simplest smf automotive battery charger

A very simple battery charger circuit is described in this article, which uses just a single transistor for the voltage detection as well as for automatically disconnecting the battery from the supply when it gets fully charged. This adjustment becomes the high voltage trip point of the circuit and is used to switch OFF the charging voltage to
simplest smf automotive battery charger - schematic

the battery when it gets fully charged or its voltage reaches around 14 volts. Typically a general purpose transistor like the one which is shown (BC547) when adjusted to switch ON at 14 volts may have the lower threshold of around 10 volts, when it might get just switched OFF. The lower threshold of 10 volts is dangerously low and we cannot wait for the circuit to restart the charging process until the battery voltage falls to this dangerous 10 volts level. Therefore to eliminate this issue the circuit needed to somehow reduce the hysteresis level. This is done by introducing a couple of diodes at the emitter of the transistor. By inserting the two diodes in series with the emitter of the transistor, we force the transistor to switch off 1. 4 V earlier than its normal specified limit of 10 volts. Therefore now the lower operating threshold of the circuit becomes 10 + 1. 4 = 11. 4 volts, which may be considered just OK for the battery and for the automatic restart of the charging process. Having both the thresholds updated as per the standard charging requirements, we now have an automatic automotive battery charger that`s not only cheap to build but also smart enough to take care of the battery charge conditions very efficiently.

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