in a non-immersive environment. And yes the contact pressure is designed to carry the rated current in both static and dynamic (vibe and drop test) conditions. You can weld if you want, but this is a solution for those who don’t want to weld and want to have a recyclable pack.
      Tim

  5. It seems a bit odd to use a wireless bus for monitoring a system thats purpose is to be otherwise wired. Also, won’t that potentially cause issues in EV use where there will likely be lots of shielding in the form of the chassis and interference from the inverter?

    1. Not at all. If you have ever troubleshot a pack with lots of small voltage and temperature sense wires, you will appreciate the wireless part. The wired part will just be the large current conductors and busbars, hard to lose or break those!

      Tim

  6. I would have to pick up and whack on the street my e-bike every time it winked out, just like the aptly named “flashlight” with it’s springs and series contacts. Then there is the magic battery dance. The device is dead, take the battery out twirl around and put back in. Hocus Pocus now it works.

    A rounded contact touches another or a flat surface at a very tiny point. Nothing is really flat so it fails. The design might be more robust if multiple springy fingers in a radial pattern engage the cell ends.

    1. So this is really nothing like your “flashlight” spring. This cell is constrained to not move (much) in place, so it really doesn’t bounce around inside the block. Also the current rating holds for dynamic conditions, like you would see on your ebike. But because of the lower peak current specification, as I said in the Why DKblock webpage, this block is better suited to lower power density applications, like cars and trucks (more room for batteries) and power walls, which means you might still wanna use other designs for your ebike. (https://dkblock922508958.wordpress.com/why-dkblcock/)
      Hope that helps!
      Tim

  7. Are thse DIY batteries actiually legally allowed on the road in an EV vehicle? Do these EVs have a sticker: DIY LIIONPACK. I have been in the LiION Battery business for many years. Seen a few fires. These batteries will have never seen a drop test, or a UN test.

    1. Its a cool concept but I too have worked on a BMS system and a lot of this stuff needs to have proper QA/Testing as lithium battery fires are very dangerous, and the smoke to these things is very toxic. My concert is a cell shifting and shorting something

    2. Thanks partly to DIY’ers we have lithium-ion packs in cars. Some of the first lithium cars were converted “lead-sleds”.
      Do it yourselfers don’t always wait for those of us in the battery business to make commercial products.

      Tim

  8. Crap, there goes my weekend. I need this, but I need it for 2170’s :-) gotta love open source.

    I’d be worried about spring contacts over time as well, but maybe some Dow4 in each connection would improve things.

  9. I dunno, these people seem informed of safety issues and typical mitigation, but some stuff was not considered for both static and vehicular applications.

    Interrupt rating for fuses.
    post-mold CTI, tracking, and MFI for plastic (having a UL94 flame rating for the raw material does not mitigate end-use design implementation).
    Charger limit Type Tests (IEC61851).
    Functional safety (IEC61508).
    EMC and RF immunity.

    The last one on this list had become an increasingly common failure point for the chargers (that I tested and submitted for certification) for a former employer, where we did battery stuff for the military/police/fire market.

    And the big problem with BMS. There is effectively no safety or EMC standard that is scoped for battery charge equipment. There are very few competent people at third-party test labs (NTRLs, NBs, TCBs, etc) that understand chargers, so a ‘certified’ or ‘listed’ charger is not necessarily any more safe than one made in your garage.

    1. UL458 is a good battery charger safety standard and also FCC Class B is required for commercial products. Actually both are required for renewable energy products, as well as CE marking which requires both safety and EMC testing.
      Tim

    1. Hey Serial, If I get enough interest, I’ll put some of the stuff on ebay, maybe a kit so that you don’t have to make your own from scratch. It might have the plastic cell holders, the boards, the hardware, basically everything but the cells. Or it could just be some of the parts. People can let me know what they need, and I’ll open up an ebay auction for that. Or you can make it yourself …it’s open source!!
      Tim

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