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Description:
Battery level indicators generally detect for the voltage levels of the batteries to give a result. So, tester circuits must not be a heavy load during the measurement process. This tester circuit draws very low current. A short duration LED bright will show you battery has still enough voltaga level to operate devices. This light occurs after decharging of C1 on D1 LED, this happens only when the battery provides enough voltage.
When you close the S1 switch, Tr1 transistor makes C1 to discharge through R3 current limiter. Minimum required battery voltage level can be determined by using voltage divider R1/R2. Values of R2 and R3 must be calculated as shown below;
R2 = (0.6 x R1) / (Vbmin - 0.6) ohm and R3 = (Vb - 1.4) / 0.2 ohm
For example, for 6.5Vbmin value (to test a 9V battery) R2 = 10k and R3 = 39 ohm. R4 must be between 10k to 1M values.
For higher values of R4, circuit becomes more economic but this causes lenghten the test period. When R4 is 100k, battery can be tested after about 10 seconds.
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