[davidl]

I have a battery pack with 17 AA 2000 mAH NiMH batteries. I built a charger with two LM317T chips. The first one was set to limit the current to 250 miliamps (5 ohms} per hour and the second to set the voltage at 22 V, a simple adaptation following the datasheet. I used a 270 ohm(220 and 47 in series) and 4700 ohm resistors to get the 22V. I left it on for about 10 hours to find completely fried batteries. They aren't just merely dead, they're really most sincerely dead.

So the first question is, if the charger did what I intended for it to do, would it have fried the batteries? The shrink wrap around it was overheated. My guess is that the real capacity of these batteries would be more like 1600 than the claimed 2000. So I sat it at about C6, which may have been too much.

I know one member indicated that he would put the voltage regulating chip followed by the current limiter. Would that design have been better? Why?

How can I test the charger circuit before I fry the next pack? I can put a resistor of say 500? ohms to test the voltage limiting. With an open circuit, the charger tests at 30V, which is the nominal value for the wall wart. But if I go go lower ohm resistor to check the current limiting feature I need a power resistor. What size would be appropriate?


I intend to at least double the resistance of the current limiting stage, which should take me to about C12, which is probably more reasonable. Should I go higher and take it closer to C20?

 

[ppl]

check out this http://www.batteryuniversity.com/partone-11.htm

 

[tangent]

Quote:

Originally Posted by davidl ...2000 mAh...limit the current to 250 miliamps per hour...left it on for about 10 hours

Ouch! That's C/8...you should not allow a dumb charger to provide more than trickle current (C/10 at most) if you're going to run it unattended. C/20 is safer.

I used a charge controller chip on the PPA battery board for a reason...