The battery circuit scheme is designed as a source of constant voltage with negative temperature coefficient. Transistor Q1 (BD 140) is used as a temperature sensor. transistor Q2 is used to prevent the battery from discharging through R1 when electrical power is unavailable. Charging circuit is designed based on the LM350 voltage regulator IC. The output voltage of the charger can be adjusted between 13-15 V by varying the POT R6.
LM350 will try to keep the voltage drop between the input pin and output pin at a constant value of 1.25V. So there will be a constant current flow through resistor R1. Q1 act here as a temperature sensor with the help of R6/R3/R4 components that are more or less controls the base current of Q1. As connection emitter / base of transistor Q1, the same as other semiconductors, containing the temperature coefficient of-2mV / ° C, the voltage output will also show a negative temperature coefficient. This one is just a factor of 4 large, because the variation of the emitter / base of Q1 is multiplied by a factor of division P1/R3/R4. This leads to some-8mV / ° C. LED will light whenever power is available.