Using rechargeable batteries to power circuits is a proven method for providing energy to mains-independent equipment. A major disadvantage of this is that the battery usually turns out to be empty at the most inopportune moment. As a user, you are unexpectedly confronted with the fact that the circuit suddenly doesn’t work anymore. Sometimes this is only a minor inconvenience, but at other times it can be a catastrophe. For instance, just imagine what happens to a model airplane if the radio receiver stops working in flight due to an empty battery. We can assure you that the consequences are anything but pleasant.
The solution to this problem is actually quite simple: use two batteries! When one of the batteries becomes discharged, the second one can take over and continue supplying power. Of course, all this must happen automatically, so we need a handy circuit that takes care of everything for us. The design presented here is intended to be used with circuits (such as receivers used in models) that use NiCd batteries composed of four cells. The circuit is quite compact, and thanks to the accompanying PCB populated with SMDs, it is easy to fit into existing equipment.
The operating principle is simple: IC2 measures the terminal voltage of battery A. If it drops below 4.38 V, the RESET output goes low, and otherwise it remains high. IC4 does the same thing, but for battery B. Both signals go to a flip-flop consisting of ICla and IC3d, which determines which of the batteries is to be used. If the voltage across battery A is too low, the output of IC1a will always be high. As a consequence, battery B will be active. The same thing applies in reverse to the output of IC3d. When both batteries are discharged, they will both power the circuit, in keeping with the motto ‘better a little bit of juice than no juice at all’. Components D3, R8 and C3 provide a switch-on delay that causes battery switch-on to be delayed somewhat. This is because it is undesirable to have both batteries power the circuit at the same time during the switchover from one battery to the other. That would cause large equalization currents to flow due to the difference between the terminal voltages of the two batteries.
The best choice for the switching device is a FET instead of a bipolar transistor. This saves energy since no base current is nec
essary. A disadvantage of a MOSFET is that it always has an intrinsic diode. This diode is quite annoying in this circuit, since the one battery can charge the other battery via the diode. A simple solution would be to wire a diode in series to prevent this. Unfortunately, a diode always has a voltage drop (approximately 0.3 V with a Schottky diode).
To solve this problem, we use a second MOSFET wired in the opposite direction. The underlying trick here is that the channel of a FET conducts in both directions when it is switched on. This eliminates the effect of the forward voltage of the internal diode.
LEDs D5 and D6 indicate which battery is in use.
The circuit is very easy to use. Connect a four-cell NiCd battery to each of the battery inputs (tC2 and K3). Then connect output K1 to the circuit to be powered.
Switch on the supply voltage with switch 51. The LEDs now indicate which battery is in use. If things every get so far that both batteries become deeply discharged (Heaven forbid!), this can be recognised by the fact that both LEDs are lit.