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By Spiro
#57190
I would like a to build a system that can charge 6cell nicad packs @ about 1.0-1.5amps with peak detection, trickle charge, and also discharge @ about 1 amp. I would be perfectly happy with any of the many commercially available solutions in the RC world that take care of this problem, except that I need to be able to do this for 12 battery packs simultaneously and even the least expensive commercial unit gets expensive when purchasing 12 of them.

Right now I am just in the information gathering stage. I am sure a basic nicad peak charger/dischager/trickle charger has been designed and built a couple of times before :) I see no need to reinvent the wheel, but I was hoping perhaps some of you folks could help point me in the right direction.

Links to other applicable projects, schematics, or advice are all most appreciated.

Thanks in advance,
Spiro
By theatrus
#57195
The easiest way to approach it is to use a charge controller IC. One common example is the MAX712.

http://www.maxim-ic.com/quick_view2.cfm/qv_pk/1666

Its available in DIP for easy prototyping. Just needs an external drive transistor, a few caps/resistors, and a DC power supply. For 12 packs, you'll need lots of current though. Perhaps an old computer power supply?

Looking at digikey pricing, you'll probably spend $10 per charger circuit, not counting power supply.
By Spiro
#57199
Thank you for the suggestions. I will take a look at the MAX712 IC as well as that Bluebird charger.

I have several old computer power supplies that I keep just for this sort of purpose.
By lyndon
#57228
Hah! That's funny: I built exactly that system using a MAX712 for the charger about 3 years ago. Discharge current was up to 30A -- fried a bunch of power resistors & FETs until I got it right :-)

Not much to it: about $15 in parts IIRC.
By Spiro
#57232
lyndon,

Do you have a build thread or any drawings for this project? I would be interested to learn more about it.

Thanks much,
Spiro
By lyndon
#57291
Nothing online, but I should still have the Eagle CAD schematics. I'll look around. Until then, here's what I remember:

The charger was the Max712 design right out of the appnotes. It worked like a champ,: just had to adjust for the # of cells I was using.

The discharger was a Proportional feedback loop using a Linear Technology precision opamp and a HEXFET (IRFZ44 I think, mounted on a MASSIVE heatsink) sinking the current with a 0.05 ohm power resistor to close the loop as a current-measuring shunt and a manual trimpot for the current discharge setpoint. There was a comparator output so an attached CPU would know when a certain discharge level was reached and stop the current, but I never used that portion for anything. Pretty straightforward stuff. Except for the MAX712, it was all stuff I had sitting in the junkbox.

I'll look for those schematics.