AAA Battery charger

[cinic]

Hello! I tried to build this aaa battery charger circuit. Can someone explain the purpose of:
-D7 and D8 diodes
-C3 capacitor
And how the charging of the batteries is actually stopped when they are charged? I think that due to the negative voltage in the emitor of T1 when they are charged, T1 opens, but I am not sure about this. Thank you!
*Sorry for bad english

 

[Audioguru]

Are rechargeable alkaline batteries made anymore? I saw them 30 years ago and they were almost useless.
Ni-Cad rechargeable batteries were also used about 30 years ago. Today Ni-MH and Lithium rechargeable batteries are used and a charger that charges, detects a full charge then then shuts off properly.

This circuit does not detect a full charge, instead it detects low charging current through R6. The 1.6V is much too high for a modern Ni-MH battery and nothing limits the current so the charging current will always be too high which will destroy the battery.

 

[cinic]

I can easily lower the voltage. How do I limit the current though? And what is the optimum current for Ni-Mh (i am a beginner, obviously). It's for a school project, it doesn't have to work perfectly. Just to generate the necessary voltage and current and to stop when I apply 1.5 volts or so, simulating the charged batteries

 

[cinic]

Anyway, the purpose of C3, D7 and D8?

 

[Audioguru]

I can easily lower the voltage. How do I limit the current though? And what is the optimum current for Ni-Mh (i am a beginner, obviously). It's for a school project, it doesn't have to work perfectly. Just to generate the necessary voltage and current and to stop when I apply 1.5 volts or so, simulating the charged batteries

You found a very old circuit made to charge ancient obsolete battery cells. It has nothing to adjust the charging voltage and current. Since the LM317 and resistors have minimum and maximum values then this charger circuit could have a low voltage of 1.3V at low current of a high voltage of 1.8V at a high current.

Energizer battery company has Technical Info at the bottom of their online home page and when you enter Ni-MH it gives a list of all their datasheets for Ni-MH cells and a Applications Manual that tells you all about the sophisticated charger features that are in a modern Ni-MH battery charger integrated circuit that they recommend. It does not simply apply voltage and current, it actually detects when the cell is fully charged. A stupid charger circuit will not detect that a cell already has some charge in it so it will overcharge it. The smart integrated circuit charger detects a charge and detects a full charge so it never overcharges. It can charge slowly at a low current or make a fast charge with a high current. A stupid simple circuit cant doo dat.

 

[cinic]

I wasn't allowed to use integrated circuits so i tried a simple stupid circuit. I will not use it for battery charging, i just want to understand what is happening there since the teacher was somehow pleased with this one. The purpose is for us to understand circuits and how they work, and the role of every single component. I do not understand what those three do.

 

[Audioguru]

R9 crudely limits the maximum charging current. D7 and D8 limit the voltage across R9 which causes even higher maximum current which is wrong. D7 and D8 should be removed then the maximum current is limited by R9.

C3 prevents LED2 from flickering on and off when the battery is near a full voltage.

 

[cinic]

Thank you very much!