pulls down the voltage at the base of transistor q2 and q2 conducts as we 11 the collector of transistor q2 is coupled to ground through biasing resistor r3 current flowing from the collector of transistor q2 flows through diode d3 raising the voltage on the base of transistor q3 with charging of capacitor c3 coupled between the base of transistor q3 and ground transistor q3 then turns 34 on 34 the collector of transistor q3 is coupled to the positive terminal of battery bt3 when the engine is off and is also connected to bt1 when the engine is on the emitter of transistor q3 is coupled to ground through the coil of relay k1 therefore when transistor q3 conducts relay k1 is energized and changes state from that shown in fig1 relay k1 thus decouples batteries bt2 and bt3 from their pre existing series configuration and couples them in parallel for charging current from the collector of transistor q2 also flows to the base of transistor q4 thereby turning transistor q4 34 on 34 the base of transistor q4 is coupled to the collector of transistor q2 through series resistor r4 and two series coupled forward biased diodes d5 d6 transistor q4 turns 34 on 34 when the voltage at the anode of diode d5 exceeds about 2 1 v the collector of transistor q4 is coupled through biasing resistor r7 to the base of pnp darlington pair q5 and the emitter of q5 is coupled to the positive supply voltage at the emitter of q2 the collector of transistor q5 is coupled to ground through the coils of relays k2 and k3 which are in parallel therefore when transistors q4 and q5 conduct relays k2 and k3 conduct the contacts.