DIYBMS GIVES EVERY BLOCK OF RECYCLED LAPTOP CELLS ITS OWN MONITOR, FOR ABOUT EIGHT DOLLARS A BLOCK
One small board per block of secondhand lithium cells, watching voltage and bleeding off the excess so the weakest cell does not get quietly overcharged.
EnergyOpen-hardware
- difficulty
- ●●●●○
- time
- a weekend-plus
- license
- CC-BY-NC-SA-2.0
- repo
- repo ACTIVE1,129 stars
●●●●○ · a weekend-plus · CC-BY-NC-SA-2.0 · 1,129 stars · repo ACTIVE
WHAT YOU’LL NEED
- 3D printer + filament — printable parts — files are in the repo
- Electronic parts — full list with part numbers in the repo BOM
- Dev board / microcontroller — runs the project firmware
Partner
COMPAREE VERDICT
diyBMS version 4 solves the central problem of building a lithium pack from mismatched secondhand cells: they drift apart in voltage, and without active balancing the weakest block ends up being overcharged. Each small module sits on one cell block, measures voltage and temperature, bleeds off charge when a block runs high, and reports to an ESP32 controller serving a live web page with a bar per block and history logs. The repository has everything to order boards and assemble them — PCB files, firmware, and wiring diagrams — and one builder documented the entire chain from laptop pack to finished wall for under seventy dollars. What makes this a weekend-plus rather than a weekend is the harvesting: stripping cells out of discarded packs, testing each one for capacity and internal resistance, and sorting them into matched groups is slow, repetitive work, and a single swollen or shorted cell is a fire risk. The firmware and the boards themselves are well-documented and stable (1,129 stars, active maintenance), but this is not a kit — you order PCBs from a fab house, source the components, solder the boards, and wire the blocks yourself. The single thing most likely to go wrong is not the electronics; it is harvesting cells that should have gone to recycling or misjudging which packs are safe to strip. If you already have a tested pile of 18650 cells and want open balancing hardware that does not cost per-cell what a new cell costs, this is the project. If you are starting from zero and the phrase 'test each cell for internal resistance' sounds like a research task, start smaller.
IN THE REPO
NOT IN THE REPO
- —Full PCB designs in KiCad format and Gerbers for both cell modules and controller board.
- —Bill of materials present with component references, but no consolidated per-module cost sheet in the repo.
- —Firmware lives in a separate repository (diyBMSv4Code), compiles in PlatformIO, and includes web interface.
- —Documentation covers assembly, wiring and configuration; no step-by-step cell harvesting guide.
- —Licensed Creative Commons Attribution-NonCommercial-ShareAlike 2.0 UK — free for personal builds, explicitly not for commercial resale.
- —The builder's published cost (eight pounds per module, under seventy dollars total) is from a real build and includes PCB fabrication, but moves with component prices and exchange rates.
Can I build this?
Build at your own risk. Projects involve tools, electronics and sometimes mains voltage — follow the creator’s safety notes.
Partner · KickstarterHeyGears G1: 10M+ colors and transparent parts in one print, plus UV printing on flat objects. Figures, parts and labels — no painting.
Videos
Laptop cells, blocks and the diyBMS after a year
no official channel; the best walkthroughs are community builds
Gallery
Start here
Navigation into the creator’s own docs — we don’t rewrite the guide, we route you to the source.
- 1.Read the repository README and the assembly documentation to understand the full system before ordering anything. (The main repo covers hardware; firmware is in diyBMSv4Code.)
- 2.Order PCBs from a fab house using the Gerber files in the repository.(You need one module board per cell block and one controller board.)
- 3.Source components from the bill of materials and solder the boards.(Most parts are generic; the ATtiny and ESP32 are the only programmable chips.)
- 4.Flash the firmware to the modules and controller using PlatformIO. (The code repository includes instructions for compiling and uploading.)
- 5.If harvesting cells, test every single one for capacity and internal resistance and discard any that are swollen, dented, or read outside safe limits.(This step is slow and non-negotiable — one bad cell can ruin the pack or start a fire.)
- 6.Wire the modules to the cell blocks and connect them to the controller, then configure the voltage limits and balancing thresholds in the web interface.(The documentation includes wiring diagrams for different pack configurations.)
Resources
Documentation, files and community threads for this build — we link straight to the original sources and never rehost the creator’s files.
Partner · KickstarterCORE POWER BANK S: 0.2" thin, 2.8 oz, 5000 mAh with 22.5W fast charging. Semi-solid battery, carbon fiber, IPX7 waterproof.
KNOWN ISSUES
- The licence is Creative Commons Attribution-NonCommercial-ShareAlike 2.0 UK, so you cannot sell a pack or a product built with diyBMS — this is for personal builds only.
- Harvesting cells from laptop packs is the step most likely to go wrong: packs from unknown sources may have been stored discharged for years, and a cell that reads normal voltage can still have high internal resistance and fail under load.
- The builder's cost of eight pounds per module was from 2020 or earlier; component shortages and PCB minimums mean your first build may cost more unless you order in a batch.
- Testing cells properly requires a charger or tester that measures internal resistance, not just voltage — a five-dollar USB tester will not catch a cell that is about to fail.
- The firmware and hardware are for passive balancing (bleeding off excess charge through a resistor), which is slow; if your blocks differ by more than a few hundred milliamp-hours, balancing after every charge cycle will take hours.
- The repository assumes you understand lithium cell safety and does not include a harvesting guide — if you have never stripped a laptop pack before, watch community videos first and work in a fireproof container.
Can I use this with new cells instead of recycled ones?
Yes — diyBMS works with any lithium cell chemistry (LiPo, LiFePO4, 18650, pouch cells) as long as you configure the voltage limits correctly in the firmware.
How many cells can one system manage?
The controller can handle up to 24 modules daisy-chained together, so a 24S pack (about 100 volts fully charged) is the upper limit without adding a second controller.
Do I need one module per cell or per block?
One module per block — the builder used 20 cells in parallel per block, giving 50 amp-hours, and one module balanced the whole block.
What happens if a module fails?
The controller logs the last reading from every module, so a dead module is obvious in the web interface; the system can trip a relay to disconnect the pack if voltage goes out of range.
Can I sell packs built with diyBMS?
No — the licence explicitly prohibits commercial use. You can build it for yourself, document your build, and share the design, but you cannot sell a product that uses it.
Community builds
No community builds yet — be the first, we feature the best ones.
Discussion1
FROM THE COMPAREE TEAM
The builder tested every cell for capacity and internal resistance, found an average of 2.5 amp-hours, and grouped twenty per block — and a year later every block still reads within a few thousandths of a volt. Where would you source your cells, and what capacity would you aim for?
Stuart Pittaway
Stuart Pittaway built diyBMS to manage his own off-grid battery system and released it as open hardware in 2017. Version 4 is the current stable release, with active maintenance and a community of builders documenting their own packs.
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DISCLAIMER
- Comparee is not the author of the projects featured here. All rights to each project belong to its creator — every page links to the original source, and we never host creators’ files.
- Information is provided without warranty and may become outdated as projects evolve. Prices are indicative bands only — always check the creator’s parts list for current costs.
- Building and operating any project is at your own responsibility. Protective equipment, safe workshop practice and compliance with local regulations are the builder’s responsibility.
CompareeTEAM19d ago
Practical notes from our verification: the footage in the reel is from Adam Welch's build, which used an earlier generation of diyBMS hardware — the current repo is version 4, with updated firmware and a web interface served by an ESP32. There is no official build video from the creator; the best walkthroughs are community-made. The builder's cost sheet (eight pounds per module, under seventy dollars total) is real but from 2020 or earlier, and PCB prices and component availability have moved since then. The single biggest variable is not the electronics — it is whether you already have a tested pile of 18650 cells or whether you are starting from laptop packs, because harvesting and testing cells is slow, repetitive work, and one swollen or shorted cell is a fire risk. The repository has everything to build the system, but it assumes you understand lithium cell safety and does not hold your hand through harvesting.