Let me start by saying you've done some really good work here! I'm a retired NASA guy with 20 years experience with this kind of thing. I have some advice that would help if you are interested in hearing it? If yes, read on. But if you're sick of people being critical of this project and/or giving you advice, simply ignore the following comments. After completing the list below, I realized that I have written a novel. My apologies!!!!! 1) I really like that you've measured the capacity of each cell. That is so important in order to capacity match EACH of the seven series sections to each other. Bravo! Its very time consuming to do this and many people don't bother. Your reward for such dedication is that the 7S battery (and specifically, the voltages of each section in series,) will automatically stay balanced without the BMS constantly having to fix the mismatched voltages. Each of the seven sections will charge and discharge in sync AUTOMATICALLY due to the capacities being matched!!! In my opinion, this step is mandatory. Congratulations for doing this! 2) Please don't get mad at me for what I'm about to say..... Laptop batteries are typically used very "hard" and are NOT the best choice for building your own, large Lithium ion batteries. Yes, of course they will work... however, due to the temperatures they typically endure and the number cycles they have already seen prior to getting into your hands... you'll find that they are often thrown away or recycled when their performance has decreased or is lackluster when compared to a new lithium laptop battery. (e.g. - this laptop battery used to last me 3 hours, but now it only lasts 1 hour.) Even if the cells' capacities are measured to be nearly the same as a brand new cell, their internal resistance will likely be higher than a new cell (of the same brand and model). Higher internal resistance is actually ok for most projects... as long as you won't have large current demands from your battery. It sounds like you will have high current demands in your proposed usage. Might I suggest “Less Used” 18650’s? Many types of medical equipment use lithium batteries for backup (or portability) reasons. I have found medical packs to be far superior sources of practically brand new 18650 cells! The number of cycles is low and their capacities are almost the same as brand new cells. Another source of great 18650’s is modems with backup batteries. Those cells might get cycled 3 times (total!) before you buy them for a fraction of the cost of new cells!!! I have used batteryhookup dot com for many years with great success. Their inventory is always changing due to the nature of their business. Another great source is Power2Spare dot net. You can choose if you want to tear into the plastic enclosures yourself (and save money), or just buy the raw cells from them. If you’re going to all the trouble of building a big 84 cell lithium battery, these would be far better to use. 3) Prior to connecting the cells together, it is imperative that you check the voltages on EVERY cell! They absolutely MUST all be at the same Voltage +/- approximately 0.1 Volt. (E.g. - 4.12 Volts and 4.02 Volts is ok. But.... 4.15 Volts and 3.81 Volts is NOT OK!) 4) I typically charge my 18650's to 4.20 Volts and let them sit for a 7 days. It's normal for them to drop back to about 4.13 Volts over that time, but if they drop down below 4.10 Volts during those 7 days, you might have a problem called "Self Discharging". Self Discharging is exactly what it sounds like; the cell is losing Voltage while it is just sitting on your desk, not connected to anything else. I would suggest that you set aside any cells that self discharge for a different project, not a big battery, as they will slowly drag down all other cells wired in parallel with them. This will lead to an unbalanced 7S battery. For example: 4.14V, 4.15V, 3.82V, 4.14V, 4.15V, 4.14V, 4.13V. The third contributor in the 7S series above is 3.82V, while all others are up around 4.14 Volts. One bad, self discharging cell can lead to this issue. Simply charge the cells up to 4.20 Volts and then wait a week to spot the possible self discharging cells. Separate those cells out and don't use them on this project. P.S.- those self discharging cells would be perfectly fine to use on a solar light that gets charged up to full everyday. You might not even notice that they are self discharging if they are getting charged to full 4.20 Volts everyday. 5) Go back and watch the accidental movement of the nickel strip at 13:43. Without the Capton tape, you can easily short out cells. In this case, you DID have the Capton tape in place, (and the other side wasn't connected yet) but this small mistake is why I always tape off each section as I go instead of waiting until I complete one whole side. (If you're wondering....... Yes, I have accidentally shorted out cells. LOL.) 6) At 15:10 you're commenting about loose welds. Try applying just a tiny bit more down force prior to the welding. Any gap at all will certainly mess things up. 7) The cells in each section are connected in parallel, so tapping into the top left corner (for negative) and the top right corner (for positive) seems fine, however, you're better off using opposite diagonals. For your battery: Bottom Left for Negative, and Top Right for Positive. EVEN BETTER: Use multiple wires spaced along the buss from bottom to top. E.g. - three 16 AWG wires for the Negative battery terminal... one connected at the bottom of the buss, one in the middle of the buss, one at the top of the buss. Repeat for the Positive buss, using three wires spaced out along the positive buss. Be sure to cut these wires the same length to ensure they each carry the same current when you're asking the battery for maximum Amps. 8) At 18:05 you state the battery is fully charged at 25.1 Volts. It's far from fully charged. Each cell contributes 4.20 Volts when fully charged.... hence 7S string should be 4.20 * 7 = 29.4 Volts. The "NOMINAL" Voltage is typically 3.63 Volts per cell, thus the 7S string will measure about 25.4 Volts when its State of Charge (SOC) is approximately 50%. 9) I believe the DALY BMS that you're using is a 40 Amp model. Your nominal voltage of 25.4 Volts * 40 Amps Max = 1016 Watts Max. But I wouldn't push it that hard for fear of thermal issues. Perhaps 50% of Max would be ok (500 Watts) for continuous use, with peaks at 1000 Watts. P.S. - if you are going to ask for that much power from your battery, you'll want to increase the number of wires, or the AWG used for positive and negative leads. 10) I wanted to estimate your battery's overall capacity, based on some guesses from your verbiage. If I assume your average cell is 2000mAh, then placing 12 cells in parallel is 24Ah. Multiply that by the nominal Voltage of 25.4V to get Watt hours..... 24Ah * 25.4Vnom = 610 Watthours. 11) You had mentioned that you were hoping for 650 to 850 Watts... Let's be careful about the units here. If you are talking about the battery's energy storage CAPACITY, we need to use Watthours. If you are talking about how much power you are drawing from the battery, (the instantaneous power draw,) we need to use Watts. To summarize: Looking above, your 610 Watthour battery can likely deliver 1000 Watts at any given moment. Do you see how I used Watthours and Watts in the same sentence because they are referring to two different things? Great, now I'll point out that the inverter that turns DC battery power into 120 Volts AC power will likely consume about 10% of the power it delivers. So, those 1000 Watts coming from the battery will drop to about 900 Watts at the load due to losses in the system. 12) At 19:57 you mention that the battery has fallen to 8Volts. That's about 1.15 Volts per each of the 7S sections. I wish that I had seen this video about a year ago..... you can EASILY charge those cells back up if you do it reasonably soon after they are depleted.(< 4 weeks) Its the DURATION that the cells stay at 1.15 Volts that causes permanent decreases in capacity, as well as an increase in the self discharge likelihood. Letting them sit for a year at 1.15 Volts will create unknown permanent damage, but don't be discouraged... I've have resurrected hundreds of 18650's from their 1.0 Volt graveyards after they sat for 2+ years. OVERALL SUMMARY: I’m very impressed with your project. Ok, you made some mistakes… but don’t let that detract from your successes. This is a fantastic project! And you learned from your mistakes. That’s what matters. Keep building and posting your projects. Every project builds experience & knowledge. Perhaps a future project could be a solar generator? Those are fantastic and they build upon everything you’ve already learned!

Damn, sorry to hear about the cells. It was still a good video, and glad you decided to upload it. I imagine that most people messing around with 18650's and making their own battery packs run into similar problems. I didn't see where you made any mistakes, but I got nervous AF after you flipped the pack over to begin nickel stripping the other side. That's when it's so easy to accidentally short out the cell groups.

Magnets can be your best friend when spot welding unruly nickel strips

most BMS always go to sleeping mode when connected. simply connect a charging voltage to wake up the BMS

Always be very careful when connecting the wires because one mistake and there can be a thermal runaway which can cause a fire. When the time comes to connect the BMS, you must immediately check if the latter heats up. If so disconnect it immediately.

Did you check each cell's voltage and resistance before the assembly? Just a few bad ones can be a huge resistor.

Hi there,do you have another power source, say two 12 volt batteries to test the bms. Nice battery pack,I do have a few of those 18650's,and some still in laptop batteries. You've given me bit of inspiration there. What a cool thing to make and play with, thank you.

Hi there, I really liked the spot welder and nickel strips you used, can you share a link with the details to order? Thanks for the awesome video.

i have a 24-18650 battery pack with a readout of 45volts. fully charged as a 42 volt battery for a gm electric bike, so you probably have a loose wire on one of those series or too many batteries hooked up to each row, maybe try 4 in each series and should give you a higher wattage with only two connected for the end of positive and ngative leads

There’s nothing wrong here. You just need to put a charge to the pack via the bms to activate it. Just for a few seconds and it will start putting out your voltage

Put a charge on it to turn the bms on

Hi there did you ever get this up and running ? I am looking to make one for my wife's mobility scooter

Can I use 2 BMS parallel.. ? I have two 7s battery packs(Ah not same). I need connet to my inverter

Yep dude, try waking up the bms

It looks as though you mention that you know what you did wrong with the BMS. Was it connecting the S1 through S7 wires in the opposite positions that they should be? (Wire #1 accidentally connected to where wire #8 should go? Then Wire#2 connected to where Wire #7 should go? etc etc...) Even knowing the correct locations..... my gut tells me that connecting them to the correct locations in the WRONG order can goof things up. I always connect them to the BMS in order from lowest voltage to highest voltage.

It happened to me yet I installed everything correctly so what I did is I charged for about 10sec and read correct on the multimeter

From hundreds person's only two person's shows what's they are connecting PARALEL & SERIES.Not this one either.

Wrong BMS. S7 is 28.7v or 30.8 You're thinking is that its 25.7 (nom) A S6 is closer to 25volt (22.6v) 24v BMS for li-ion won't work. And I would think maybe you have a LFP battery BMS. 7*LFP This is 25.2-5 chrg to 21v dis