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Fast Battery Tester Schematic: 3V-15V Test

Description

This circuit facilitates a rapid battery evaluation without requiring a power supply or expensive moving-coil meters. It incorporates two ranges: when SW1 is positioned as depicted in the circuit diagram, the device assesses batteries within the 3V to 15V range. When SW1 is switched to the alternative position, only cells rated at 1.5V can be tested.

Circuit diagram:

Circuit diagram

Parts:

  • R1 = 2.2K
  • R2 = 3.3K
  • R3 = 10K
  • R4 = 4.7K
  • R5 = 33K
  • R6 = 100K
  • R7 = 100K
  • R8 = 220K
  • R9 = 330K
  • R10 = 500K
  • C1 = 10nf-63V
  • C2 = 10nf-63V
  • C3 = 100nF-63V
  • C4 = 100nF-63V
  • C5 = 100nF-63V
  • C6 = 100nF-63V
  • C7 = 100nF-63V
  • C8 = 220uF-35V
  • Q1 = 2N3819 FET
  • Q2 = BC337 NPN
  • Q3 = BC337 NPN
  • D1 = 5mm Red LED
  • D2 = 1N4148
  • D3 = 1N4148
  • D4 = 1N4148
  • D5 = 1N4148
  • D6 = 1N4148
  • D7 = 5mm Red LED
  • IC1 = 7555
  • IC2 = 7555
  • SW1 = DPDT Switch
  • BUT = Battery Under Test
  • P1 = SPST Pushbutton

Testing 3V to 15V batteries:

  1. Switch SW1 as shown in the circuit diagram.
  2. Place the battery under test in a suitable holder or clip it to the circuit.
  3. Wait a short period to allow C8 to fully charge.
  4. LED D1 illuminates with a constant intensity, independent of battery voltage from 3V to 15V.
  5. If D1 illuminates dimly or is completely off, the battery is unusable.
  6. If D1 illuminates brightly, press P1 and observe LED D7. If D7 remains off, the battery is in a very good condition.
  7. If D7 illuminates brightly for a brief duration, the battery is weak. This condition is confirmed by a noticeable reduction in D1 intensity.
  8. If D7 illuminates dimly for a brief duration but D1 maintains its brightness, the battery is still usable, yet it is not brand new.

Testing 1.5V batteries:

  1. Switch SW1 to the position opposite that shown in the circuit diagram.
  2. Place the battery under test in a suitable holder or clip it to the circuit.
  3. Wait a short period to allow C8 to fully charge.
  4. LED D1 illuminates dimly solely in the presence of a new battery, otherwise it is off.
  5. Press P1 and observe LED D7. If D7 remains off, the battery can be in a very good condition.
  6. If D7 illuminates brightly for a brief duration, the battery is weak.
  7. If D7 illuminates dimly for a brief duration, the battery is still usable, yet it is not brand new.
  8. If you suspect a 1.5V cell to be completely discharged, a better test can be performed by wiring two 1.5V batteries in series, then running the 3V test.

Circuit Operation:

FET Q1 provides a constant current biasing for LED D1 and Q2's base. Consequently, D1 illuminates with a constant intensity, irrespective of battery voltage from 3V to 15V, and Q2 (when P1 is closed) applies a constant current load of approximately 120mA to the battery. IC1 operates as a square wave generator oscillating at around 3KHz. IC2 acts as an inverter and drives, together with IC1 but in anti-phase, Diodes D2-D6 and Capacitors C4-C7, generating 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's Base is biased to maintain LED D7 in the off state. Closing P1 applies a 120mA load current to the battery under test. If the battery is not fully charged, its output voltage begins reducing: when this voltage falls 0.6V below the battery's nominal voltage, Q3's Emitter becomes more negative than the Base, the transistor is hard-biased, and D7 illuminates.

Obviously, this state of affairs will last for 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 below a 120mA loading current, so LED D7 will remain off. When testing 1.5V batteries, the circuit formed by Q1, Q2, D1, and R1 & R2 does not function optimally at this supply voltage, so a 100mA load current is applied to the BUT via R3 (10 Ohm) after switching SW1A. Q3 bias is also modified via SW1B.

Notes:

  1. To set up this circuit, apply a 6 to 7.5V voltage source to the input and adjust R10 until LED D7 is completely off (without pressing P1).
  2. 1.5V test position requires no setup.
  3. CMos 555 ICs must be used for IC1 & IC2 because they are the only inexpensive devices capable of oscillating at 1.5V supply or less.
circuit from http://www.extremecircuits.net/2009/12/self-powered-fast-battery-tester.html