# Battery Tester Circuit Schematic

Is the battery depleted, or is there an issue with the device itself? When your Walkman or any other battery-operated device appears non-functional when you attempt to power it on, determining the exact problem can be quite challenging. Your initial step should involve examining the battery or batteries before considering professional service. This process, naturally, requires access to a reliable battery tester. However, it also implies that the potential cost might be limited to replacing one or two batteries, coupled with a one-time investment of time and money to acquire an appropriate tester.

### Circuit Diagram:

Many commercial battery testers consist of just a resistor, a small and unassuming meter, and a push-button. Some manufacturers offer an even simpler tester with a pack of batteries. This type of tester comprises a plastic strip coated with an electrically conductive substance that changes color as an electric current flows through it. When you place this strip between the positive and negative terminals, a fully charged battery will result in a more pronounced color change compared to a partially depleted one.

The design of the constant-current circuit was intended to function with voltages as low as 0.9 V. Creating a circuit that operates at even lower voltages with standard transistors can be quite challenging. The active constant-current element in this circuit is Transistor T1. It maintains a constant current by comparing the voltage across resistor R1 in the collector circuit with a relatively stable reference voltage provided by diode D1. This comparison is achieved using the differential amplifier T3/T4.

The voltage across Schottky diode D1 tends to be naturally stable, but it can be further stabilized by using FET T5 as a straightforward constant-current sink. At somewhat higher voltages, T5 also limits the current, especially when multiple batteries are connected in series. The differential amplifier T1/T2 transfers the constant voltage across D1 to resistor R12, resulting in a constant current flowing through R1 from the battery or batteries under test. Because R1 has low resistance, the current it draws is higher than the total current drawn by the circuit.

Consequently, the quiescent current, which remains reasonably consistent, is minimal. This ensures that the test current remains relatively stable while testing the battery or batteries. T5 sets the maximum battery voltage the tester can handle, which, in this case, is 30 V. To prevent overheating of T1 at high battery voltages, it’s advisable to keep the test duration as short as possible. Additionally, a push-button switch is used as a test switch to ensure that the battery under test is not inadvertently left connected to the load.

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