Auto Trickle Charger


THIS simple 12-volt automotive battery booster/trickle charger provides between FULL CHARGE (approximately one ampere) and TRICKLE.  It provides a choice of charging rates to suit CHARGE (50-mA). Indicator LED1 is in battery condition. Set to FULL CHARGE, series with its current limiter, R4. The it will restore a partially discharged battery overnight; when set to TRICKLE CHARGE, it will maintain the battery at full charge.  The fuse protects against short circuits.




The entire circuit can peak capacity for an extended time. It can be mounted in a small metal enclosure, built-in LED glows only when the using multi lug terminal strips. Point-to-point wiring can be used. The ac line cord and the

cable delivering current to the battery should be passed out of the enclosure via grommetted holes. A plug that fits the vehicle’s cigarette lighter should be connected to the output cable. Make sure the polarity is correct.


(The circuit is shown in Fig. 1.)




Plug the ac line cord into a convenient outlet and plug the output cable into the cigarette lighter connector. Select either a FULL or TRICKLE charge via S1, and verify that the LED glows in either position of S1.


Circuit Operation.


The output of T1 is rectified by diodes d1 and D2. Pulsating dc is delivered to the battery via a cable to the cigarette lighter connector in the vehicle. If the LED does not glow clean the contacts on the plug and the cigarette lighter and try again. If this fails check for a wiring error. Should the battery be completely discharged (dome light does not light up), use the TRICKLE CHARGE position of S1 for one to two hours. Less than 8 to 9 volts from the battery at the end of this time means that it must be replaced. If the voltage is about 12 volts, place S1 in the Full CHARGE position.





Copyright by Bill Bytheway, K7TTY February 2012