Charging those Ni-Cad sets

LoopyByteloose

I purchased two sets of Ni-Cad batteries intended for wireless home phone replacements.
They seemed very attractive because they are rated 4.8volts at 700ma/hr.

I jumped into The Art of Electronics and read about recharging at a proper rate and I built a constant current charger using a LM317 to charge at about 65ma.·

I put the whole unit on a 6volt DC 500ma supply.· The voltage would come up to 4.8 in 14hours, but it would not hold a charge when used with one yellow LED and a 470ohm resistor.

Out of frustration, I tried charging with 7.5volts DC and now they hold a charge better·but read 5.4volts.· The current is still regulated at 65ma.

I am very confused and wonder what I have done.· It seems to me that that 4 cells rated at 1.2volts should not climb to 1.35volts each.

I cannot seem to get a clear picture from the literature
Have I done any damage or am I doing okay?

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"When all think alike, no one is thinking very much.' - Walter Lippmann (1889-1974)

······································································ Warm regards,····· G. Herzog [noparse][[/noparse]·黃鶴 ]·in Taiwan

SteveW

"I put the whole unit on a 6volt DC 500ma supply" - I think this is the fatal flaw. The LM317 needs far more than (6-4.8)V to do its thing. Try again with 9V or so.

And yes, 1.2V cells are quite entitled to charge at 1.35V. Heck, if you're not charging them with more than their open-circuit voltage, there's no incentive for the current to flow through the cell.

Steve

SteveW

Right, a quick dig through the LM317 datasheet. At 50mA, 25oC, it needs to be able to drop out (throw away) at least 1.6 volts or so.
If it takes 6V to push 50mA through a stack of 4 NiCd cells, you need your power supply to be providing at least (6+1.6)V. Any extra voltage will be regulatred away by the LM317 as it maintains the constant current. As long as you don't exceed the power rating (which, at a 50mA load, you shouldn't), then it'll all just work.

(The LM317 datasheet is well worth reading, if you just used the application circuit without reading the rest)

Steve

RickB

Kramer:

When charging, it is normal for the voltage to rise. On my battery analyzer, a 7.2 v pack will rise to 8-9 v at 600 mA depending on its' condition. At 65 mA the rise will be less. Are you sure you have 65 mA for the entire duration? If the max voltage from the charger is too low, the charge rate will drop off as battery voltage rises. That may be why you had better results with 7.5 v.

1.2 v per cell is nominal, good quality charged cells will be higher, even under load. ( for a while) Cycle the batteries a couple of times. A full charge followed by a discharge to 1 volt per cell. New cells tend not to achive full capacity untill after a few charge/discharge cycles.

Try building a constant current load controlled by a stamp. Monitor the voltage using the r/c function and turn off the discharger when the battery voltage drops to 1 volt/cell. By measuring the time and knowing the current you can calculate its' capacity.

Rick

LoopyByteloose

Thanks all, it seems with Ni-Cads the writers are so concerned with discussing the concepts of Constant Current that they omit clarifying what is an acceptiable voltage limit.

I googled and googled and google, but once again this Forum resolves the question at hand.
Incidentally, the higher voltage is still in range for providing an unregulated DC source for BasicStamps.

So I am quite pleased with the possiblity of using these in some kind of remote solar charged application.

Apparently the Ni-Cads prefer to yo-yo up and down rather than merely 'float charge'. At 4.5volts, the BasicStamp will shut down until a charge is regained. The Ni-Cads prefer to be recharged at 1.1volts or 4x1.1=4.4volts. I just have to figure out a way to keep it shut down until enough sunlight comes along. I suppose a low-power 555 triggered to wait 24hours could keep a FET turned off.

The BS2 is far more attractive at 3ma than the BS2p at 40ma. The snail wins the race.

Of course I should take a look at sleep mode again - something like 15seconds on, 5 minutes off would be enough to collect a bit of data and communicate. Continous real time·data would just be too much·to analize·anyway.

BTW -·am cycling the batteries with a yellow led and a 470 ohm resistor - supposed to be a very slow discharge.· Maybe I will move that up by a factor of 5 or 10 as the cells seem to be healthy.



▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
"When all think alike, no one is thinking very much.' - Walter Lippmann (1889-1974)

······································································ Warm regards,····· G. Herzog [noparse][[/noparse]·黃鶴 ]·in Taiwan

Post Edited (Kramer) : 11/16/2005 5:33:44 PM GMT