# Dual Battery Schematic Circuit Diagram

Employing rechargeable batteries to energize circuits is a reliable technique for supplying power to equipment that operates without mains connection. However, a significant drawback is that the battery often depletes precisely at the most inconvenient time. As a user, you are caught off guard by the sudden malfunction of the circuit. In some cases, this inconvenience is minor, but in others, it can lead to disastrous outcomes. Consider, for example, the scenario where a model airplane’s radio receiver ceases to function mid-flight due to a drained battery. We can assure you that the aftermath of such situations is far from pleasant.

#### Solution

The solution to this issue is surprisingly straightforward: employ two batteries! When one of the batteries runs out, the second one can seamlessly step in and continue providing power. Naturally, all of this should occur automatically. Hence, a practical circuit is necessary to manage these tasks on our behalf. The design provided here is specifically crafted for use with circuits, like those in model receivers, employing NiCd batteries consisting of four cells. The circuit is compact, and with the help of the PCB populated with SMDs, it’s effortlessly accommodated into existing equipment.

#### Simple

The operational principle is straightforward: IC2 monitors the terminal voltage of battery A. If it falls below 4.38 V, the RESET output goes low; otherwise, it remains high. Similarly, IC4 performs the same function for battery B. Both signals feed into a flip-flop composed of IC1a and IC3d, determining which battery is in use. If the voltage across battery A is too low, the output of IC1a stays high, activating battery B. The reverse holds true for the output of IC3d. When both batteries are depleted, they both power the circuit, adhering to the principle of ‘some power is better than none at all.’

Components D3, R8, and C3 introduce a switch-on delay, ensuring a pause in battery activation. This delay prevents both batteries from simultaneously powering the circuit during the transition, which could otherwise cause substantial equalization currents due to the voltage disparity between the two batteries.

#### Switch

Opting for a FET as the switching device is more efficient than using a bipolar transistor because it doesn’t require any base current, conserving energy. However, an inherent drawback of a MOSFET is its built-in diode. In this circuit, this diode is problematic as it enables one battery to charge the other through it. A simple solution would be to add a series diode to prevent this. Unfortunately, diodes always have a voltage drop (around 0.3 V with a Schottky diode).

To address this issue, we employ a second MOSFET wired in the opposite direction. The clever trick here is that the channel of a FET conducts in both directions when switched on. This negates the impact of the forward voltage of the internal diode.

LEDs D5 and D6 serve as indicators, showing which battery is currently active.

#### Use

Using the circuit is straightforward. Connect a four-cell NiCd battery to each of the battery inputs (labeled TC2 and K3). Then, link output K1 to the circuit requiring power.

Turn on the supply voltage using switch S1. The LEDs will indicate which battery is currently active. In the unlikely event that both batteries are deeply discharged, both LEDs will be lit, indicating the situation.

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