Voltage Regulators in NiCd charging circuits

  • Thread starter mishima
  • Start date
  • #1
543
33
Hi, I have setup the circuit here: http://www.next.gr/circuits/nicd-battery-charger-l36249.html [Broken] 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?
 
Last edited by a moderator:

Answers and Replies

  • #2
1,187
224
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.
 
  • #3
543
33
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.
 
  • #4
1,187
224
Datasheet for the adjustable voltage regulator:
https://www.fairchildsemi.com/datasheets/LM/LM317.pdf [Broken]
[PLAIN]https://www.fairchildsemi.com/datasheets/LM/LM317.pdf[/PLAIN] [Broken]
Pin 1 is the control pin, Iadj=Vadj/R2, Iadj< 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, Vout= V'out+IoutR3. 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 Vin=25V
 
Last edited by a moderator:
  • #5
543
33
Thanks for the clarification on that, I had been using a 28 V DC source so I guess I'm ok.
 
  • #6
davenn
Science Advisor
Gold Member
9,545
8,569
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
 

Related Threads on Voltage Regulators in NiCd charging circuits

  • Last Post
Replies
3
Views
2K
  • Last Post
Replies
15
Views
4K
Replies
6
Views
180
  • Last Post
Replies
5
Views
2K
Replies
11
Views
3K
Replies
4
Views
736
Replies
5
Views
2K
Replies
6
Views
1K
  • Last Post
Replies
1
Views
1K
  • Last Post
Replies
11
Views
10K
Top