Is the battery empty, or is there something wrong with the device? That’s always a difficult question when your walkman or some other battery-powered device appears to be dead when you switch it on. Before you take it to the shop for servicing, the first thing you should do is to test the battery or batteries. Of course, this means you need a reliable battery tester, but it also means you can limit the damage to the cost of a battery or two and a one-time investment of time and money in building a suitable tester.
A battery tester is an electronic device intended for testing the state of an electric battery, going from a simple device for testing the charge actually present in the cells and/or its voltage output. Various types of dry and rechargeable batteries (with an e.m.f. of ≤2.7) can be tested with the circuit shown in the diagram below. It is based on the well-known Type LM 3914 LED display driver from National Semiconductors. The battery tester circuit compares the e.m.f. of the battery with a reference voltage that is derived from an internal source. The reference potential (pin 8) can be set between 1.5 V and 2.7 V by R1-R2-P1.
The voltage at pin 8 refers to the maximum scale value of the LED series. That is, if that voltage is 1.5 V, each LED represents 150 mV. It is recommended to set it to 1.5 V for NiCd batteries. Resistor R1 arranges a current of 12.5 mA for each LED. Uses the adjustable load test as a gauge to decide whether your battery is still usable or needs to be discarded and replaced. These testers measure the voltage drop when a voltage load that is half the battery’s CCA rating is used.
Battery Tester Circuit Diagram:
It is advisable to test dry batteries on load since the terminal voltage depends on the residual capacity. And, of course, the e.m.f. of even an almost flat battery is still close to its specified value. Rechargeable batteries retain their specified e.m.f until they are virtually discharged when it drops fairly rapidly. It is therefore of not much use to check the residual capacity of these batteries on the basis of their e.m.f the test is limited to an indication of whether the battery is fully charged or nearly flat
