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.
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.
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.
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).
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
current to 1 or 2 ma to just equal the self-discharge rate.
It is generally accepted that you can charge sealed NiCads at 1/10th
capacity on a continual basis. ( 500mA battery on a 50mA charger)
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.
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
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
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
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
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
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.
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.
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
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.
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