Simple Battery Charger With Overcharge Protection

This battery charger circuit uses the classic LM317 adjustable voltage regulator as the core power-regulating element to safely deliver current to a multi-cell battery pack. The LM317 adjusts its output voltage based on the resistance configuration connected to its adjustment pin, which includes a trim potentiometer (VR1) for fine-tuning the target charging voltage. To prevent the battery from being damaged by overcharging, the circuit incorporates a shunt-style overcharge protection mechanism driven by a TL431 precision shunt regulator and a BD140 PNP transistor. The TL431 acts as a voltage reference and comparator; once the battery voltage reaches the fully charged threshold set by VR1, the TL431 begins to conduct. This triggers the BD140 PNP transistor to divert excess charging current away from the battery cells or regulate the control path, effectively halting the overcharge condition and providing basic cell balancing capabilities. Additional components like standard diodes, current-limiting resistors such as R3, and an LED indicator work together to show operational status, protect against reverse current flow, and ensure the circuit remains stable during the charging cycle. As the battery approaches full capacity, the discrete component network ensures that charging tapers off safely, protecting the chemistry of the multi-cell pack from harmful voltage spikes.