Where This Build Guide Comes From
Every step in this article comes from one source video — Mr. Electron - Alternator’s Stop Buying Expensive Batteries - Build This 100Ah LiFePO4 Instead!, which has been watched more than 179,000 times. In the video, the creator takes four 3.2V 100Ah prismatic LiFePO4 cells, checks their voltages, compresses them with foam, connects them in series with copper bus bars, wires up a Bluetooth active-balance BMS, charges the pack to full and then load-tests it with a 100W headlamp and a 500W DC motor. If you would rather watch the build before reading the breakdown, the video is right here:
Why Build a 100Ah LiFePO4 Pack Yourself?
A ready-made 12V 100Ah LiFePO4 battery is convenient, but it is also one of the most expensive ways to buy capacity. The cells inside are commodity parts — the video uses four prismatic 100Ah cells that cost a fraction of a finished pack — and assembling them yourself lets you choose the BMS, the bus bars and the enclosure. The trade-off is time and care: a pack is only as good as its weakest connection, and the video is essentially a checklist of the details that separate a safe build from a dangerous one.
The build in the video targets 12V: four 3.2V cells in series give a nominal 12.8V, which is the same voltage as a drop-in lead-acid replacement and works with standard 12V inverters, chargers and RV wiring. That makes a DIY 4S pack useful in exactly the places a 100Ah AGM battery used to sit — but with more usable capacity, less weight and a far longer cycle life.
Cell Selection and Voltage Matching
The build starts with four 3.2V 100Ah prismatic cells. Prismatic cells are the right choice for a 12V 100Ah pack because they are rigid, rectangular and easy to bolt together with bus bars — unlike cylindrical cells, which need spot-welded nickel strips. Before anything else, the creator measures each cell’s resting voltage: 3.254V, 3.255V, 3.255V and 3.255V. Four cells that close in voltage are effectively pre-matched, which matters because mismatched cells force the BMS to work harder and shorten pack life.
The cells are also functional-tested before assembly with a 55W car headlamp, one at a time, to confirm they can actually deliver current. This is a cheap but effective sanity check: a cell that reads full voltage but cannot hold a load is a cell that will fail inside your pack. If you replicate this build, test every cell under load before you bolt anything together.
Compression: How Much Pressure Is Right?
LiFePO4 prismatic cells behave better under light compression, but the exact number is debated — the creator found recommendations ranging from about 80kg to 150kg in different sources. His approach is practical: he places thick foam between the cells, which provides compression and acts as an insulator, and applies roughly 70kg to stay on the safe side. The cells are then held together with zip ties.
The important point for your own build is not the exact kilogram figure but the principle: prismatic cells expand slightly during charge and discharge, and controlled compression keeps the internal layers in contact and slows capacity fade. Too little compression leaves the cell free to swell; too much can squeeze the jelly roll. A firm, even clamp with a compliant layer — foam or a thin rubber pad — between cells is a reasonable middle ground, and the creator openly flags that he is still refining the design based on viewer feedback.
Bus Bars and Series Connection Order
For connecting the cells in series, the video uses copper bus bars cut from a single piece of 20mm by 4mm copper bar, in 75mm lengths — sized to handle up to 300A. The bars are cut to equal lengths, drilled, and the contact surfaces sanded to remove the oxide layer. The battery terminals get the same treatment: a light sanding with sandpaper cleans off oxidation so the connection resistance stays low. This is a step many hobby builds skip, and it is exactly where hidden voltage drop and heat come from.
The series order follows the standard alternating pattern: minus, plus, minus, plus, so that each bus bar connects one cell’s positive to the next cell’s negative. The creator’s assembly trick is worth copying: loosely fit both bolts on a bus bar before tightening either one, because tightening one bolt first lets the electric screwdriver rotate the bar and short an entire cell. Fit both, then tighten both.
Wiring the BMS: Balance Leads and Sense Cables
The BMS in this build is a Daly-style active balancer with Bluetooth, and its wiring is the heart of the pack. The B-minus cable bolts to the overall negative of the battery; the black cable from the BMS connects to B-negative; the balance leads then attach one by one to each bus bar — the blue wires go to the intermediate cell connections in order, and the final blue wire connects to B-positive. Getting this order wrong is the classic DIY battery mistake, so label every connection and double-check the sequence against the BMS diagram before powering anything up.
One viewer’s comment on the video makes a good practical point: clamp the sense cables to the battery terminals can put the BMS measurement at the mercy of the main-current connection. Reading the BMS manual and keeping the balance leads on their own clean terminals, or at least verifying the readings after assembly, is the safer habit. The creator confirms he followed the manual, and the green status LED on the BMS — which lights only when the connections are correct — is the moment everything clicked.
First Charge and BMS Monitoring
After assembly, the pack reads about 13V and needs a full charge. The video charges it through an SMPS power supply set to 14.3V DC, with a clamp meter on the line so the current can be watched in case it climbs too high. One hour in, the pack has risen from 13V to about 13.4V, and the BMS app shows state of charge at 88%, charging current of 11.9A, and cell voltages beginning to spread — the top cells sitting around 3.52–3.53V while the pack finishes its top-up.
When the pack reaches full, the BMS cuts charging at 14.2V and the app reads 100% SOC with the pack resting at about 14.05V. This is exactly the behavior you want from a BMS: the app shows maximum cell voltage, minimum cell voltage, which cell is highest, and the charge/discharge current limits — all adjustable over Bluetooth. The creator also demos the remote-monitoring side: the same BMS supports Wi-Fi and 4G modules, so a pack in a shed or a caravan can be watched from anywhere.
Load Test: Headlamp and 500W Motor
The finished pack is load-tested twice. First with a 12V 100W car headlamp bulb, which draws roughly 8A — easy duty that confirms the main wiring and BMS pass current without drama. Then the serious test: a 500W permanent-magnet DC motor from an e-bike, which at 24V rating pulls about 26.7A and is a far more realistic inverter-style load. The motor spins up cleanly, which confirms the bus bars, the BMS and the connections can all deliver real current without tripping protection or dropping voltage.
For your own build, this two-step load test is the right pattern: start with a small resistive load to validate the wiring, then step up to the kind of load the pack will actually see — an inverter, a motor, a charger. If anything is loose or mis-wired, it shows up as heat or a tripped BMS at small current, long before it becomes a fire risk at full load.
Safety Rules for Your Own Pack
A 100Ah LiFePO4 pack stores enough energy to do real damage if assembled carelessly, and the video’s details encode the safety rules: sand terminals before connecting, always fit both bolts before tightening either, keep the BMS balance wires in the right order, add insulation over exposed bus bars and sense cables, and never charge a pack without a BMS in the loop. The creator’s choice of foam between cells doubles as an insulator, and the completed pack is taped and zip-tied into a tidy assembly with no bare copper reachable.
Viewers raised two legitimate concerns in the comments — that foam can compress over time, and that it is flammable — and the creator responded by redesigning the pack in a follow-up video. If you copy this build, treat the foam layer as a first attempt: a firmer, fire-resistant spacer is a genuine improvement, and the principle of light compression with insulation between cells remains sound. When in doubt, use the BMS’s own cutoffs as your last line of defense and keep the pack in a vented, non-flammable enclosure.
If you would rather buy a professionally built pack than solder your own, the same 12V 100Ah class is exactly what we make. 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. If you do build your own, we’d still suggest matching the video’s care — and if you’d rather not, tell us your voltage and capacity and we’ll engineer the pack for you.
What Viewers Are Asking (Top Comments on the Video)
Where the creator replied, we quote his answer verbatim; where he didn’t, we give our own practical answer — the way we’d answer a customer on the shop floor. We kept only comments with real practical value and dropped the pure thank-you notes.
Source video: Mr. Electron - Alternator — “Stop Buying Expensive Batteries - Build This 100Ah LiFePO4 Instead!” (youtube.com/watch?v=PH6mIXQvK4A). Comment excerpts are quoted verbatim from the video’s top comments; creator replies are quoted where they exist, and unanswered threads carry Dajiu Energy’s own answers. 179K+ views. Four 3.2V 100Ah prismatic cells become a 12V pack with a Bluetooth active-balance BMS, then power a 500W motor.