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YOU CAN BUILD A READER THAT TELLS YOU WHY A DEAD POWER TOOL BATTERY IS NOT ACTUALLY DEAD

A reader that tells you why a dead power tool battery is not actually dead — and sometimes lets you bring it back.

by The Repair Forge

FULL CAD BOM FIRMWARE DOCS

WorkshopOpen-hardware

Built withESP32Arduino

difficulty
●●●○○
time
a weekend
license
NOASSERTION
repo
repo ACTIVE126 stars

WHAT YOU’LL NEED

Videos
1
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COMPAREE VERDICT

PocketOBI is a handheld diagnostic tool for Makita LXT 18V battery packs, built on an ESP32-C3 with a 2.4-inch TFT screen and a rotary encoder. It speaks the OneWire protocol that the Open Battery Information project reverse-engineered, so it can read what the pack is actually reporting: per-cell voltages with colour-coded health and imbalance, both temperature sensors, charge count, manufacturing date, model, and error state, and it gives a traffic-light verdict before a staged repair wizard tries to clear a false lockout. The repository publishes firmware, a wiring diagram, a parts list and a bench-validated carrier PCB (Gerbers, schematic, BOM), and the project is actively maintained. There is no enclosure. The single biggest trap is not the build, it is the safety: these packs hold lithium cells and up to roughly 21 volts, the ESP32 must never touch the pack's positive terminal, and a pack with a genuinely bad cell must not be forced back into service because that is a fire risk. The licence is non-commercial (PolyForm Noncommercial), so it is for your own repairs, not a product to sell. For someone who repairs Makita packs and wants to see what the BMS is actually saying, this is exactly the tool. For someone unfamiliar with lithium pack safety or ESP32 wiring, this is a weekend that could go very wrong.

GOOD TO KNOW

  • —Firmware is published and actively maintained, for ESP32-C3 (and, since v2.2, the CYD board); builds with the Arduino IDE or PlatformIO.
  • —Parts list is in the README (5 items) and a 6-part carrier-PCB BOM is in hardware/; no vendor links.
  • —No enclosure files or 3D models; build it on a breadboard or order the published carrier PCB.
  • —Licensed under PolyForm Noncommercial 1.0.0 (hardware files CC BY-NC-SA 4.0) — free for personal and educational use; commercial use needs a separate licence from the author
  • —This is beta software with a blunt safety warning: lithium cells, up to 21V, never connect the ESP32 to the pack's B+ terminal
  • —The project can reset a false BMS lockout, but forcing a pack with a genuinely bad cell back into service is a fire risk

Parts to buy

6 items

From our check of the build. Exact quantities and part numbers are in the creator’s BOM.

  • ESP32-C3 SuperMiniFind
  • 2.4-inch ST7789 TFT with integrated EC11 encoderFind
  • Makita BL1830 LXT adapterFind
  • 2× 4.7 kΩ resistorsFind
  • Breadboard or the published carrier PCBFind
  • USB-C power bank or chargerFind

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Can I build this?

Printnothing required — no enclosure published
BuyESP32-C3 SuperMini, 2.4-inch ST7789 TFT with integrated EC11 encoder, Makita BL1830 LXT adapter, 2× 4.7 kΩ resistors, breadboard or the published carrier PCB, USB-C power bank or charger
ToolsPlatformIO or Arduino IDE, soldering iron if building on prototyping board, multimeter for wiring checks
SkillsESP32 wiring and basic soldering; lithium pack safety is non-negotiable — if you do not know how to safely probe a multi-cell lithium pack, this is not the project to learn on
Timea weekend if you have built ESP32 projects before; longer if you are learning PlatformIO or troubleshooting display wiring
Cost$ — an ESP32-C3 SuperMini, a 2-in-1 TFT + encoder module, a Makita LXT adapter and two resistors; the project lists no prices.
SafetyReal lithium-cell risk: packs hold up to about 21 V on B+, the ESP32 must never be connected to the pack's positive terminal, and a pack with a genuinely bad cell must not be forced back into service — the README calls that a fire risk. Thermistor replacement inside a pack requires opening the case and working around live cells. If you are unfamiliar with lithium pack safety, do not start here.

Build at your own risk. Projects involve tools, electronics and sometimes mains voltage — follow the creator’s safety notes.

Videos

This makita battery isn't dead. A 20 cent part killed it !16:37

The creator, The Repair Forge, uses PocketOBI to track down a failed thermistor in two Makita packs. For the build itself, see his original build video linked in the README (youtu.be/57KsQQ7-Qd0) and the v2 walkthrough (youtu.be/8Qe2dTXrJ8g).

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Gallery

Start here

Navigation into the creator’s own docs — we don’t rewrite the guide, we route you to the source.

  1. 1.Read the safety section in the README (This is not optional — the project works with live lithium cells and up to 21V, and the wrong connection can damage the ESP32 or worse)
  2. 2.Order the parts from the bill of materials (The core parts are an ESP32-C3 SuperMini, a 2.4-inch ST7789 TFT with built-in EC11 rotary encoder, a Makita BL1830 LXT adapter and two 4.7 kΩ pull-up resistors)
  3. 3.Wire the ESP32 to the display and encoder following the pinout in the README (The project assumes you know how to connect an SPI display and a rotary encoder; if this is your first ESP32 display project, find a basic ST7789 tutorial first)
  4. 4.Flash the firmware with PlatformIO (The README gives step-by-step instructions for both the Arduino IDE (ESP32 core 3.x) and PlatformIO.)

Resources

Documentation, files and community threads for this build — we link straight to the original sources and never rehost the creator’s files.

KNOWN ISSUES

  • There is no enclosure — build it on a breadboard following the wiring diagram, or order the published carrier PCB from the hardware/ folder
  • The ESP32 must never be connected to the battery pack's positive terminal (B+); the README is explicit about this, but it is the single most dangerous wiring mistake
  • The project can clear a false BMS lockout, but forcing a pack with a genuinely bad cell back into service is a fire risk — the tool shows you the data, but the decision is yours
  • Replacing a faulty thermistor inside a Makita pack requires opening the case and working around live lithium cells; if you have never opened a lithium pack before, this is not the project to learn on
  • The licence is non-commercial (PolyForm Noncommercial 1.0.0) — fine for repairing your own packs, but selling units or using it in a paid repair business needs a separate licence
  • The OneWire protocol is Makita LXT-specific — this will not work with other brands' battery systems

Does this work with other battery brands?

No. The OneWire protocol is specific to Makita LXT packs. Other brands use different communication protocols, and this tool will not read them.

Can I use this to revive any dead battery?

No. It can clear a false BMS lockout if the cells are genuinely healthy, but a pack with a bad cell must not be forced back into service. The tool shows you the data; you still have to make the safe decision.

Do I need to build a custom PCB?

No — it works on a breadboard following the wiring diagram. But a bench-validated carrier PCB is published in the hardware/ folder (Gerbers, schematic, BOM and ordering notes) if you want a tidier build.

What kind of fault can cause a false lockout?

One failure the creator documents in his video: a failed thermistor. In two Makita packs with healthy, balanced cells, one temperature sensor reported minus thirty degrees; the thermistors measured open circuit and 2.6k instead of 10k. The part costs about twenty cents. He also found that the BMS stops charging when the two sensors drift about thirteen degrees apart.

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Discussion1

FROM THE COMPAREE TEAM

A bench-validated reader, a repair wizard and a blunt safety warning about lithium cells and 21 V — and it tells you what a Makita pack is actually reporting, which no charger will. Have you ever binned a pack that turned out to be a twenty-cent part?

CompareeTEAM1mo agoedited

Practical notes from our verification: the YouTube video (Fxc9aUqT9bo) is by the project creator, The Repair Forge, and shows a pack that was killed by a 20-cent part; the README links a newer walkthrough of the v2 interface. The repository holds the firmware plus a fabricated and bench-validated carrier PCB with Gerbers, schematic and BOM in the hardware folder. The current release, v2.2, includes a staged repair wizard that checks for broken sense wires, weak cell groups, imbalance and implausible thermistor readings before it will clear a lockout. Note the licence: the firmware is PolyForm Noncommercial and the hardware CC BY-NC-SA, so personal repair use is fine but selling devices is not. The single most important habit is not the hardware — it is learning to safely handle a lithium pack before you connect anything, because the README is blunt: never connect B+ (up to about 21 V) to the ESP32. Correction (4 October 2026): we re-checked this page line by line against the project's own repository, documentation and videos, and fixed errors in earlier versions.

The Repair Forge

The Repair Forge built PocketOBI to diagnose Makita LXT battery packs without a computer, using the OneWire protocol that the Open Battery Information project reverse-engineered. The project is maintained on GitHub and documented on YouTube.

GitHub

Star the project on GitHub

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.
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