wiring and diagram

With a suitable load, the terminal voltage of a NiCd or lithium-ion battery is proportional to the amount of stored energy. This relationship, which is linear over a wide range, can be used to build a simple battery capacity meter.

 

Voltage Tester for Model Batteries Circuit Image 

This model battery tester has two functions: it provides a load for the battery, and at the same time it measures the terminal voltage. In addition, both functions can be switched on or off via a model remote-control receiver, to avoid draining the battery when it is not necessary to make a measurement. The load network, which consists of a BC517 Darlington transistor (T2) and load resistor R11 (15 Ω /5 W), is readily evident. When the load is active, the base of T1 lies practically at ground level. Consequently, T1 conducts and allows one of the LEDs to be illuminated.

Voltage Tester for Model Batteries Circuit Diagram

The thoroughly familiar voltmeter circuit, which is based on the LM3914 LED driver, determines which LED is lit. The values of R6 and R7 depend on the type and number of cells in the battery. The objective here is not to measure the entire voltage range from 0 V, but rather to display the portion of the range between the fully charged voltage and the fully discharged voltage. Since a total of ten LEDs are used, the display is very precise. For a NiCd battery with four cells, the scale runs from 4.8 V to 5.5 V when R6 = R7 = 2 kΩ. The measurement scale for a lithium-ion battery with two cells ranges from 7.2 V to 8.0 V if R6 = 2 kΩ and R7 = 1 kΩ.

For remote-control operation, both jumpers should be placed in the upper position (between pin 1 and the middle pin). In this configuration, either a positive or negative signal edge will start the measurement process. A positive edge triggers IC1a, whose output goes High and triggers IC1b. A negative edge has no effect on IC1a, but it triggers IC1b directly. In any case, the load will be activated for the duration of the pulse from monostable IC1b. Use P12 to set the pulse width of IC1a to an adequate value, taking care that it is shorter than the pulse width of IC1b.

If the voltage tester is fitted into a remote-controlled model, you can replace the jumpers with simple wire bridges. However, if you want to use it for other purposes, such as measuring the amount of charge left in a video camera battery, it is recommended to connect double-throw push-button switches in place of JP1 and JP2. The normally closed contact corresponds to the upper jumper position,while the normally open contact corresponds to the lower position.

Voltage Tester for Model Batteries PCB Layout

This cable tester allows you to quickly check audio cables for broken wires. Because of the low power supply voltage, batteries can be used which makes the circuit portable, and therefore can be used on location.

The design is very simple and well organ-ised: using the rotary switch, you select which conductor in the cable to test. The corresponding LED will light up as indication of the selected conductor. This is also an indication that the power supply volt-age is present. If there is a break in the cable, or a loose connection, a second LED will light up, corresponding to the selected conductor. You can also see immediately if there is an internal short circuit when other than the corresponding LEDs light up as well.

You can also test adapter and splitter cables because of the presence of the different connectors.

Two standard AA- or AAA- batteries are sufficient for the power supply. It is recommended to use good, low-current type LEDs. It is also a good idea not to use the cheapest brand of connectors, otherwise there can be doubt as to the location of the fault. Is it the cable or the connector.

This circuit uses the popular and easy to find LM3914 IC. This IC is very simple to drive, needs no voltage regulators (it has a built in voltage regulator) and can be powered from almost every source. This circuit is very easy to explain: When the test button is pressed, the Car battery voltage is feed into a high impedance voltage divider. His purpose is to divide 12V to 1,25V (or lower values to lower values).

This solution is better than letting the internal voltage regulator set the 12V sample voltage to be feed into the internal voltage divider simply because it cannot regulate 12V when the voltage drops lower (linear regulators only step down). Simply wiring with no adjust, the regulator provides stable 1,25V which is fed into the precision internal resistor cascade to generate sample voltages for the internal comparators. Anyway the default setting let you to measure voltages between 8 and 12V but you can measure even from 0V to 12V setting the offset trimmer to 0 (but i think that under 9 volt your car would not start).


There is a smoothing capacitor (4700uF 16V) it is used to adsorb EMF noise produced from the ignition coil if you are measuring the battery during the engine working. Diesel engines would not need it, but Im not sure. If you like more a point graph rather than a bar graph simply disconnect pin 9 on the IC (MODE) from power. The calculations are simple (default)
For the first comparator the voltage is : 0,833 V corresponding to 8 V
* * * * * voltage is : 0,875 V corresponding to 8,4 V
for the last comparator the voltage is : 1,25 V corresponding to 12 V
Have fun, learn and dont let you car battery discharge... ;-)

author: Jonathan Filippi
e-mail: jonathan.filippi@virgilio.it