Express Battery Tester without Power Supply by 555 Timer

Part List

The circuit was designed without the use of a power supply or a moving coil voltmeter to create a quick battery tester which can test 1.5 V to 15 V battery cells with a two-LED display.

• BC337 – a small signal NPN Silicon AF medium power transistor used for general purpose switching and amplifying applications with features such as TO-18 manufactured package, suited for AF driver stages and low power output stages, and divided into three group types
• 2N3819 – an all purpose JFET which offers good performance at mid to high frequencies and a low cost type; has low noise and leakage, very high system sensitivity, high speed switching capability, wideband high gain at 100 MHz, high low-level signal amplification and high quality of amplification
• TS555 – a single CMOS timer which offers very low power consumption and high frequency where the timing remains very accurate in monostable or astable mode while providing reduced supply current spikes during output transitions, which enables the use of lower decoupling capacitors and with added features of high output current capability, high input impedance, wide voltage range, and output compatible with TTL, CMOS & Logic MOS

The position of witch S1 determines the two voltage ranges that the circuit can test. One position can test 3V to 15 V while the other position can test 1.5V only. During the measurement of 1.5V batteries, C8 is allowed to charge fully after a few seconds. The device is turned OFF initially and in the presence of a new battery, the LED D1 will illuminate very weakly. The life of the battery can be revealed by pressing the SPST push button P1 while keeping an eye on LED D7. The battery is in good state if the indication in D7 remains fully OFF. A weak battery is indicated with a bright illumination of D7 for a few seconds while a weak illumination of D7 will indicate that the battery is still in good condition but not new.

Same thing happens with the testing of 3V to 15V batteries where C8 is fully charged. During this time, the LED D1 is illuminating at a constant intensity that is separated from the battery voltage. The battery is said to be unusable if D1 illuminates very weakly. The same thing happens when P1 is pressed and D7 provides all the indications.

The base of Q2 and LED D1 is provided with a constant current generator biasing by FET Q1. A constant intensity is being illuminated by D1 at this stage while a constant current load of 120mA is applied to the battery by Q2. A square wave oscillation is being generated by IC1 with a frequency of 3KHz while an inverter and driver is performed by IC1. The amplification of voltage is done by D2-6 and C4-7. This causes C8 to charge while the base of Q3 is biased by a voltage divider formed by R8-10.

The fast battery tester circuits can be seen widely in car fleets, paint & body workshops, automotive workshops, used car dealers, and roadside assistance for the test of charge state, battery diagnosis, starter system state, and charging system state. They are also found in all forms of instrumentation, backup power systems, alarm systems, alternative energy, controls, and small electric vehicles. Some designs are easy to use for measuring the battery performance and flexible since it can be programmed for different types of batteries.

Source:redcircuits.com/Page43.htm