Charging Nicad batteries is done many times by just using a serial resistor.
It is by far not the ideal way of doing it, and it makes the life of such a battery seriously shorter.
Using current regulators is one option, but also then, when charging is done, the charger puts the battery to a much too high voltage.
So you need an alarm clock with it, which tells you when to take the battery from the charger.
But for many years already , good systems , packed in simple Integrated Circuits, are available to do the charging the right way.
One of these IC's is the Telefunken TEMIC U2403B.
This is article is mostly based on the Datasheet, available from the manifacturer.
The printed circuit, presented here, is mainly ment for charging the battery of a Faller Car.
Every charger for Nicad's is best to be adapted to the used battery.
With the choice of three resistors and one capacitor, the charge time and current in this charger can be chosen over a wide range.
The best option for charging a nicad is charging it with 1/10C this 1/10 of the capacity.
So a Nicad of 500 mAH is best to be charged with 50 mA.
But that will take at least 10 hours! Mostly we are not that patient.
In practice, modern batteries have no problem with a charge time of 2 to 4 hours.
This means that a 500 mAH nicad charging in 2 hours , will need 250mA.
This is just an ideal figure. In practice with that current it will take a bit longer.
For that purpose, this charger has a 'trickle' option.
The charger itself will stop after the set charge time, and then this trickle charge will complete it.
This also means that leaving the battery in the charger is not a problem.
For timing and charge current, please refer to Table 1 (current) and Table 2 (time), or the Datasheet of U2403B.
The user manual is simple: after assembling the unit with the right resistors and C4, just plug in the power to J1 and connect the battery to J2.
The red Led will show that charging has started, and after charging, the green Led will indicate that trickle charge has started.
JP1 is to check the charge time.
Putting the jumper on, the charge time will be shortened with a factor 512.
This means that a choice of charge time of 4 hours, will make it 28 seconds with jumper Jp1 set.
That is the easy way to check if timing is correct.
The necessary voltage to power for charging Faller Cars should be minumum of 9,5 Volt dc or 8V ac, due to some voltage loss in the rectifier.
Higher voltage (up to 16V ac) is not a problem. The IC U2403B has internal protection for overload and overheat situations.
After writing this article and testing the unit, it approved that the U2403B is very hard to find.
I ordered a few some years ago at Conrad under #144819, but that is no longer available.
The prototype is doing exactly what is is supposed to.
|Selection table Charge Current||240 mA||150mA||100 mA||50 mA|
|Selection table Charging time||2 uur||4 uur||6 uur||7 uur||12 uur|
|2||R3,R5||See Table 1|
|1||R4||See Table 2|
|2||C1,C5||Elco 100 uF 25V rad|
|1||C4||See Table 2|
|1||Led1||Led 3mm green|
|1||Led2||Led 3mm red|
|1||J2||header 3 pins|