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Fast Battery Tester Circuit Diagram

Tests from 1.5 to 15 Volt cells

A two-LED display is present, requiring no external power supply

Circuit Diagram

Self-powered Fast Battery-Tester-Circuit diagram

Parts:

  • R1 2.2kΩ 1/4W Resistor
  • R2 3.3kΩ 1/4W Resistor
  • R3 10kΩ 1/4W Resistor
  • R4 4.7kΩ 1/4W Resistor
  • R5 33kΩ 1/4W Resistor
  • R6, R7 100kΩ 1/4W Resistors
  • R8 220kΩ 1/4W Resistor
  • R9 330kΩ 1/4W Resistor
  • R10 500kΩ 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

Device Purpose:

This circuit performs a rapid battery test without needing an external power supply or costly moving-coil voltmeters. It provides two ranges: when SW1 is positioned as shown in the circuit diagram, the device tests batteries from 3V to 15V. When SW1 is switched to the other position, only 1.5V cells can be tested.

Testing from 3V to 15V Batteries:

  • Set SW1 as depicted in the circuit diagram.
  • Place the Battery under test within a suitable holder or clip it to the circuit.
  • Allow some seconds for C8 to fully charge.
  • LED D1 illuminates at a constant intensity, independent of battery voltage.
  • If D1 illuminates very weakly or is completely off, the battery is unusable.
  • If D1 illuminates brightly, press P1 and observe LED D7. If D7 remains completely off, the battery is in a good state.
  • If D7 illuminates brightly for a few seconds, the battery is weak. This condition is confirmed by a noticeable decrease in D1 brightness.
  • If D7 illuminates weakly for a few seconds but D1 maintains the same light intensity, the battery is still good but is not new.

Testing 1.5V Batteries:

  1. Switch SW1 to the position opposite to that shown in the circuit diagram.[/p>
  2. Place the battery under test within a suitable holder or clip it to the circuit.
  3. Allow some seconds for C8 to fully charge.
  4. LED D1 illuminates very weakly only in presence of a new battery, otherwise it is off.
  5. Press P1 and observe LED D7. If D7 remains completely off, the battery can be in a good state.
  6. If D7 illuminates brightly for a few seconds, the battery is weak.
  7. If D7 illuminates weakly for a few seconds, the battery is still good but is not new.
  8. If you suspect a 1.5V cell to be fully discharged, a better test can be made 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. Therefore, D1 illuminates at a constant intensity, regardless of battery voltage between 3V and 15V, and Q2 (when P1 is closed) applies a constant current load of approximately 120mA to the battery. IC1 functions as a square wave generator oscillating at around 3kHz. IC2 serves as an inverter and drives, in anti-phase with IC1, Diodes D2-D6 and Capacitors C4-C7, producing a voltage multiplication. C8 is charged by this elevated 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 to decrease; when this voltage falls 0.6V below the nominal battery voltage, Q3’s Emitter becomes more negative than the Base, effectively biasing the transistor, and D7 illuminates. Obviously, this state persists for just a few seconds: the time required for C8 to reduce its initial voltage to the new one, which is proportional to the voltage of the loaded battery. If the battery under test is in a good charging state, its output voltage doesn't fall below 120mA when this load current is applied, so LED D7 remains off. When testing 1.5V batteries, the circuit using Q1, Q2, D1, and R1 & R2 doesn't work well at this supply voltage, so a 150mA load current is applied to the BUT via the 10 Ohm resistor R3 after switching SW1A. Q3’s bias is also altered via SW1B.

Notes:

  • 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 on P1).
  • 1.5V test position needs no set-up.
  • CMOS 555 ICs must be utilized for IC1 & IC2 due to their capability as the only inexpensive devices capable of oscillating at 1.5V supply or less.
author: RED Free Circuit Designs
circuit from http://www.redcircuits.com/