Where This Guide Comes From
Every step in this article comes from one source video — Well Done Tips' 12V lead acid to lithium conversion, watched more than 324,000 times. In it, he takes a flat 9Ah sealed lead acid battery, cuts it open, removes the old plates, and rebuilds the same case with recycled Samsung 18650 cells in a 3S7P layout, protected by a 3S 40A BMS. If you prefer to watch the build before reading the breakdown, the video is embedded below:
The build answers a question that comes up constantly in the DIY battery world: is it worth converting an old lead acid battery to lithium instead of throwing it away and buying new? The video's answer is a confident yes — 50% more capacity, less than half the weight, and a case that would otherwise go to recycling. Below is the full breakdown, with every number taken from the on-camera build.
Why Convert a Lead Acid Battery Instead of Buying New?
The video opens with the most common sealed lead acid battery type in the world — the 12V block battery used in alarm systems, emergency lights, scooters and a hundred other devices that need a constant 12V supply. Sooner or later every one of them dies, and the default response is to buy a new lead acid battery. The video's alternative: recycle and improve, by converting the dead case into a lithium-ion pack.
The numbers make the case. The original battery is rated 9 amp hours; the rebuilt pack holds 14 amp hours — roughly 50% more capacity in the same case. And lithium weighs less than half as much as the lead plates it replaces. You get a battery that lasts longer per charge, weighs less, and reuses a case that otherwise goes to the recycler. For anyone who already has a dead lead acid battery and access to good 18650 cells, the economics are hard to argue with.
Disassembly: Getting the Old Battery Ready Safely
The first safety rule in the video is absolute: before any disassembly, make sure the battery is completely flat — no more than one or two volts of charge. If the battery still holds voltage, drain it with a 12V car bulb or an equivalent load. Cutting into a charged lead acid battery is asking for a short, a spark and an acid mess; a flat battery makes the job dramatically safer.
With the battery flat, the case is opened by cutting along the lid seam with a hacksaw. Inside, the six cells of a classic 12V lead acid battery are visible. In the video, the better-looking battery banks came out easily in one piece, while the crusted brown ones had oxidized and expanded, trapping themselves in the compartment. Those needed to be scratched and picked out piece by piece until the case was empty. Next, an internal divider is removed — thin snips for the bulk, a chisel for the remainder — leaving a clean plastic case ready to be rebuilt.
Cell Selection: Recycled Samsung INR 18650-20R Cells
The heart of the conversion is a pile of Samsung INR 18650-20R cells. Each is rated for 2000 mAh of capacity and can deliver up to 22 amps of maximum discharge current — a high-drain cell well suited to 12V power tool and device loads. In the video these cells were already spot welded together, because they were recycled from a pair of faulty DeWalt 36V battery packs bought cheaply online, at an average cost of about 80 euro cents per cell.
The critical detail is that every cell was tested before reuse, and its state recorded on the wrapper — the voltage at recycling time and the measured capacity. The video links to a full explanation of how to recycle, test and separate good cells from trash. That discipline is non-negotiable: an untested 18650 is a lottery ticket, and a mismatched cell inside a pack is how packs fail early and sometimes dangerously.
The 3S7P Configuration Explained
The pack is built in a 3S7P configuration: three groups of cells connected in series, with seven cells in each series group connected in parallel. Three groups in series give roughly 12V nominal — the same voltage as the lead acid battery being replaced — and seven cells in parallel multiply capacity: 7 × 2000 mAh = 14,000 mAh, which is the 14 amp hour rating in the title.
One detail the video stresses is capacity matching between series groups. Each group of seven cells should have as close to identical capacity as possible, so the cells were rearranged until every group measured nearly the same. Why it matters: in a series string, the group with the lowest capacity defines the pack's usable capacity and gets discharged the hardest. Mismatched groups mean one group ages faster and the whole pack fails early.
Spot Welding Nickel Strips and Building the Pack
To hold the cells in place, a simple battery holder is 3D-printed — a similar holder can be bought online, but printing it saves waiting on an order. Before connecting anything, the video checks that all cells are within a close voltage of each other, then starts spot welding with pure nickel strips 0.15 mm thick.
First, all the parallel cells in each group are welded together. Then the three groups are connected in series, which should give around 12V if everything was done right. Finally, the main positive and negative terminals get a triple layer of nickel strip, specifically to avoid heating and resistance issues when the pack pulls maximum amps. That triple-layer reinforcement on the high-current terminals is a detail a lot of first-time builders skip, and it is exactly where packs develop hot spots under load.
Wiring the 3S 40A BMS With Balancing
Protection is handled by a 3S 40A BMS module, designed for 3S configuration and rated for a continuous 40A load with short peaks up to 60A. That is plenty for most 12V devices. The video contains a genuinely useful warning: two very similar-looking 3S BMS modules exist, and they are not the same. One has a balancing function; the other only protects against overcharge and over-discharge. The visual difference is three big resistors on the balancing version — if you see those, you have the right board. Buy the one with balancing.
The BMS is glued onto the pack with an insulating sticky spacer, and the pack terminals are soldered to the board one by one, starting from the most negative connection. A pair of silicon wires is then soldered to the main BMS terminals. The wiring order matters — working from negative to positive is the standard way to avoid accidentally shorting the partially-connected pack during assembly.
Reassembly: Gluing, Insulating and Sealing the Case
With the pack built, it goes back into the old battery case. Both surfaces are sanded for a perfect joint, and small plastic parts are glued around the battery perimeter to keep the pack sturdy inside. All exposed cells are covered with insulating sticky paper — never leave bare cell wrappers touching each other inside a case, because vibration can wear through and cause a short.
The battery wires are soldered to pre-soldered terminals, and the case is sealed with 5-minute epoxy. The result looks like a factory battery until you pick it up: a recycled 9Ah lead acid case now holding a 14Ah lithium-ion pack at less than half the weight. The video is honest that the original 9Ah rating is now 14Ah — 50% more capacity — and the whole rebuild reuses the original plastic case that would otherwise be scrap.
Charging and Discharge Limits of the Finished Pack
The finished pack can be charged with a smart charger like the iMAX B6 or any constant-voltage power supply set to 12.6V — the standard full-charge voltage for a 3S lithium-ion pack (3 × 4.2V). Because the BMS limits output, the pack delivers up to 40A continuous and 60A peak through the BMS for any 12V device.
For users who need more, the video mentions a bypass option: connect directly to the battery terminals through a fuse, which pushes the maximum discharge current up to a stunning 154 amps. That bypass is worth a strong caution — bypassing the BMS removes over-discharge and low-voltage protection, so it should only ever be done with a properly rated fuse and an external understanding of the pack's state of charge. For most applications, the BMS path is the right one.
Safety Checks and Common Mistakes
A few rules from the video and its comments deserve repeating. First, never weld or solder with cells at wildly different voltages — check them all first. Second, when using parallel cells, consider a fuse or fusible link per parallel group; a single shorted cell in an unfused parallel group can dump the whole group's current into it and start a fire. The comments thread raises exactly this concern, and it is the most important safety point in the whole project.
Third, understand what you are building: a 3S pack of 3.6V-nominal cells sits around 10.8V when partially discharged, not 12V — devices that expect a solid 12V from a lead acid battery may behave differently. Fourth, use a BMS with balancing, not just protection. And finally, cover every exposed cell and terminal with insulation before sealing the case. Each of these is a one-time habit that prevents a lifetime of problems.
Is a Lead Acid to Lithium Conversion Right for You?
This conversion is the perfect project if you already have a dead lead acid battery, access to tested 18650 cells, and a device that needs 12V. The payoff is real: 50% more capacity, half the weight, and a case rescued from the recycler. It is also a complete education in series/parallel math, spot welding and BMS wiring that transfers directly to larger battery projects.
If, on the other hand, you need a reliable battery without the disassembly, testing and welding — or you want the safety of a purpose-built pack with a properly sized BMS and enclosure — a drop-in lithium replacement is the simpler path. Both routes end with the same chemistry doing the same job; the difference is how much of the work you want to do yourself. We help customers on both sides of that question every day.
And if you would rather not cut open old battery cases at all — or you need a drop-in 12V lithium replacement that already has the BMS, enclosure and terminals handled — that is exactly 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.
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 all answers below are Dajiu Energy's own. Thanks-only comments were left out on purpose — these are the ones with actual information in them.
Source video: Well Done Tips — "How To Convert 12v 9Ah Lead Acid Battery Into 12v 14Ah Lithium-Ion Battery Pack" (youtube.com/watch?v=9ACafobCNNM). 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.