Is the battery dead or is there a problem with the device? When your walkman or other battery-powered gadget looks to be dead when you turn it on, that’s always a difficult issue to answer. The first thing you should do is test the battery or batteries before taking it to the shop for service. Of course, this necessitates the use of a dependable battery tester, but it also implies that the damage can be limited to the expense of a battery or two, as well as a one-time investment of time and money in developing a proper tester.
A resistor, a modest tiny meter, and a push-button are all that many commercial battery tests include. With a pack of batteries, some manufacturers offer an even simpler tester, which consists of a strip of plastic with a coating of electrically conductive substance that changes color as a current passes through it. A fully charged battery will produce a more vivid color shift than a partially depleted battery when this strip is placed between the positive and negative terminals.
The constant-current circuit was created with a voltage as low as 0.9 V in mind. With regular transistors, it’s extremely difficult to make a circuit operate at even lower voltages. Transistor T1 is the active constant-current element. By comparing the voltage across resistor R1 in the collector circuit with a reasonably constant reference voltage across diode D1, the current through it is kept constant. T3/T4 differential amplifier provides this contrast.
The voltage across Schottky diode D1 is rather stable by nature, but it may be stabilized by employing FET T5 as a simple constant-current sink. At somewhat high voltages, T5 also restricts the current (with several batteries in series). 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 being tested. Because R1 has a low resistance, this current is more than the total current drawn by the circuit.
As a result, the quiescent current, which is also a reasonable constant, is minimal. As a result, the test current remains rather consistent while the battery(s) is/are being tested. T5 determines the highest battery voltage that the tester can withstand, which is 30 V in this case. Keep the test as brief as possible to avoid T1 becoming too heated at high battery voltages. As a test switch, use a push-button switch to ensure that the battery being tested is not accidentally left under load.