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Looking for advice on how to build battery pack

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    Looking for advice on how to build battery pack


    New to the EV world here and am looking for advice on how to build a battery pack for a 35kw motor. Motor can be found here. Would like to either use These 18650 cells, or These LifePo4 cells.

    ​Not really sure if I should use a 24s, 28s, or 30s configuration. Guessing the 30 would be best, but does this cause issues finding other stuff I need like a BMS or ESC??

    If anyone could help me design the pack, I think I can build it from there.

    Last edited by Dthaller; 11-24-2022, 06:20 AM.

    It all depends on how long you want to stay in the air. How heavy is you and your rig. How much power it will take to spin your prop . What kind of controller would run that motor? It might have its own voltage and current limits. There's just too many variables to say with any certainty. This forum is very familiar with the power needs of getting bikes in motion. But nobody know for sure of the needs to make bikes fly. Except briefly off a ramp. Hah. . Normally you get your system figured out, then get a battery that will run it. About the only thing that can be said is that LifPo4 batteries are much bigger and heavier than LiIon. Oh and another thing that can be said is that those 21700 batteries in your link are 4 to 5 years old. Its a bad idea to make a giant battery with those old old cells. any one going bad will bring the whole pack down.

    You need to find other people doing this and find out the details on their batteries and the performance they are getting.


    • Dthaller
      Dthaller commented
      Editing a comment
      Appreciate the honesty. I'm having issues with finding a group that is building what I'm trying to do. This is going to go on my little john boat to use as an air boat. Only reference I have is a guy on YouTube, but he is trying to sell his so doesn't disclose information. His is claimed to be 25kw motor, and it has enough thrust. SOOO I figured, hey 35kw should definitely have enough then lol. Im honestly just not sure how many in parallel I need to say run it for a good half hour. Is a pack that size to big? How do I configure the pack? Not really worried about size of the battery pack. Just want it made correctly. Planning on using individual fuses on each cell and overall BMS and ESC.

      Lets say I use the 18650 cells and go with a 28s, and the motor is rated at 120a continuously. 120a/5000mah=24 so would a 28s24p be sufficient to run it for an hour?

      I understand the items in this project are cheap/old, but on a limited budget and honestly just trying to have some fun.

    Yup, you got the math right. But 18650s dont go up to 5000mah. Only 3500maH. So 21700s would have to be the choice. But its so many batteries that I would still avoid old used ones especially since you plan on high current drain for most of the drain cycle. Its such a hassle to deal with dying cells that seem ok from the start. Bike packs of those would only use 52 cells, So easier to try to fix than 672 cells. Yikes. Especially when the pack weighs about 200 pounds. Super yikes. You would have to build a steel frame to hold those so the batteries wont distort and short out from sag. In fact it would be better to build many smaller packs. But these batteries all charge up to over voltage of 4.2v. So you cant be thinking 28s or the full charge will be too much for the motor rating. So have to pick out a controller so you know the max controller voltage. So with just motor, 100v/4.2v is rounded down to 23 cells to keep the top voltage in the safe zone. So your down to a 552 cell pack. But if you make it into multiple 36volt packs, you can find cheap chargers for that. And 2 in series is a top voltage of 84volts. Thats 2 packs of 10s 24p. A cheap charger will charge that, but VERY slowly. So then you would need a 12 amp charger to charger each smaller pack for 6 hours which are not cheap chargers. Then there is the BMS. It has to be way over 120a, which is a monster BMS. Not cheap or easy to find. Motor starting current is always huge. Say 400 amp in your case. You cant be tripping the BMS when you start up. I see 100 amp BMSs for $65 on Ebay. So now we break up 2 packs into 8 packs to do that. Thats 8 packs of 10s 6p. So 8 of those BMSs. Yikes. But you can still use the pricey charger to charge 4 packs at a time for 6 hours for a total of 12 hours to charge everything up.

    Opps, but what if the battery itself cant handle 15amp discharge at its peak? The ones you link to are only good for 7000mah. so now we got 8 packs of 10s 12p, or even 10s 18p, since tired old cells cant give even 7000mah for very long. And now the pack is simply ginormous. And the pricey charger takes 2 to 3 times as long to charge all that. But even the very latest 21700 cells only put out 9amps. So its clear that MANY more cells will be needed to cover for the surge currents needed to run motors.

    That's a ton of building, a ton of money, and a ton of ways for something to go wrong. This is NOT a newbie pack project. This is a custom professional pack project. A fire from this kind of pack can light up anybody near it in a flash. And it has to be made water proof. The person selling their setup while keeping secrets is probably scared of their frankenstein and are looking to off load it. Hah.

    After all that, I still have no clue what the controller wants. So all this may be useless math to what you really need. But it gives you an idea of the rabbit hole you want to jump into. Lifpo4 could work here too to make this simpler, but its more expensive, bigger and heavier. Way heavier than 200 pounds. With much pricier chargers and BMSs.
    Last edited by stts; 11-24-2022, 09:47 AM.


      I don't really understand what you're specifically trying to do but, here are a couple places I would look.
      Grin technology has a marine engine replacement kit made out of a hubmotor.
      We’ve also adapted our All Axle hub motor with special high RPM windings and mounting hardware to power propeller shafts in marine applications. This system is primarily geared at those retrofitting sailboats with aging 8-12 horsepower inboard diesel engines. This kit is currently in development / pilot stage.

      And there are some 40A 21700 batteries on sale here. Maybe 3x48V.
      I own a 48V 24AH battery but haven't seen them since 13s 5p 21700 will do it. Someone on Aliexpress will probably build some for you.

      Running a lot of electricity (30Ax120V?) in an aluminum boat sounds kind of risky to me. Aircraft propellers don't make it any safer either.
      Maybe just get one of these and call it a day.