BATTERY POWERED TESTER

Behind the pressing demands of a friend who works as accepting officer at a service center I have designed a fast battery tester.

You can not imagine how many come with remote controls do not work because of dead batteries, more often than those who arrive and you see replace the batteries to test the remote control says with annoyance that the air has changed recently. The use of simple multimeter to check the voltage is not reliable because ultimately the batteries tend to increase their internal resistance. With absorptions of the order of microamperes (current absorbed by the multimeter) the read voltage is not a reliable test, It must be tried putting it under load, only then the voltage reading from acceptable reports. To avoid the only proven system protests is to take the batteries and test them in front of the customer stating that "so we exclude the hypothesis of a faulty battery", the problem is that for the most, a multimeter is machinery of which do not know the use, much less know how to interpret the numerical report, You need a visual indication easily interpreted even by non-experts. In the past I would have solved with a pointer instrument in which the displacement of the same would be understood by anyone, those who can not interpret the fuel gauge in a dashboard!

Now the pointer instruments are rare, It should be used accordingly an LCD or LED, I opt for the latter because it is easily manageable without using special integrated. Given this is a spontaneous problem, a 1.5V battery fails by itself to turn on the LED, need an additional battery which powers the testing circuit and display. The first circuit that I can think of is to use a LM3914 with inside all already circling, Here an extract of the datasheet

Apart from the extension of the view that interests me between 0,9V fully discharged battery to 1.6V with battery fully charged I was interested in something that could be supplied at a lower voltage and a lower number of LEDs, notwithstanding the extremely valid internal circuitry

I decided to take a cue from definitely tried and tested set up and running as well as simple.

Of course, with appropriate modifications and simplifications to make it always functioning and easily replicable at low cost, Another key element was the total absence of additional battery to avoid the risk of ending up with a dead battery when you need it, view my proverbial ability to forget the tools turned to find them useless then when you actually need. The only acceptable solution for me is to use the same battery to power the test circuit, the booster and visualization constitute the load to be applied to the battery for a reliable test. With such voltages without using special integrated circuit choose the Joule Tieff, Also this circuit widely proven and low-cost, in my case I added some feedback to stabilize the output voltage and make it independent of the load until the absorptions are kept low.

The result was that

In a few components I solved, U1 and U2 are two low-cost LM324 quad operational to drive the 8 led, T1 Q1 and R1 is the JOULE circuit TIEFF base, I added my D1, R2, Q2, D2 e C1.

The operation is easily explained D1 serves several purposes, the key is to avoid that in the absence of the capacitor voltage does not return to the battery Q1 piloting, second function is that in the event of reversal of the power does not reach the integrated, the two transistors are more robust and negative 1.5V bear them, a diode in series to the input would have been possible otherwise would not have more voltage to drive the circuit. With Battery, to about 0,9V if I used a protection diode in the input series would find me after it 0,2-0,3V, too few to be used!

C1 and C2 in addition serve to make the supply voltage stable, C2 could also be optional but I prefer not to risk it and use it in parallel to the power of the integrated.

Q2 , R2 and the zener D2 serve to turn off the transistor Q1 when the voltage rises over the established level, R2 from a stable ground reference for the Q2 base to avoid to leave it floating when the zener is non-conductive, a transistor with a floating base is never a good idea can start conducting with some disturbance at unpredictable times.

Now I have a stable voltage to 5,8V (5,1V of the Zener diode added to 0.7V Q2 of BE junction) to feed the test circuit and display, it first thing feeds R2 via the red LED D3, used as a reference voltage to 1.8V for the rest of the circuit. The resistors R4 to R12 fix the voltage levels of the comparators with levels written in the schema. The resistors R13 to R20 limit the current in the LED while R21 serves to pick up the voltage of the battery for testing.

The LEDs are positioned so that only one LED is lit in the display at a time so as not to absorb excessive current from the battery.

The scale running from 0,9V to get to 1.6V is also useful for nickel-cadmium batteries with a nominal 1.2V. For those interested here it is visible from a print size of about 4,5cm x 4,5cm, more or less the length of a AAA battery

Some bridge has served to limit the size and to avoid using the double-sided printed.

One last thing remained in the construction of the transformer description, I recovered the ferrite ring of an energy-saving lamp and I made a bifilar winding from 20 coils such as to achieve simultaneously the primary and secondary.