Charging a Ni-cad battery

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Agree. I don't believe in total discharge. I was involved with recharging banks of NiCad batteries. We never deeply discharged and had them last for 20 years or more. Of course those were big commercial units. We never let them get below 50% state of charge.
My theory is that memory only shows up if you repeatedly discharge to the same level of state of charge. Varying the depth of discharge or even just one deep one with wipe out any accumulated memory effect.
I think more important is the number of discharges.
For tools and the like I keep them on a curcuit where I turn on a switch occasionaly for a few minutes. A timer would work too.

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Rich256 wrote:

There may be something to that. I will add one additional personal observation. It appears that the earlier (like 1970) NiCds did suffer from the memory effect much more than today's do. It may be due to different chargers, but I suspect they made some improvement to battery formula and design over the years. Those old NiCd batteries in the flashes for the Polaroid 100 cameras all seemed to have the problem. I recovered many of them for irate customers back then. Two or three deep cycles did the trick. That and instructions to allow them to discharge fully from time to time, seemed to make happy customers out of un-happy ones.

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Joseph Meehan

Dia duit
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Ken wrote:

Lead acid battery chargers are voltage dependent (i.e. 13.5v to charge a 12v battery). Ni-Cad battery chargers are current dependent (i.e. ~ 500ma charging current). So the voltages you measured are somewhat irrevelant. You can take a 12v battery charger and hook a 12v lightbulb that draws 300-500ma in series with your Ni-Cad battery and it will charge just fine. I've done it for years.
Bob S.
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From my experience in using ni-cads for the past 30 years or so, you did good. they do not last forever.

No it is probably just fine. Some chargers give a high reading under no load. Others can expain it better than I can. Stick with what you have.
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I have a Dustbuster that the first pack lasted for 10 years,the cells are designed to be charged 24/7/365,while most NiCds are not. The Dewalt repair center charged less to replace the pack than for what I could have bought new cells from Digi-Key(of the same charge type).
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Jim Yanik
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It is generally accepted that you can charge sealed NiCads at 1/10th capacity on a continual basis. ( 500mA battery on a 50mA charger)
DJ
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Andy writes
I disagree. It is generally accepted that the 1/10 rate is the desired rate to charge a NiCad without overheating. Once the cell is fully charged, you can THEN overcharge it if you leave it there..... So 1/10 for about 15 hours is a good rule of thumb to get to full charge...... If you want to "float" it, drop the charging current to 1 or 2 ma to just equal the self-discharge rate.
Andy
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DJ wrote: It is generally accepted that you can charge sealed NiCads at 1/10th capacity on a continual basis. ( 500mA battery on a 50mA charger)
========= DJ,
Yep - that is the 16 hour charge rate and it is the basis for many/most dumb chargers. Unfortunately, many of us are in a hurry and some manufacturers have put out equally dumb quick chargers which will begin toasting battery packs once they have reached full charge. Sadly, the general public doesn't realize why their cordless items need new battery packs every year or so.
Gideon
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Andy writes:
Since the best way to charge a NiCad is with a constant charging current, it looks like the wall wart has been design to do that. It probably has a rectifier inside and a series resistor set to approximately:
7.5 - 1.2 ---------------- ohms or around 1200 ohms .05
in which case it is a very inefficient supply, but will provide a fairly constant charging current of, in this case, 50 ma.
This is a cheap way to do it, but it is foolproof , works very well, and doesn't have a failure mode that results in a battery explosion....
You can verify this by reading the open circuit voltage (which you said was 7.5 volts, and shorting the output to ground and reading the short circuit current. That gives you the internal Thevinen resistance. If it is anywhere in the vicinity of 1K-2K, you know that is what they are doing...... I doubt that there are any electronics inside the wall wart other than a rectifier and a resistor, and probably not even a filter capacitor since it isn't needed......
If this is the case, Meehan is correct when he says you should not substitute another wall wart since it will not have the current limiting resistor.
Andy
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Ken,
Without getting into the chemistry of ni-cads, keep in mind these facts:
(New) ni-cads are typically good for about 1000 charge / discharge cycles. Shelf life depletes the charge about 1% a day. Occasional deep discharge does help you get your 1000 cycles. Batteries may be kept on the charger continuously in some cases. Let me explain.
Assume your "AA" battery has a listed current rating of 1100 milliamp hours (Mah). This means it should deliver 1100 milliamps for 1 hour or 110 milliamps for 10 hours or 11 milliamps for 100 hours. You get the idea. The 1 hour rate (1100) is known as it's "1R" parameter.
Now, regarding chargers, you can charge a totally dead cell at the 1R rate (1100 milliamps) and expect a complete charge in just over 1 hour. You can also charge at the 1/10R rate for about 14 hours and get the same results. The important factor to remember is that once a cell is completely charged, a high charge rate will cause it's temperature to rise and that's a "bad thing". The trick is to know when a cell is "full" so the charger current can be reduced or stopped. Two ways exist to determine a full charge. Carefully monitor the cell voltage or carefully monitor the internal temperature (rectal and oral have no meaning here).
In specialized cells (think Motorola commercial walkie-talkies) where quick charging is desirable, custom batteries are built with internal temperature sensors. That's why these batteries have more than two contacts. Customized (smart) chargers use these sensors to know when to reduce the high charge rate (1R) to the 1/10R rate.
Finally, a ni-cad cell can be left connected to a 1/10R charger indefinitely with no ill effects. The off-gassing that occurs is offset by the re-combining chemistry within the cell so overheating does not occur at the 1/10R rate. How do you know if your wall wart is designed to deliver at the 1/10R rate? Easy. If the owners manual indicates a complete charge may take 14 - 16 hours, you have one. If it says it can charge a dead battery in substantially less time, you don't.
Nickel metal hydride (Nimh) and lithium ion cells have different rules. Another time, perhaps.

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I do not believe this to be true. Digi-Key lists NiCds *specifically designed* to be left on the charger when not in use. If what you said was true,this would not be necessary. My own experience also shows it to be false.
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Jim Yanik
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Digi-Key does not make the batteries; they only sell them. This claim of unique capability is only sales talk. They claim a special benefit which is, in fact, common to all ordinary NiCads -- a widespread practice with sales people.
SJF
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Bulls..t. It's not "sales talk",it's in their printed catalog.
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Jim Yanik
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Thanks for all the info. I don't recall the exact number, but I do recall the manual saying something about the charge taking the better portion of a day. Basically, I try to trim my beard some morning, find that it is dead, plug in the charger, and then unplug it either that night or else the next morning if I forget.
Ken
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