Voltage Regulators in NiCd charging circuits

[mishima]

Hi, I have setup the circuit here: http://www.next.gr/circuits/nicd-battery-charger-l36249.html in LTspice, and am trying to understand it. If I actually build it, its purpose will be to charge 19.2 V NiCd batteries.

My question is if the regulated voltage coming from the LM317 output needs to be close to 19.2 V, or if only the regulated current (which should be around 150 mA) actually matters. The reason I would guess affirmative is if I say regulated at 9 V, then when the battery was above that voltage (like if I tried to charge a battery that wasn't completely depleted) a greater current would flow through the voltage divider than what is coming out of the regulator and so none of the charging current would reach my battery. Is that correct?

 

[zoki85]

It appears the minimum voltage from the regulator output depends on the number of NiCd cells in a string. Rated voltage of NiCd is 1.2 V and I would take at least 10% higher value per cell. In a slow charging mode, with I ≈ mAH/10, mAH being capacity of individual cell, you can't damage ("overcharge") the battery anyway.

 

[mishima]

Maybe this is a better way to state my problem: In a normal LM317 setup, there are two resistors that determine the ultimate output voltage of the regulator. In the schematic I linked, it is my understanding that R2 and the equivalent resistance of the transistor network fill this role. In my specific case, I wish to charge a series connection of 16 1.2 V NiCd cells for an equivalent voltage of 19.2 V (or 14.66 via your suggested 10%). The problem is finding a R2/R1 combination that provides sufficient potential difference to light the LED until the point of reaching max charge (19.2, then the LED switches off). Experimenting in spice led me to believe that perhaps the power source DC voltage had the most significant effect on achieving this behavior. I was curious if I was on the right track.

Altering R2/R1 in the real world circuit to be 100/10000 ohms respectively has achieved a good result, but only up to around 15 V.

 

[zoki85]

Datasheet for the adjustable voltage regulator:
https://www.fairchildsemi.com/datasheets/LM/LM317.pdf
[PLAIN]https://www.fairchildsemi.com/datasheets/LM/LM317.pdf[/PLAIN]
Pin 1 is the control pin, I_adj=V_adj/R₂, I_adj< 0.1 mA. In the circuit you propose, it controls voltage of battery string V'_out. Output voltage is higher by voltage drop on R₃, which is part of transistor circuit, V_out= V'_out+I_outR₃. For 16 NiCd 1.2V cells with 10% increase up per cell charging voltage for proper charging should be at least V'_out=16x1.2x1.1=21.2V. Since LM317 has about 3V drop during operation, I would power circuit with at least V_in=25V

 

[mishima]

Thanks for the clarification on that, I had been using a 28 V DC source so I guess I'm ok.

 

[davenn]

You may do well to read this and other information on NiCad charging requirements ...
you cannot treat them like, say, car batteries when it comes to charging

http://www.powerstream.com/NiCd.htm

Dave