Various types of dry and rechargeable battery (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.
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