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could it be this simple? - 2/12/2004 5:43:17 PM   
ZAGNUT



Posts: 2436
Joined: 2/26/2003
From: tel-aviv, ISRAEL
Status: offline 		been looking for a fast field charger and came up with this schematic. i suppose i could modify it a bit and make three of them, one each for 4, 5,and 8 cells.

any reasons not to use this $1 circuit?


dave

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RE: could it be this simple? - 2/13/2004 12:51:25 AM   
Geistware



Posts: 12946
Joined: 4/9/2002
From: Locust Grove, GA, USA
Status: offline 		If I read this correctly this circuit will put 4.6 to 4.8 to the pack. You need a higher voltage to charge the pack, I would remove both diodes to time charge and remove one diode to make it an indefinate charger.

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RE: could it be this simple? - 2/13/2004 1:28:19 AM   
mr.rc-cam



Posts: 536
Joined: 6/6/2002
From: West Coast, CA, USA
Status: offline 		The two diodes will raise the output voltage of the 6V-Vreg to a little over 7VDC. If you use this circuit, I suggest you introduce some series resistance with the output to ensure that you limit the end of charge current to no more than C/10.

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RE: could it be this simple? - 2/13/2004 1:44:40 AM   
Flying Geezer



Posts: 1049
Joined: 4/12/2003
From: Bowling Green, KY, USA
Status: offline 		This circuit will function, however:

A. The part is obsolete, but you may be able to find one.

B. It is a constant voltage, not constant current circuit. Nicads and NiMh cells need constant current.

C. Heat is going to be a REAL problem due to regulation from 12 + volts to about 6 volts.

D. This IC is protected by thermal shutdown. and you'll have to engineer the current draw and heat sinking (lots of sinking) to prevent shutdown.

E. It'll cost a lot more than a dollar by the time you get into heat sinking and an enclosure.

F. Unless you are using small packs it won't be a fast charger.

Having said all this. I recommend that you play with this circuit a little, if you are interested in electronics. Worse case, you might burn your finger on the heat sink.

Don't mean to discourage you, if you are interested in electronics, you'll have fifty dollars worth of fun from it. Just go into it not expecting a practical charge system to materialize. Keep us posted, I'm interested. This could lead you to more ambitious projects.

The Geezer

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RE: could it be this simple? - 2/13/2004 5:18:58 PM   
Rodney



Posts: 4774
Joined: 12/8/2001
From: FL
Status: offline 		I'll have to agree with Flying Geezer. This circuit has to many possible problems to be reliable and any failure mode will likely cost you a battery or even a fire.

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RE: could it be this simple? - 2/13/2004 8:58:12 PM   
ZAGNUT



Posts: 2436
Joined: 2/26/2003
From: tel-aviv, ISRAEL
Status: offline 		thanks guys, i've just read up a little bit on charging nicds and nimhs and current limiting does look like a better method. i've also read that an 8 cell wall-wart charger can charge a 4 cell pack no problem, the charge current would just double.

does all this mean that the pack doesn't care about how many volts you give it? does it just draw the voltage it needs for charging? a bit confusing to me. if this is all true then couldn't i just charge directly from my 12 volt lead-acid by using an lm317 in current limiting mode? it would be just as simple as the other circuit.

thanks,
dave

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RE: could it be this simple? - 2/17/2004 1:09:26 PM   
ZAGNUT



Posts: 2436
Joined: 2/26/2003
From: tel-aviv, ISRAEL
Status: offline 		c'mon guys! please give me an answer to my last post before i start frying my battery packs

and yes, i am interested in DIY electronics! right now i'm in the middle of building a pair of MICROSTAR2000 transmitters that require a lot of SMD work and PIC burning from my PC so i'm up to anything you guys throw at me as long as it's SIMPLE...


dave

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RE: could it be this simple? - 2/17/2004 1:57:37 PM   
Flying Geezer



Posts: 1049
Joined: 4/12/2003
From: Bowling Green, KY, USA
Status: offline 		Hi Dave,

First the wall wart. The lower resistance of the 4 cell pack will probably draw twice the current from the wall wart. The Battery pack will pull the voltage on a small OEM wall wart down to an acceptable level, then proceed turn burn out the wall wart.

Charging from the 12 volt battery? A battery pack can be damaged by too high a voltage. It can cause damage to the seperaters I think. There are those who know more about this than me. If you want to use a 12 volt battery to charge a 4 cell pack through an lm317 you will want to drop the voltage to 6.5 - 7 v. By maintaining a constant voltage at a relatively low current, your current will be somewhat constant, varying from a slightly higher current in the depleted state, to a lower current in the fully charged state. It will work pretty much like the wall wart does unless you get into a more complicated circuit. You will want to use the LM317t-ADJ. There are many circuits diagrams available on the National semiconducter website. I think it's www.national.com. . Personally, I think you are better off with the selection of a good wall wart. Home made chargers at currents higher than 1/10 of the battery packs capacity are an invitation to battery pack destruction, unless you have a peak sensing cut off circuit. This gets substantually more complicated. At the National website do a product search under battery charging. They have peak sensing chips with circuit diagrams. They sense on negative peak, but are quite practical.

Hope this helps.

The Geezer

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RE: could it be this simple? - 2/17/2004 10:51:01 PM   
Bax


 

Posts: 12451
Joined: 4/26/2002
From: Champaign, IL, USA
Status: offline 		To clarify a point....batteries (or other devices) don't "draw" voltage. It's the amount of current flowing through the wires that does the work, whether charging cells, or operating equipment. Voltage is just the "pressure" driving the current.

When you charge a battery pack, its voltage "pushes back" against the charger. the greater the voltage differential between the charger and the battery, the more current that will flow. You limit the voltage of the charger so that you can get a certain amount of current to flow into the battery pack. It's current, not voltage you're worried about. Large-capacity battery packs can charge at higher current rates, so the voltage differential will be more than if you're charging small battery packs.

As far as constant-current versus constant-voltage is concerned, both types of chargers have been used very effectively for charging NiCd and NiMH packs. The "wall wart" type of charger is a constant-voltage type. The simple high-rate charger with the wind-up timer is also a constant voltage device.
The high-tech peak chargers hold a selected charge current until they detect the peak. The battery pack really doesn't "care". As long as it can accept the current flow coming into it the actual type of charger is irrelevant.

We've used all kinds of battery chargers over the years, and have found that the only real constraint is that you don't overload the wiring and don't overload the battery pack.

bax

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