Where This Guide Comes From
This guide follows Rob Cameron’s "How to Build an eBike Battery" video, which has over 300,000 views. The project is a 52V replacement pack for a Rad Power RadRunner 2, built from scratch with 112 cells. What makes the video worth following is the order of operations: pack math first, cell choice second, then layout, welding, wiring and only then the first charge. We walk the same sequence here with the numbers, so you can size your own pack instead of copying this one blindly.
Plan the Pack: Voltage and Capacity Math
The build starts on paper, not at the workbench. The bike needs roughly 52V nominal, and 14 cells in series give 3.7V × 14 = 51.8V — that is the "14S" part. For capacity, eight cells in parallel give 3,200mAh × 8 = 25,600mAh, the "8P" part. Multiply the two numbers and you get 112 cells total. This is the standard way pack sizes are written: series first, parallel second. A 14S8P pack is a rectangle of 14 groups, each group holding 8 parallel cells. If you are sizing for your own bike, start from the controller’s voltage window, then decide capacity from your range target — the math is the same.
Pick the Right 18650 Cell: Capacity vs Discharge Rating
Cell choice in the video comes down to three numbers: capacity, continuous discharge current and price. The EVE 33V cells used here are rated 3,200mAh and 10A continuous. The 10A number is a limit you should stay away from — at full discharge current the voltage sags hard, sometimes from 4.2V down to 3.2V, so you are not getting the voltage you think you are. Staying at half the rating or less is the safe working zone. The original RadRunner 2 controller draws up to 25A; upgraded controllers go to 35A. An 8P pack can theoretically deliver 80A, but again the working target is around half that — about 40A — which comfortably covers 35A.
Layout: How the Cells Fold Into a Rectangle
The video’s layout explanation is the clearest part of the build. Connect cells in parallel groups and you can fold the groups over each other so the positive and negative terminals alternate, current flowing through the group in sequence. Measure across the whole folded block and you get the final pack voltage, with series on one axis and parallel on the other. The result is a compact rectangle that actually fits on a bike frame. The cells go into a plastic 18650 holder — here a 4×5 holder with a couple of spare slots — and the holders lock together into one rigid block once the nickel strip is welded on top.
Test Cells Before You Weld
Before any welding, every cell gets a voltmeter check. The factory cells in this build all read around 3.46V, so they are safe to parallel immediately. That matters because a parallel group is a network: the moment the strip is welded across cells at very different voltages, the higher cells dump current into the lower ones, and a big voltage gap can mean a lot of current moving through a thin strip — genuinely dangerous. If you are using harvested cells from laptop packs, you have no idea what the voltages are, so test everything and group cells with similar voltages. Outliers should be charged or discharged to match the group before welding.
Spot Welding: Strip Choice and Technique
Voltage is not the only test worth doing. Internal resistance varies between cells even from the same box, and a high-resistance cell in a parallel group takes more than its share of current and ages faster. If you have a tester, match cells within a few milliohms for the same parallel group; if you do not, at least check voltage and pick cells from the same batch. A cheap cell tester pays for itself on the first pack, and it is the difference between a pack that stays balanced for years and one that needs cell replacement mid-life.
The pack is connected with 0.15mm nickel strip — the "2P" style that welds two parallel cells in one pass — and cut to length with scissors. Strip choice gets a specific warning in the video: a lot of "nickel strip" is actually steel with a nickel coating, which conducts far worse. Pure nickel is what you want, and a quick salt-water test (it rusts = steel) settles it. The welding is done with a battery-powered spot welder at 30 joules per pulse, roughly 18ms and over 1,100 amps per weld. The technique points that actually matter: firm pressure, strip centred on the cell, two welds per tab, and watch the cables — they heat up after a long run and need a cool-down break.
Wiring the BMS: Balance Leads and Main Cables
Once the block is welded — with a second nickel layer added over the first for current capacity — the BMS goes on. A 14S 35A BMS matches this pack: it covers the original 25A controller and an upgraded 35A one. The balance connector wiring is where most first builds go wrong. Each of the 15 balance wires lands on one tab along the pack, starting at pack negative (0V) and stepping up 3.7V each time — 3.7, 7.4, 11.1, 14.8 and so on to about 51.8V. That lets the BMS read every series group and balance them to the same 4.2V after charging, because after many cycles groups drift apart. The main cables are 12AWG: B- connects to pack negative, P- is where the load comes from.
Connectors: XT60 and the Charging Port
The pack exits through an XT60 connector, the standard in the eBike world. The number is the current rating: XT30, XT60 and XT90 handle 30, 60 and 90 amps. Here 60A is comfortably above the 35A maximum, and the connector lets you unplug the battery for charging or testing. The video’s smart detail is soldering the main wire to a short strip first and then to the cell tab, instead of heating the battery directly. The charging port gets its own 3D-printed bracket, wired through the same connector, with crimp terminals joining the cables instead of one awkward triple solder joint.
Insulation, Heat Shrink and Bike Installation
The finished block is wrapped in a large sheet of heat-shrink tubing — the kind you usually think of only for cables, but it comes in huge sheets. Heat shrink contracts in one direction, so the ends curl slightly and can be trimmed. Underneath, a couple of layers of EVA foam cushion the pack because this bike has no suspension and gets ridden hard over curbs; the battery sits on foam instead of bare metal. The XT60 on the bike side is wired to the controller, the pack is strapped in, and the whole assembly is a solid, well-insulated block.
First Charge and the Voltage Reference Table
The first charge also sets the storage habits. A lithium pack that sits fully charged ages faster than one stored at about 50-60% charge, so if the bike will be parked for weeks, charge it to around 52V and top it up later. Never charge a pack unattended on the first run: watch the balance lead voltages climb together and confirm no group races ahead. A pack that finishes with one group much higher than the others has a wiring or cell problem you want to find on the bench, not on the road.
The first charge is the verification step. A 5A charger is plugged into the XT60, the output voltage is monitored on a voltmeter, and the charge voltage rises steadily — in the video roughly one volt in nine minutes on a 49V start. The output settled around 48.4V at the pack’s initial state before charging to full. If you only have voltage to judge state of charge, a 48V/52V reference table showing each voltage from 0% to 100% is genuinely useful, because eBike batteries rarely ship with a usable fuel gauge.
Before You Buy Cells: Match the Pack to the Bike
The whole build comes back to matching the pack to the bike’s controller. Check the controller’s maximum current first, then pick cells whose parallel group capacity and discharge rating cover it with a margin. Cheap high-capacity cells with low discharge ratings are the classic failure: the pack has the amp-hours but sags under load. If you would rather not build at all, a purpose-built battery from a manufacturer removes the welding, BMS wiring and balancing risk entirely — and for a bike you ride daily, that reliability is worth comparing against the DIY saving. For more on lithium behaviour and sizing, our battery guides cover the rest.
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 and industrial stackable systems, with CE, UN38.3, MSDS and ISO9001 certified builds and OEM/ODM support. Browse the full product range or read more battery guides before you decide.
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.
Source video: Rob Cameron — "How to Build an eBike Battery" (youtube.com/watch?v=ZxhIs_xxVY0). 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.