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Ni Cad chargers help

Posted: Sat Jun 28, 2003 11:16 am
by Larry Woods
My friend wants to build an 8.7 Volt 1.4 amp charger for a ni cad battery. I said I would help him but I only know a little more than him which is "just a little" We talked about using a LM 317 T chip and circuit that Russ K. suggested for voltage regulation. Are we on the right track or should we do something different. This is a learning project so any ideas are appreciated.We see how to control the voltage but how is the steady current maintained??(assuming the 317 is the approach.)<p>Thanks

Re: Ni Cad chargers help

Posted: Sat Jun 28, 2003 2:00 pm
by Rodney
If you are relatively new to using NiCads, check out www.rcbatteryclinic.com for some good info on the feeding and care of all types of batteries. A very good site.

Re: Ni Cad chargers help

Posted: Sat Jun 28, 2003 2:38 pm
by bridgen
Larry, you mention a circuit for voltage regulation. <p>The generally accepted wisdom for charging nicads is that it should be done form a constant current source not constant voltage. <p>Reaf more about them before you proceed.

Re: Ni Cad chargers help

Posted: Sat Jun 28, 2003 4:45 pm
by skrallman
What about using one of the charge controller ic's?
The max712/713 are one example. They'll send you a free sample and there are several examples out on the internet of chargers people have made that can help guide you.
I've been working on a 13.2V charger.....haven't got it working yet.
Scott

Re: Ni Cad chargers help

Posted: Sat Jun 28, 2003 7:14 pm
by k7elp60
Larry,
I believe I can help you. First I need to know the following information:
A. Are the batterys the fast charge type?
B. What is the ampere hour rating on the cells?
C. The number of cells in series.
You said 8.7 volts, that doesn't really say how many cells. NI-CAD batteries are rated at 1.2 volts per cell. 6 cells would be 7.2 volts. 7 cells would be 8.4 volts.
The recommended charge scheme is to charge the cells with a constant current. This is easily accomplished with a 3 terminal regulator.
Awaiting your reply
Ned :)

Re: Ni Cad chargers help

Posted: Sat Jun 28, 2003 9:12 pm
by Larry Woods
From yours and the earlier post replys I have been educated a lot. On closer inspection, the battery pack is a Ni MH with a 2000 mah rating and is a 7.2 volt battery( 6 cells). It can be charged quickly with a 8.7 volt/ 1.4 amp charger output. I realize now that the charging methods for different batteries are quite different. <p>I find this information absolutely fascinating and can't wait to start building.<p>Thanks<p>Larry

Re: Ni Cad chargers help

Posted: Sun Jun 29, 2003 10:04 am
by k7elp60
Larry,
If you want to fast charge the pack I would recommend you follow the link that Scott provided,as when they are fast charged, the temperature of the cells is important and that the charge voltage has to be removed when they are
fully charged.
If you want to charge them in about 10 hours and not have to worry about removing the charger when they are fully charged, I can provide you with a circuit diagram and parts list. You can use a LM317T, 3 terminal regulator.
Please let me know your preference.
Ned :)

Re: Ni Cad chargers help

Posted: Sun Jun 29, 2003 5:54 pm
by Larry Woods
The advice Scott gave is excellent and am reading about it. Fast charging isn't critical to me and the direction you suggested might me easier for me. If you could give me that information I'd like to try starting that way.<p>Thanks,<p>Larry

Re: Ni Cad chargers help

Posted: Mon Jun 30, 2003 9:00 am
by k7elp60
I figure you will need 14 volts DC at 200MA to charge the 7.2 volt battery pack. First off the cells reach very close to 1.5 volts when fully charged.
By connecting a 6.8 ohm resistor between the Vout and Adj. terminals of the 317T the current will be limited to a little less than 190MA. Adding a diode to prevent the cells from discharging in case the AC power goes off, the circuit is:
The + of the 14 volt source is connected to the Vin terminal of the 317T. The 6.8 ohm 1/2 watt resistor is connected between the Vout terminals and the Adj terminals. At the junction of the Adj terminal and the resistor connect the anode of a 1N4000 series diodes. The cathode of the diode go to the + terminal of the battery pack. The - terminal of the dc source is connected to the - of the battery. I would use a small heat sink on the regulator, and a wall wart that supplies 200MA or more at 14 volts DC or more as the DC source.
With this charger the battery will never overcharge. Manufacturers recommend for normal charging use capacity of the cells/10. I have found thru practice to use capacity of the cells times .095, then use the next standard higher value of resistor with the LM317T.
Ned :)

Re: Ni Cad chargers help

Posted: Mon Jun 30, 2003 1:40 pm
by Larry Woods
Thanks,<p>I'm off to get the parts I don't have presently. I'll build the unit you suggested and as I get the parts Scott suggested Ill try a quick charge version.<p>Thanks to all and I'll let you know how I make out.<p>Regards,<p>Larry

Re: Ni Cad chargers help

Posted: Thu Jul 10, 2003 9:15 pm
by Larry Woods
I have the parts for the slow charge project except the power supply is 16 volts /300. This was the closest I could get at the store. Is this acceptable. Is it like resistors where you get close?? <p>Do I need to to changes. How do you know what tolerences are acceptable???<p>
Thanks<p>Larry

Re: Ni Cad chargers help

Posted: Fri Jul 11, 2003 12:04 pm
by desterline
Larry, first let me aplaud your 'do it yourself' spirit, it's the thing that makes america great.<p>But let me ask, is there a specific reason your building this one? Is there some feature you need that you can't get elsewhere? <p>The reason I ask, the specs you gave for the battery is the same as standard sub-c size packs in RC cars, there are litteraly hundreds of comercial chargers that are designed for this application. I personaly have about 10 of them varying from 14 hour wall warts that I paid less than a dollar for, to a full blown computer conroled one that retails over $200. <p>Learning is great, and I've certainly built thing I could have bought cheaper, but # 23-333 from the rat shack catalog retails at $10.99 and will do what you need.<p>
Good Luck
Denny

Re: Ni Cad chargers help

Posted: Tue Jul 15, 2003 8:21 pm
by Larry Woods
Thanks Denny,<p>You're right, reinventing the wheel isn't the plan but originally I couldn't find a charger with the specs I thought I needed. <p>Since I've learned a lot, even how tought it is to package the items in a decent finished state.<p>I will look up the completed package, but is rat shack Radio Shack???<p>Thanks all

Re: Ni Cad chargers help

Posted: Tue Jul 15, 2003 10:03 pm
by desterline
Yeah Radio shack, I thought that was a fairly common slang term, but...<p>
Let's not forget the slogan:
You've got questions? We've got blank stares.<p>Denny<p>BTW; I'm not neccesarily endorsing thier charger, I've never tried it, I just pointed it out to show you how common they were. If I was to buy one, I'd probably look to ebay or maybe Tower Hobbies.

Re: Ni Cad chargers help

Posted: Wed Jul 16, 2003 6:22 am
by k7elp60
Larry,
The 16V/300Ma power supply will work fine. No other changes are necessary. The extra 2 volts
will be dropped across the LM317T regulator.
Ned :)