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Don't rush to throw away your expensive drill or wrench battery if it still runs the tool perfectly (good discharge) but the charger light stays green and won't charge! In this video, I will guide you step-by-step through how to inspect and fully fix this issue. In reality, most cases are not caused by dead internal battery cells, but rather a faulty Battery Management System (BMS) board. You only need to replace it with a new, low-cost BMS circuit board to make your battery pack work like new again. 📍 Video Timestamps: 00:00 - Checking the issue (Discharging fine, but not charging) 00:31 - Opening the case and testing individual cell voltages with a multimeter 01:09 - Desoldering to remove the faulty old BMS board 01:46 - Replacing and soldering the new protection circuit (BW 5S-90A V2 adapter charging board) 03:05 - Testing the charger (Red light indicates success) and final reassembly 🔧 Parts and Tools Used: BMS Board: BW 5S-90A V2 adapter charging circuit (For 5S 21V battery system) Tools: Multimeter, soldering iron, screwdriver... 🔔 Don't forget to SUBSCRIBE and turn on the notification bell so you won't miss any upcoming repair tips, electronic DIYs, and battery restoration videos! If you have any questions about the wiring diagram (B-, B1, B2... B+) or how to measure battery cells, feel free to leave a comment below. I'll be happy to help! #batteryrepair #BMSboard #5S21Vbattery #electronicsrepair #DIY #batteryrestoration #drillbattery #makitabattery