Is the power supply depleted, or is there an issue with the device? That’s often a puzzling question when a portable device, such as a music player or another power-driven apparatus, appears unresponsive upon activation. Prior to taking it to a repair shop for servicing, the initial step should be to evaluate the power source or sources. Of course, this necessitates a dependable power measurement tool, but it also allows for minimizing potential expenses related to replacement batteries and a singular investment of time and funds in constructing a suitable measuring device.
Many commercial power testers consist solely of a resistor, a basic meter, and a toggle switch. Some manufacturers incorporate an even simpler tester consisting of a series of batteries, featuring a plastic strip with a layer of electrically conductive material that exhibits a noticeable color change as electrical current flows through it. When this strip is placed over the battery between the positive and negative terminals, a fully charged battery will produce a more pronounced color shift compared to a partially discharged one.
Naturally, assessments of this nature typically don’t furnish highly reliable or precise outcomes. The underlying principle behind the circuit detailed here is to apply a steady current to a single battery, a collection of batteries connected in series, a rechargeable battery, or even a small button cell, employing a separate multimeter or voltmeter module (M1) to monitor the voltage. A rapidly decreasing voltage suggests that the battery or batteries require immediate replacement. If a current-limiting circuit is utilized for the load, the current cannot be excessively large, rendering adjustments for the number of cells unnecessary.
The current-limiting circuit is specifically designed to function with a voltage as low as 0.9 V. It’s generally difficult to fabricate a circuit that operates reliably at even lower voltages utilizing conventional transistors. The key constant-current element is transistor T1. The electrical flow through it is maintained constant by comparing the voltage across resistor R1 in its collector circuit with a stable reference voltage across diode D1. This comparison is provided by differential amplifier T3/T4.
The voltage across diode D1 (a Schottky type) remains reasonably stable due to its inherent properties, and it is further stabilized by employing FET T5 as a simple constant-current sink. T5 also limits the current at higher voltages (when multiple batteries are connected in series). The constant voltage across D1 is transferred to resistor R12 by differential amplifier T1/T2, resulting in a consistent current flow through R1 from the battery or batteries under test. R1 possesses a relatively low resistance, so this current is considerably larger than the current drawn by the rest of the circuit.
The static current, which incidentally is also reasonably constant, is negligible. The test current thus remains consistently stable while the battery or batteries is/are being assessed. The maximum battery voltage that the tester can handle is determined by T5, and it is set to 30 V here. To prevent transistor T1 from overheating at high battery voltages, keep the test duration as brief as possible. Utilize a push-button switch as a test switch so that the battery being tested cannot remain under load unintentionally.