This charger uses the nonlinear resistance of lamp filaments for controlling charge current. When the battery is low and tends to draw large currents, the lamps light up and their resistance increases, safely limiting the charge current.
When the battery is mostly charged and drawing small currents, the resistance drops back down as the lamps dim, providing a faster charge time. As an extra feature, the battery charge status is displayed by the lamps. Be aware that the vehicle’s engine must be running for this charger to function. To prevent accidental discharge, a simple switching circuit is incorporated to connect the battery only when the input voltage is above about 13.8 VDC (adjustable with a trim pot). A LED is mounted next to where the wires emerge on the back, and indicates that charging is in process. A relay with low hysteresis is preferred for this application.
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