Simple Current Limiting Circuit for 24V 500mA Gel-Cell Charger from 28-32VDC Source

The discussion addresses the design of a simple, reliable current limiting circuit for charging a 24V gel-cell battery backup system from a 28-32V DC source derived from an engine alternator on an airborne platform. The key requirement is to limit charging current to 500mA while safely bringing two series 12V batteries to full charge (~28.2V). Due to weight, heat dissipation, and fire risk constraints, a robust and low-maintenance solution is needed without extensive redesign. The LM317 regulator is discouraged due to thermal limitations and failure risk under continuous load. Instead, a current-limited constant voltage source tailored to gel-cell chemistry (approximately 2.35V per cell) is recommended. Switching power supplies are suggested for low heat and weight, though EMI and transient protection per MIL-STD-704F and MIL-STD-461F standards must be considered for airborne applications. The LM338 regulator was considered for its higher current rating, but its dropout voltage (~1.8V at 1A plus additional voltage for current regulation) is too high to fully charge the battery from a 28.5V source. A low dropout current source with about 0.3V voltage drop is necessary. A practical solution involves using a rail-to-rail operational amplifier, a current sense resistor, and a P-channel power MOSFET to achieve precise current limiting with minimal voltage drop.
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