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How to build a MASSIVE 72V DIY battery pack for high power e-bike

20s14p Li-ion battery pack

So we’re going to be building this 20s14p battery pack out of samsung 30q cells this is a 72 volt 42 amp power battery pack it’s capable of 220 amps continuous discharge with a peak discharge of 300 amps which is bms limited and fully charged you’ll be capable of 25 kilowatts of peak power the whole thing is built out of these modular groups of 14 cells in

Parallel and the cell holders are custom designed and 3d printed to be more compact and save a little weight design credit goes to lottie yankmenek who has a youtube channel i’ll put a link in the description the original design was for 12 cells in parallel so i modified it for 14. i’ll put a link to the sdl file in the description as well so here i am putting

These parallel groups together keep in mind that before i did this i made sure that each one of the cells were within a few hundredths of a volt of each other this is important because as soon as you connect them in parallel the cells will begin to charge each other and equalize in voltage and any sufficiently large difference in voltage between two cells might

Result in a charging current that exceeds what the cell is specified to handle i’m also adding these insulator rings to the positive terminals of each cell and this is technically an optional step but it does help mitigate the risk of an internal short and with a battery this large and expensive it’s a good idea to take every precaution i’m using the kwel diy spot

Welder to spot weld the nickel strip i tried a number of spot welders before and k weld is definitely the best although it does have some drawbacks that i’ll mention a bit later so this is a chart describing the minimum amount of nickel strip needed for a given amount of current you can neglect the length of the nickel strip because it’s so short and offers little

Resistance so it’s only the cross sectional area that’s significant if you check the math between the two highlighted columns you’ll find that you need a minimum of 0.15 square millimeters of nickel strip per amp that it’s going to be carrying since we’re designing for 220 amps max continuous discharge that current will be distributed over 14 connections in the

Parallel groups which means that we have to plan for 15 amps of current per connection and since i’m using 0.2 by 6 millimeter pure nickel strip that means we’re going to need at least two nickel strips per connection but wait the series connections will have to handle the entire 220 amp load and since this design results in seven series connections between parallel

Groups we would need to stack four nickel strips on top of each other for each one of those connections and that’s just too much spot welding so what we’re going to do instead is pick up some of this 0.2 millimeter thick copper plating and use that for the series connections which will also reinforce our parallel connections such that we don’t need multiple layers of

Nickel strip anywhere so here i am spot welding this single layer of nickel strip and i mentioned that the k-welt spot welder had some drawbacks well one of them is that if you’re spot welding pretty continuously the electrodes tend to get really hot i covered mine with a couple of layers of electrical tape but even then you’ll see that i’m putting the electrodes

Down every few spot welds to let them cool so here i’m connecting all the parallel groups together in series using the copper plating it’s nearly impossible to spot weld copper because of its very low resistance so i’ll be soldering the copper onto the nickel strip i’m also using extra flux on the solder points to help with faster adhesion of the solder which

Will minimize the amount of time the cells are exposed to high heat now we’re going to fold these 5s groups in line so that we could stack them on top of one another to make the complete battery pack i also added some heat resistant foam as padding between each parallel group to better align the module and also prevent the groups from knocking into one another

When they’re inside the pack i think this would help with any possible wear mechanically but it also prevents uneven distributions in the flow of currents as two groups randomly touch and create a connection one issue i found with this design is the folding over of copper plates to series connections creates these jagged protrusions that need to be accounted for

To reduce the risk of a short i covered them up with electrical tape and adhesive fish paper but later i found that they needed even more wrapping with electrical tape and heat resistant foam to be sufficiently mitigated against so here i’m stacking the 5s modules on top of one another keeping track of the orientation and adding heat resistant foam in between i

Tried using some of this heat resistant glue as well but i found that it wasn’t really necessary and just kind of made things messy the modules are mainly held together mechanically with kapton tape which is a heat resistant type of tape with good tensile strength and here’s the entire battery pack before each of the 5s modules are connected electrically and now

It’s time to connect the 5s modules in series using the same copper plates from before next we’ll be setting up the bms this is the and bms 20s model with 300 amps maximum discharge current came with two temperature sensors and on off switch and an lcd screen but i won’t be using the lcd screen the first step is to connect the balance wires i left a notch of

Nickel strip at the end of each parallel group for this purpose here they are folded over the cell holders on the 5s blocks here i’m just measuring out the length of each balance wire then i’m trimming and soldering them to the notches of nickel strip then i’m checking that i didn’t make any mistakes and each one of the balance wires is connected to the right

Parallel group i’m doing this by connecting the negative probe of a multimeter to the negative terminal of the battery and using the positive probe to test the voltage of each one of the balance wires at the connector if all the balance wires are connected correctly you should see the voltage increase by about 3.7 volts for each successive group in series next

We’ll connect the temperature sensors there’s two of them and they connect to the bms like this you should place the temperature sensors directly on the cells of the pack you can use heat resistant glue to attach them and they should be placed somewhere on opposite sides of the pack to get a more or less average sample of the temperature next we’re going to make

The main discharge connector i’m using a ec8 connector which is rated for up to 190 amps continuous i know we’re designing for 220 amps continuous for this pack but this battery is for an e-bike with a 200 amp controller so in reality we’re only going to be hitting those 200 amps when we’re doing heavy acceleration i also have this four into one setup going through

A butt connector on the positive side this is because the bms is designed with four 12 gauge wires for the b minus and c minus leads and i wanted to stay consistent on the positive discharge connector so as not to create any bottlenecks or imbalances in the flow of current i’m not really sure if this is necessary but it just made sense so here i’m just attaching

The butt connector to the c minus lead of the bms so that i can attach the negative terminal of the discharge connector to it later and now it’s finally time to connect the bms i’m just using hot glue to physically attach the bms to the battery pack and here i’m soldering the b minus leads to the negative terminal of the battery i later did the same thing for the

Positive terminal but i didn’t record that part now i can connect the balance wires note that for most bmss you want to connect the b minus leads before you connect the balance wires otherwise you get some low voltage readings and sometimes it doesn’t work and here’s the almost finished pack with bms connected and heat resistant foam on the positive and negative

Terminals the two small red and black wires coming out of the front of the pack that’s the on off switch for the bms if you short those wires for a few seconds the bms will turn on although it can only be turned off using the app i later added a small push button switch between these two wires as a hard on switch but i’ve never actually found myself using it i also

Wrapped the rest of the pack in heat resistant foam using hot glue and electrical tape for extra padding and here comes the fun part wrapping it in shrink wrap this is the absolute biggest shrink wrap i could find i think it’s like 400 millimeters and that’s it that’s the complete pack i hope you guys enjoyed it and please like and subscribe i’ll be posting more videos in the future

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How to build a MASSIVE 72V DIY battery pack for high power e-bike By Gaius Garage