Hallo all,

I am a little bit unhappy about this discussion. Is it a must that discussing such a topic ends in offendings and insultings? That the one is right, having the highest ranking or who is typing the comments in biggest lines?

Where are the real arguments? If someone actually knows that Charles' circuit is dangerous, so please explain in detail, instead of using phrases like 'looking an ass', etc. If we always tell that a topic has to be handled by a professional we do not need such a forum. One line then would be enough: 'You are an idiot, call a professional, anyway what problem you have.'

To me it seems that Ayman is less experienced. Or would an experienced person post such a question? He needs help, and we should give him help. In small lines, whithout offending repeatings...

I want to tell now, why I think that this circuit is dangerous. And perhaps we can have a discussion now, which is adequate for professionals?

NiCd must be charged with constant current. Quasi constant current can be fabricated by having a constant voltage, slightly higher than battery voltage at full charge condition and having some resistance for current limiting.
Such small transformer is not a constant voltage generator, at all! A 9V 100mA rating transformer has open cirucit voltage of about 12...13V effective. This give about 18V maximum voltage. Even if there are current limiting resisitors, this voltage is just to high, for my believe. What guarantees, that charging current is limited to secure level, when NiCd battery is fully charged? Nothing. May be that with certain transformer, a stiffy one like it is used for halogen lamps there would be a limiting to a secure level. But then a certain transformer type has to be recommended, too.
In the link given, there is an interesting advice. User should feel temperature, and when is getting hot, he should cut the line. But this is not possible in Ayman's application! He will have a built-in battery and a connector to it's 'wall wart' power supply. So, it's possible that he has power supply connected many days, and will not cut the line.
The consequence of overloading is, beside heat dissipation, the produce of H2 gas. If battery has a safety opening, PROBABLY nothing bad will happen. But he will suffer a big loss of capacity. After some cycles battery is damaged and has to be exchanged. If battery has no safety opening H2 production inside would cause an extremely increase of pressure. At a certain time NiCd battery can really explode. This has happened in the past, this happens today and this will happen in future. Many people have lost their eyes and to tell that nothing wrong can happen when overcharging is just criminal. Especially for a professional...

But even when cirucit is well designed, so not leading to dangerous overload, it will not work! Why?

A NiCd battery has to be charged with a current of C/100...C/10. C/10 is only allowed for 1000h. Exceeding this limit will cause loss of capacity. NiCd batteries used in older motherboards for retaining data of BIOS used a charging current of about C/50. If charging current is less than C/100 then battery cannot be charged fully, and also there will be a loss of capactiy after some 'cycles'.
So, the acceptable window for charging current is rather small, when NiCd batteries shall be handled adequately. And for me it's very unprobable that the recommended circuit would fullfill this requirement.

But even if it would, it's very probably that the circuit will not work. Assuming that charging cirucit is designed correctly, than charging current is <=C/10. A normal 9V NiCd battery, consisting of 7 cells à 1.2V, has a capacitiy of about 110...120mAh. It depends a little bit on manufacturer. So, maximum charging current is about <=11mA. Such current would allow that charging circuit is connected permanently to battery, although I would choose a little bit smaller charging current.
When now Ayman switch-on his interface circuit a supply current will flow to his circuit. Now, two cases have to be distinguished:

1. Supply current is smaller than 10mA. Then NiCd battery is charged with 11mA minus supply current. This condition is ok. But supply current must not rise above 10mA, because then only 1mA is left for charging battery. Remember, minimum charging current is C/100=1mA.

2. Supply current is bigger than 11mA. Then voltage at battery will decrease and finally reach a level which does NOT represent voltage of fully charged battery. Means, it's not possible to charge NiCd fully. Depending on circuit it's possible that NiCd even discharges totally!

Even if first condition is fullfilled, than at certain duty cycles (charging, discharging) it's very probable that NiCd battery cannot be charged fully, but looses all it's charge over aperiod of some days. So, supply current must be much smaller than 10mA in reality, when application will satisfy.

So, Ayman, who needs help would have to be told, that this circuit can only work, when supply current is much less than 10mA.

So, such a circuit has very carefully to be designed. It's a task for a well experienced person, but not for guy like Ayman, who from it's posting has already showed, that he has not enough experience. He will end either with a circuit which is drastically overcharging his battery or a circuit which results in a completely discharged battery after some days. It's very, very unprobable that he will end with a satisfying circuit.

Bye,
Kai