A failed NTC thermistor, costing 20 cents, is the reason perfectly good Makita battery packs get thrown away. The Repair Forge documented the full diagnosis and fix on YouTube, showing that a charger flashing its red LED and spinning its fan without ever charging is not a dead battery. It is a battery the charger believes is at an impossible temperature.
The diagnostic key is PocketOBI, an open source tool built on the reverse-engineered Makita battery protocol from the Open Battery Information project. Querying the pack directly showed one thermistor reporting negative 30 degrees Celsius while the other read a normal 28. A multimeter confirmed infinite resistance on the faulty component. A temporary resistor substitution validated the diagnosis before ordering the replacement part. The original article is worth reading in full for the step-by-step protocol query output and the second battery case, where a thermistor was stuck reading 64 degrees Celsius.
Total time from diagnosis to repair is roughly ten minutes with PocketOBI. The fix itself is a standard SMD thermistor swap. The deeper story here is that Makita's battery communication protocol is now fully reverse-engineered and open, which means this class of repair is accessible to anyone with a soldering iron and a USB interface.
[READ ORIGINAL →]