Where This Guide Comes From

Every step in this article comes from one source video — GreatScott!'s classic DIY 18650 battery pack build, which has been watched more than 1.4 million times. In it, he takes six INR 18650-25R cells (2500 mAh each), arranges them in a 3S2P configuration, connects them with nickel strip, adds a BMS and a balance connector, and finishes with a 3D-printed housing. If you would rather watch the build before reading the breakdown, the video is right here:

GreatScott! — "Make your own Li-Ion Battery Pack"
Watch on YouTube →
1.4M+ views. Six 18650 cells, nickel strip, a BMS and a 3D-printed shell — the complete pack build with the soldering pitfalls shown on camera.

The build answers a question many DIYers end up asking: can you take common, cheap 18650 cells and make a protected pack yourself, instead of buying a pre-made LiPo? The short answer, shown in the video, is yes — with a 3S2P layout, a balance lead for the charger and a BMS for protection, a homemade pack is a realistic weekend project. Below is the full breakdown, with every number taken from the on-camera build.

Why Build Your Own 18650 Pack at All?

The video starts with a practical need: two five-metre rolls of LED strip that need a portable 12V supply. GreatScott compares Lithium Polymer and Lithium Ion packs and settles on 18650 cells because they are common, cheap, and can be arranged into a pack with a useful voltage without a boost converter. Pre-made LiPo packs are convenient, but they are also a known hazard when handled carelessly, and they lock you into one shape and capacity. Building with 18650s gives you control over every part — the cells, the strips, the BMS and the housing — and that flexibility is the whole point of a DIY pack.

The same reasoning applies at a larger scale. If you are sizing a battery for a solar project, an RV or a backup system, you are making the same choices: chemistry, voltage, capacity, protection and enclosure. A pack that is built to match your actual load — rather than a generic off-the-shelf unit — usually fits the job better. That is exactly the kind of project we help customers plan when they come to our product center for a custom voltage and capacity, and the same principles show up throughout our buying guides.

Cell Selection and the 3S2P Layout

The build uses six INR 18650-25R cells, 2500 mAh each. To get close to the 12V the LED strip wants, three cells go in series — giving 12.6V fully charged and 7.5V fully discharged. A second cell is then added in parallel to each of the three, doubling capacity. The result is a 3S2P pack with 55.5 Wh — enough to run five metres of LED strip for about 1.6 hours.

Before any connection, the video stresses one step that is easy to skip: check that the parallel cells sit at nearly the same voltage. If one cell is significantly higher, it will dump an unhealthy amount of current into the other when you connect them. Matching voltage first is a cheap habit that prevents hot spots and cell damage later.

Nickel Strip: the Connection Material

To join the cells, GreatScott uses nickel ribbon, 5 mm wide and 0.15 mm thick. That cross-section easily carries the 3A the LED strip draws, and nickel is the right material because it does not react with the cell terminals over time the way some metals do. The strip is the current path between cells, so its width and thickness should be matched to your load — a rule that scales directly to bigger packs where more current means wider or thicker strips (or a busbar solution).

Spot Welding vs Soldering: Which One Works?

The video is honest about the two ways to attach nickel strip, and about the fact that one is clearly better. Spot welding is the recommended method: a burst of energy welds the ribbon to the cell in milliseconds, so the cell body barely heats up. A ready-made spot welder costs around $200 from China. GreatScott instead repurposes two modified microwave transformers from a previous project, builds a DIY spot welder with cable shoes, bolts, toothed rings and a nail electrode — and shows the failure path too: the 1.7V transformer was too weak to make a proper weld, and a 7.5V unit was overpowered even after rewinding the secondary down to 4.7V. In the end, he falls back to soldering, which is why the video spends so much time on doing it safely.

The lesson for a first-time builder: if you have the choice, spot weld rather than solder. If you must solder, the video demonstrates the safe version of it — the next section.

Soldering Cells the Safe Way

Soldering directly to a cell heats the battery, which can cause capacity loss and other problems. GreatScott's routine reduces the risk: he pre-tins short nickel strips on both sides, abrades the positive and negative terminals with fine P240 sandpaper, adds a little solder to each contact, and works quickly so heat does not sink into the cell. The sanding is important because a clean, roughened surface takes solder far faster than a glossy terminal — less time on heat means less damage.

A commenter with 162 likes under the video adds a practical refinement: use the largest soldering iron tip you have, because a big tip heats the mass of the terminal quickly and stops heat migrating into the cell. A small tip may be hot enough but takes longer, and that extra time is what cooks the cell. The negative terminal is the harder one — it has no raised button, so there is more metal to heat — which is exactly why it needs the bigger tip.

Wiring the Balance Leads

A 3S pack needs a balance connector so the charger can monitor and equalize each series cell. In the video, the arrangement uses the pack positive, the 7.4V tap, the 3.7V tap and the pack negative — four wires into a standard 3S balance plug. With that in place, the pack can be balance-charged with a suitable Li-Ion charger, and the charger will stop the moment one cell finishes.

Getting the balance connector right is where a lot of first builds go wrong, and it is worth double-checking against a wiring diagram before charging. The same discipline — matching each sense wire to the correct cell node — applies to every multi-cell pack, from a small 3S to a 16S home storage bank.

Adding the BMS: What It Actually Protects

The video then adds a BMS (Battery Management System) on top of the pack. It is glued to the pack body, and the cell wires are soldered to the circuit according to its labels. The protection list is worth reading twice:

Overcharge protection: with a 12.6V charging voltage applied, the BMS cuts charging current as soon as one cell exceeds 4.24V. Over-discharge protection: it cuts discharge current when a cell falls to 2.3V. Overcurrent protection at 26A and short-circuit protection round it out. One honest limitation the video shows: this simple BMS does not balance the cells on its own, which is why the balance connector is still mandatory for long-term health.

Housing: Insulation and a 3D-Printed Shell

A bare pack with a couple of layers of Kapton tape is still fragile, so GreatScott designs a housing in 123D Design and prints it on a Delta printer. After six hours of printing, the pack slides inside, is secured with a little hot glue, and the XT60 main connector and balance plug are mounted to the lid with component adhesive, held by two M3 bolts and nuts. The result is a pack that looks like a product rather than a science project — and, more importantly, one where the fragile weld points and sense wires are protected from being knocked around.

Enclosure design matters more than most first builds realize. Vibration, chafed insulation and loose wires are the most common causes of pack failures later. Even a simple box that keeps cells fixed and wires strain-relieved extends the life of a DIY pack dramatically.

Safety Rules Every Builder Should Follow

The video's build works, and its comments section is full of builders sharing hard-earned cautions. A few rules show up again and again: never glue a BMS directly on top of cells with hot glue — the negative can is insulated only by a thin paint layer, and the glue can soften under load. Keep high-amp wires away from cell edges, and consider a paper shim between strip connectors and the positive ends, because a hot connection melting through insulation is a real fire path. Practice soldering on dead AA cells first to learn the temperature and timing before touching a live 18650. And if you see swelling, smell electrolyte or measure an odd voltage, stop and check the cell before charging — never charge a damaged cell.

If you would rather not hand-assemble cells at all, that is also a perfectly sensible path: pre-built packs with the BMS, balance and enclosure already handled save time and risk. That is the difference between a DIY project and a manufactured pack — and it is exactly what we build at Dajiu Energy when a customer needs a specific voltage and capacity without doing the assembly themselves.

And if you're not hunting for a consumer brand but need the battery itself — a custom LiFePO4 pack at a specific voltage and capacity, an OEM or ODM project, or wholesale supply for your own power station line — that's what we do. Dajiu Energy is a Chinese manufacturer building LiFePO4 batteries for RVs, homes, golf carts and portable applications since 2017. Our product range covers LiFePO4 drop-in replacement batteries, EV and golf cart batteries, wall-mounted home storage, portable power stations, commercial energy storage and battery swap cabinets — explore the full lineup in our product center, or browse more buying guides. Tell us your voltage, capacity and size, and we'll engineer the pack to hit it.

Reviewed by Dajiu Energy Engineering Team — a Chinese LiFePO4 battery manufacturer building LiFePO4 batteries for RVs, homes, golf carts and portable applications since 2017, serving OEM/ODM projects with CE, UN38.3, MSDS and ISO9001 certified builds.

What Viewers Are Asking (Top Comments on the Video)

These are the most useful of the video's top comments, with our practical answers. The original creator did not reply to these threads, so the answers below are ours — written the way we'd answer a customer on the shop floor. Thanks-only comments were left out on purpose; these are the ones with actual information in them.

@albertpelletier9415 · 162 likes
"When soldering directly to a battery cell, it works best by using the largest possible soldering iron tip. That way there is less chance of overheating the cell. A small tip may well be hot enough, but will take much longer to heat the mass of the end of the cell, and more heat will have time to migrate into the cell... This is especially noticeable on the negative pole of the battery, which does not usually have a raised button... The positive protruding button is always easier to solder."
Our take: This is the single most useful soldering tip in the thread and it matches what the video shows on camera. A big tip delivers heat fast and stops it sinking into the cell. The negative end has no raised button, so it is a bigger thermal mass — give it the biggest tip you have and work quickly. If you can spot weld instead, still do that; this advice is for when soldering is your only option.
@GadgetAddict · 48 likes
"Nice project. I'd probably skip the hassle of tab welding or soldering and just use two of the 3-cell holders from eBay. It won't be quite as compact but it will get the job done. Maybe replace the included wires with thicker ones."
Our take: A spring-contact cell holder is a legitimate shortcut for low-current projects — no welding, no soldering, and the cells stay replaceable. Two 3-cell holders in series give you the same 3S2P voltage with less density. Upgrade the thin wiring if you pull more than a couple of amps, and remember a holder pack still benefits from a BMS and a fuse, because spring contacts can loosen under vibration.
@theonlyari · 31 likes
"Its nice to see someone on youtube using a BMS. Most people take hundreds of li-ion batteries, tie them together, and call it done. Its stupid. And as an engineer it makes me cringe, and become irate."
Our take: Agreed, and it is worth saying plainly: a multi-cell pack without a BMS is not a pack, it is a hazard waiting for a charger. The BMS's overcharge, over-discharge and overcurrent cutoffs are what stop a small wiring mistake from becoming a fire. Even the simple BMS in the video, which does not balance, is infinitely safer than bare cells in series. If you are building any pack larger than two cells, budget for a proper BMS first.
@PlasmaHH · 17 likes
"Note that a lot of LiPos don't like it when you solder to them, so use a very high soldering temperature with lots of thermal mass, work quickly, and then use some damp cloth to quickly cool the joint down to safe temperatures. If in any way you can, use welding for the tabs instead."
Our take: Solid advice that applies to both LiPo and Li-Ion cylindrical cells. High temperature, high thermal mass, fast work, then cool the joint — that combination limits heat soak into the chemistry. A damp cloth on the terminal (not on the body) after soldering speeds the cool-down. The deeper point stands: welding beats soldering whenever you can arrange it, because it never heats the cell in the first place.
@daxeshchaudhary6981 · 9 likes
"Please make video of how to make battery management system circuit it will be very helpful. And THANKS for this one."
Our take: A good request, because most builders install a BMS without understanding the circuit. The essentials: the BMS sits in the discharge path between the cells and the load, monitors each cell's voltage via sense wires, and opens a MOSFET switch when any cell goes out of range. For a first project, buy a tested BMS module rather than rolling your own protection circuit — DIY protection electronics is a specialist project in itself.
@LA6UOA · 4 likes
"First never glue a BMS on top of the batteries. The negative cylinder of the battery is only isolated by a thin layer of paint. Shorten this might be bad (Speaking of experience). Never use melting glue on the BMS. It gets so hot, the glue will melt."
Our take: This is exactly the kind of comment the video itself does not warn about strongly enough. The 18650 can is the negative terminal and carries full cell voltage; its only insulation is a thin wrapper or paint. Mount the BMS with double-sided foam tape or standoffs on a flat surface next to the pack, never hot-glued over the terminals. And keep the BMS off the cells for thermal reasons too — under load it heats up, and that heat transfers into the cells.
@jimmybleron4700 · 4 likes
"Can you please do a video on how to check if the battery is damaged so as not to charge it?"
Our take: Quick field check: measure open-circuit voltage (a Li-Ion cell below 2.5V is suspect), check for swelling or a wrinkled wrapper, smell for electrolyte, and inspect the terminal for corrosion or heat marks. If any check fails, do not charge it — dispose of it properly. The most dangerous cell is the one that looks fine but has been over-discharged or dented, so when in doubt, retire it.
@michaelparker2449 · 3 likes
"I did a lot of practice on dead AA batteries first to get the right soldering temperature and fastest time before going onto the 18650s."
Our take: One of the best beginner habits in the thread. Dead cells cost nothing, and they behave close enough to 18650s to teach you tip size, timing and tinning technique without risking real cells. When you switch to lithium, you are no longer learning — you are just executing a motion you have already drilled. Strongly recommended before a first live build.
@Huskie · 3 likes
"I did not know how to setup the balance connector, you delivered with your great content once again."
Our take: The balance connector is genuinely the least intuitive part of a first pack. The rule: one wire per series node, in order — pack positive, then each intermediate tap, then pack negative. If the wires are out of order, the charger reads nonsense voltages and refuses to start, which is annoying but safer than charging a wrong reading. Follow the diagram that comes with the charger or BMS, and re-check continuity before the first charge.
@Hobbyminecraft · 2 likes
"I built a boombox with li-ion batteries and had to find every Information by myself about charging, bms and so on. Now you answered all My questions just a Little Bit too Late."
Our take: This is the exact learning curve most builders go through — the information exists but is scattered. That is why we wrote this guide in the order of an actual build: cell matching, connection method, balance leads, BMS, then enclosure. If you are planning a boombox or any 3S project, use the same sequence and you will avoid the "find it out the hard way" stage entirely.

Source video: GreatScott! — "Make your own Li-Ion Battery Pack" (youtube.com/watch?v=hwhqn4BmC2I). Comment excerpts are quoted verbatim from the video's top comments; the original creator did not reply to these threads, so all answers are Dajiu Energy's own. Build figures are transcribed from the video's measurements.