This self powered battery tester circuit runs a fast battery test without the need of power supply or expensive moving-coil voltmeters. It features two ranges: when SW1 is set as shown in the schematic diagram, the device can test 3V to 15V batteries. When SW1 is switched to the other position, only 1.5V cells can be tested.
Testing 3V to 15V batteries:
Testing 1.5V batteries:
FET Q1 provides a constant current generator biasing LED D1 and Q2 Base. In this manner D1 illuminates at a constant intensity, independent of battery voltage from 3 to 15V and Q2 (when P1 is closed) applies a constant current load of about 120mA to the battery. IC1 is a square wave generator oscillating at about 3KHz. IC2 acts as an inverter and drives, together with IC1 but in anti-phase, Diodes D2-D6 and Capacitors C4-C7, obtaining a voltage multiplication. C8 is charged by this raised voltage and R8-R10 form a voltage divider biasing the Base of Q3. When P1 is open, a very light load is applied to the battery under test and Q3 Base is biased in order to maintain LED D7 in the off state.
Closing P1, a 120mA load is applied to the battery under test. If the battery is not fully charged, its output voltage starts reducing: when this voltage falls 0.6V below the battery nominal voltage, Q3 Emitter becomes more negative than the Base, the transistor is hard biased and D7 illuminates. Obviously, this state of affairs will last a few seconds: the time spent by C8 to reduce its initial voltage to the new one, proportional to the voltage of the loaded battery. If the battery under test is in a good charging state, its output voltage will not fall under a 120mA loading current, so LED D7 will stay off.
When testing 1.5V batteries, the circuit formed by Q1, Q2, D1, R1 & R2 does not work well at this supply voltage, so a 150mA load current is applied to the BUT by means of the 10 Ohm resistor R3 after switching SW1A. Q3 bias is also changed via SW1B.
To set-up this self powered battery tester circuit apply a 6 to 7.5V voltage source to the input and trim R10 until LED D7 is completely off (without pushing on P1).
1.5V test position needs no set-up.
CMos 555 ICs must be used for IC1 & IC2 because they are the only cheap devices able to oscillate at 1.5V supply or less.
R1 : 2K2 1/4W Resistor
R2 : 3R3 1/4W Resistor
R3 : 10R 1/4W Resistor
R4 : 4K7 1/4W Resistor
R5 : 33K 1/4W Resistor
R6,R7 : 100K 1/4W Resistors
R8 : 220K 1/4W Resistor
R9 : 330K 1/4W Resistor
R10 : 500K Trimmer Cermet
C1,C2 : 10nF 63V Polyester Capacitors
C3-C7 : 100nF 63V Polyester Capacitors
C8 : 220µF 35V Electrolytic Capacitor
D1,D7 : LEDs Red 5mm. (see Notes)
D2-D6 : 1N4148 75V 150mA Diodes
Q1 : 2N3819 General purpose FET
Q2,Q3 : BC337 45V 800mA NPN Transistors
IC1,IC2 : 7555 or TS555CN CMos Timer ICs
P1 : SPST Pushbutton
SW1 : DPDT Switch
BUT : Battery under test
Holder or clips to connect the Battery under test to the circuit
Source: redcircuits
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