The test circuit gives an indication of the capacity remaining in a battery. By noting the time in seconds that the LED remains on after you depress the test switch Sl. The circuit has proven reliable in testing NiCad-, carbon-, and alkaline-type batteries. Closing Sl activates the circuit by applying voltage from the battery under test.
Voltage V, jumps to a value V0 ~ VRR3 /(R2 + R3 ) when the switch closes and then increases with a time constant T ~ C1 (R2 + R3). The divider R4/R5 fixes V2. The reference circuit IC1 sets VR to approximately 2.5 V. The op amp"s output remains high (LED on) until V, rises to the level of V,, when the LED turns off.