in the last episode of this video series I showed you how to create your own sensored ESC in order to power an electric bike wheel with low voltages but since the achieved rotation speed was too slow and not many people want to create their own ESC for an E-bike conversion let’s rather switch back to the controller which was included in the kit to test it I got myself a new lab bench power supply which can output up to 60 volts so I connected its outputs to the input of the controller and slowly started raising the voltage and approximately at around 40.6 volts the controller started working how it was supposed to now by cranking up the voltage even more the wheel started rotating continuously faster until I reach the limit of my power supply that means we will need a battery pack which can cover a voltage range of at least 40.6 volts to a maximum of 61.5 volts a suitable choice of batteries would be 18650 lithium ion cells since they offer a great volumetric and gravimetric energy density can deliver enough current and let’s face it everyone uses them for E-bike battery packs but while searching for a compatible battery pack on eBay I noticed the rather high prices which got me wondering whether DIY-ing your own E-bike battery pack would be cheaper so in this episode of DIY or BUY let’s find out what goes into creating your own lithium ion E-bike battery pack and whether it is truly cheaper in the end LET’S GET STARTED [INTRO] this video is sponsored by JLCPCB one fact about them JLCPCB was the first PCB company that cut the price from $70 to $7 on 2-layer PCBs 10 years ago upload your Gerber files to order 10 professional PCBs for only 2 dollars when inspecting the data sheet of most lithium ion cells then we can find out that they got a nominal voltage of 3.6 to 3.7 volts and maximum charging voltage of 4.2 volts and have almost no capacity left when they got discharged to 3 volts that means we got a voltage range of 3 volts to 4.2 volts per cell thus for our controller voltage range it would make sense to put 13 cells in series to create a battery voltage range of 39 volts to 54.6 volts with a nominal voltage of around 48.1 volts which not surprisingly is the advised voltage of the controller next we need to know the maximum required current of the controller sadly though my dry test with the lab bench power supply did not present an exact answer and the product page does not mention the current as well but luckily it states 1000 watts at 48 volts which would equal a current of around 20.83 amps the next best common lithium ion cell which can output 20 amps continuously was the Samsung INR 18650-25R with a capacity of 2500 milliamp-hours but just to be on the safe side and to double the capacity of the battery pack I decided to use two of those cells in parallel which thus ultimately equaled a 13S2P lithium ion pack with a capacity of 5 amp-hours and nominal voltage of 48.1 volts and a possible constant output current of 40 amps so I went ahead and ordered 30 of those cells from a trustworthy German seller how do I know that they are trustworthy? well after receiving the cells and visually inspecting them I measured the voltage of all of them and noticed that they were all very close to one another which was not only a very good sign but also indispensable since we want to connect two cells in parallel if they would have a big voltage potential difference a parallel connection could result in a large current flow and the destruction of the cell but anyway to turn 26 of those cells into a nice looking battery pack I will be utilizing those plastic spacers which can hold 2 cells each so I connected 13 of them in series through the help of the interlocking system place two batteries with the same orientation in the first row and alter the orientation of the next two cells continuously while filling up all the spacers once that was done I added the remaining spacers to the top of the battery pairs and connected them as well through their interlocking system to connect the cells to one another I got this 7mm wide and 0.3mm thick nickel ribbon which can handle up to 30 amps so I started creating 26 smaller pieces of the nickel ribbon which were long enough to connect all the parallel cell pairs now to create the actual connections I wanted to avoid soldering this time but as you might know I recently failed at creating my own battery spot welder thankfully though a viewer sent me a solution to this problem the so called kWeld which is basically a pretty advanced battery spot welder after doing a bit of assembly it can be powered by a LiPo battery and therefore can create suitable welding spots without a problem according to its manual it is recommended to use an energy of 100 joules for 0.3mm nickel strips which I use as a standard value for all my battery pack welds and as you can see creating the welds is really not that complicated simply press the electrodes onto the metal with a distance of roughly 3mm to one another push the foot switch and THERE YOU GO! 😀 now I created two welding spot pairs for each battery terminal which resulted in a total of one hundred and four welds and once that was done it was time to measure and cut another 24 nickel strips for the series connections which need to get connected to the parallel batteries in the here shown arrangement so I created another 96 welds for the series connections in pretty much the same manner as I did it for the parallel cells and with that being done our 13S2P battery pack is basically complete and should deliver us a voltage within the previously calculated voltage range which it did 🙂 the only remaining question is: how to charge it up? the data sheet of our utilized lithium ion cells states a constant-current constant-voltage method with 1.25 amps and 4.2 volts if we multiply those values for the 13S2P battery pack we would get 54.6 volts and 2.5 amps this means I can set my lab bench power supply current limit to 2.5 amps the voltage limit to 54.6 volts and simply hook it up to the battery terminals to which I soldered thicker 10 AWG color-coded wire beforehand and not surprisingly the charging process worked like a charm but as soon as I got closed to the target voltage I interrupted the charging process to measure the voltage of each battery pair and as you can see the voltages are still pretty close to one another but let’s imagine we repeat such a charging process several hundreds of times since no two batteries are completely the same the voltage gap between the cell pairs will grow and grow until one will eventually give up what we need to prevent such an event is this a BMS aka a Battery Management System not only it keeps all cells at an equilibrium voltage at 4.18 volts but it also adds an overcharge over-discharge and short circuit protection to use it we simply must solder its balance connector wires to the battery according to its label that means B1- to the ground potential B1+ to the 3.71V potential B2+ to the 7.4V potential B3+ to the 11.1V potential and so on and on… until B13+ connects to the 48.1V potential finally we simply connect the ground wire of the battery to the B- terminal and add two more black wires to the P- and C- terminal now to charge the battery pack we reconnect the positive supply voltage but connect the ground potential to the C- terminal this way the battery now charges like before but simultaneously the battery charges itself through the BMS and once all the red LEDs lit up the charging process was complete and thus we can connect our load through the P- terminal and the usual positive voltage wire of the battery pack and just like that the creation of my DIY E-bike battery pack was complete 🙂 but one question remains… was it CHEAPER? well according to eBay prices it was in fact cheaper! but only a tiny bit 🙁 but then again if you add labor costs and the cost of a battery spot welder then it would only be cheaper if you plan to create more than just one battery pack and care for customization so all in all I hereby declare that both DIY and BUY are the winner of this episode! and with that being said I hope you’re looking forward to the final chapter of the E-bike conversion project! as always don’t forget to like, share, and subscribe STAY CREATIVE AND I WILL SEE YOU NEXT TIME!