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When a DeWalt 20V Max battery refuses to charge, it is most often an intentional protective lockout by the pack’s BMS (undervoltage, cell imbalance, thermal or handshake fault) or a charger/interface/contact issue — not sudden catastrophic cell failure. Check terminals and seating, feel pack temperature, cross-test with a known-good charger and a known-good pack, and let a hot/cold pack rest 20–30 minutes; measure OCV before further steps. Persistent refusal after these checks usually indicates BMS/PCBA or weak-series-group problems that need diagnostic logs or lab-level intervention — replacing cells alone commonly fails unless the BMS and balancing are addressed.

Charging algorithms are the firmware and control logic that turn a power supply into a safe, effective charger. For Makita-style Li-ion tool packs the algorithm must balance three goals: safely restore energy (speed), avoid actions that accelerate aging (longevity), and reliably handle protection/BMS handshakes. This note explains the common algorithm building blocks (precondition/wake, CC–CV, taper/termination, thermal compensation, balancing and adaptive aging logic), how they protect cells, and which tests you can run to verify correct behavior.

Chargers can be a primary cause of premature battery aging or failure when their hardware, firmware, or use patterns induce excess voltage, current, heat, or ripple into BL-series packs. This guide explains how chargers stress BL packs, practical preventive design/operational controls, reproducible tests (field → bench → lab) to detect charger-driven damage, and clear troubleshooting/mitigation steps you can apply immediately. Technical, non-marketing, and suitable for engineering, QA and fleet operations.

Accurate temperature sensing determines whether a charger can safely fast-charge or must derate/stop. Sensor location, thermal coupling and control logic directly affect detection latency, false trips and charger/pack lifetime. This guide gives practical placement rules, sensor choices, mounting best practices, control strategies, reproducible tests, common failures and immediate engineering actions — concise, technical and shop-floor ready.

Modern Makita-style chargers embed multiple thermal and electrical safety mechanisms on the PCBA to protect cells, users and the charger itself. These include temperature sensing and thermal cutbacks, overcurrent/current-limit, overvoltage/OVP clamps, reverse-polarity detection, input surge protection, NTC/thermistor handshakes with packs, watchdogs and hardware failsafes (fuses/TVS/crowbar). This note explains each mechanism, how it behaves, reproducible tests you can run now, bench forensic methods, common failure modes and practical mitigations.