I was just looking through the Mar/Apr issue of Wood magazine, where
they did an article on myths of rechargable batteries. One thing they
said really confused me - they said that rechargable batteries should
NOT be discharged completely, and that you should recharge them as soon
as you feel your tool slowing down, because if a cell is totally
discharged it can switch polarity and ruin the whole battery (or
something like that). I'd always heard that with NiCad batteries, you
DO want to discharge them completely, so they don't develop a 'memory'
and accept less of a charge each cycle. Does anyone have a good
explanation for why one or the other is true, or firsthand experience
trying it both ways?
Also, they said that NiMH batteries are really not superior, because
even though they can have larger amp-hour ratings, they don't last for
as many charge/discharge cycles. I'd also read elsewhere that NiMH
batteries don't develop a memory, which seems to me like it would make
them last longer. Has anyone used both types side-by-side through the
whole life of a battery?
Well, it's not about incomplete discharge, it's about frequent discharge
to the same level of discharge.
Well, it depends.
It depends on what you're using it for. A cellphone where you throw it
on the charger when it beeps, you probably want the nickel metal
hydride, because a NiCd will (if memory is real - cue debate on that
now) eventually consider anything below that point not to be there.
It's complicated. Basically, given a choice, I'll go with the Nickel
Metal Hydride, but often it's a case of "Here's what battery works with
this" and that's your choice.
Cell phones have pretty much all gone to lithium ion which gives an even
better power to weight ratio than NIMH.
For all practical purposes the memory effect doesn't exist in quality ($)
batteries like you would get for a cordless drill (i.e. it may show after
several thousand charge/discharge cycles but the drill would be worn out by
then too), the cheap nicads you get in a $19.99 cordless phone usually due
from being overcharged due to having a very simple charging circuit and
show the memory effect after a couple years.
Good point, when Black and Decker first appeared they made a fortune by
sticking cheap universal motors in their tools that were good for a
couple of hundred hourse use. If you think of the amount of time that a
drill is actually in use it is normally less than a fe3w minutes a day.
contractors who spend their time fitting decks and the like tend to use
the tools that are way up market.
When I did electronic engineering we had a lab experiment that involved
measuring magnetic remenance curves. There is a battery 'memory'
effect, over time the batteries really do loose their ability to
The remenance curves are different for different materials which is one
reason why Lithium Ion is better than NiCad. But that is a 'soft'
problem, you can cure it by simply draining the battery completely. The
bigger problem with nicads is that they don't hold a lot of charge to
start with and they tend to wear out after a relatively short number of
charge cycles compared to LiIon. The battery manufacturers have done a
lot to address these problems over the years and NiCads are much better
than they were, but there is still a memory effect and they do still
wear out over time.
I don't quite see how a battery cell could reverse its voltage. The
voltage is the consequence of the difference in the electrode
potentials. To reverse voltage the materials of the anode and cathode
would have to change their atomic structure...
I really really would discourage folk from using anything other than a
proper battery charger, particularly with Li-Ion. In the early days of
developing Lithium ion batteries the labs used to regularly blow up. If
you go by the old BBN buildings in cambridge Ma you can see a corner of
one building in a different brick where they fixed the damage from a
Li-Ion explosion. The energy density of Li-Ion is pretty high, in fact
its higher than some explosives so messing about is a really bad idea.
'shocking a battery' with high voltage might not cause the battery to
explode but they have been known to catch fire, see the tales of Mac
The cells won't be absolutely identical, and one will likely run down
just a bit faster than the others (especially with time). As you run
the battery down, that weak cell will go absolutely flat first, and
then be driven by the remaining cells in the wrong polarity. That is,
it will start charging from the other cells, but with its polarity
reversed. That's especially bad for the battery.
Well, yep, that would be good advice.
Michael and MJ Houghton | Herveus d'Ormonde and Megan O'Donnelly
firstname.lastname@example.org | White Wolf and the Phoenix
NiCad battery memory problem can only occur under special circumstances,
which never occur in how any one uses NiCads, so don't worry about it.
You do want to discharge a cell, but not reverse the polarity.
A battery has many cells. Take the battery voltage and divide by
1.3 to get the approximate number of cells. If you have a 14V
battery pack, you do not want the voltage to drop more then 1.3 Volts.
If more than that, you have a good chance of reversing the polarity
on one of the approximately 10 cells.
Nicads have a fairly constance Voltage until they reach the discharge
level, then the Voltage drops quickly. If you start the recharge
just as the Voltage drops, i.e. when the tool starts to slow, then
you recharge when the battery has mostly discharged.
People have done side-by-side testing. Let's hope they reply to
your posting with the results or references to the results.
FYI: Cell reversal is caused when a cell in a pack has a lower
capacity than the other cells and that cell reaches 0 volts before the
others. The other cells in the pack then pump current backwards
through the depleted cell causing the reversed voltage.
The NICAD memory effect, according to the history/legend that I heard,
dates to the late 50s and the Vangard space program. the US lauched
the Vanguard satelites into orbits that maintained precisely the same
amount of time time (to the second) in the sun, charging the
batteries, and in the dark, discharging the batteries. Thus, the
batteries were charged/discharged to exactly, and I mean exactly, the
same point for their entire life time. They developed a memory.
So there are 2 conditions for memory.
1. You must be using ancient NICADs (I think someone else pointed this
2. You must discharge precisely to the same point each time.
Using todays hand tools in normal everyday use, you are unlikely to
achieve these conditions.
Now, there is an effect called charge depression, or capacity
depression where the discharge curve is lowered, not shortened. The
end effect is to make it look like there is a memory. However, it is
caused by overcharging and not by repeated discharging to the same
point. This effect is what most people refer to when they say they
fixed a capacity problem by deep cycling a battery.
Also, some cells develop large crystals on their plates that can grow
large enough to puncture the insulating separator between the two
plates of the cell. This shorts out the cell and unless you can blow
out the short the battery pack will exhibit reduced voltage and will
appear to have reduced capacity. Pulse chargers, where high current
for brief periods of time (pulses) are used to charge the battery, are
sometimes good at blowing out the short but you have to realize, once
that insulator is punctured, the cell is essentially destroyed. you
may be able to bring it back to life by zapping it but the cell
essentially self discharges and the crystals grow back. You might as
well buy a new pack rather than fool around with zapping cells. Or at
least start saving your pennies because you're going to have to buy a
new pack sooner rather than later.
BTW, in order to effectively zap cells, you need a really high current
source. the best way that I found is to connect another fully charged
nicad cell across the shorted cell. positive to positive, negative to
negative. I'm talking cells here, not the entire battery. The charged
cell will discharge into the bad cell and each cell should end up half
charged. You should monitor the voltage of the pair to see that the
short really is cleared or else you're going to be dumping an extreme
amount of current into a low resistance short which can only lead to
bad things happening.
Obligatory warning time: zapping cells can result in dangerous buildup
of hydrogen gas in the depleted cell. Be careful and monitor cell
voltage to ensure the short is cleared.
On the subject of these crystals, they really only short out a cell if
the cell is discharged. On a fully charged cell, they tend to blow
themselves out with a current surge when thy puncture the separator.
Thus you should always try to keep your packs charged, especially over
the winter when they're sitting in drawers, and the incidence of
shorted cells should be reduced. They will still tend to self
discharge, but they will be at full voltage.
NiMH batteries are good for *fewer* charge/discharge cycles than NiCad.
you get more watt-hours in less space, but pay for it with reduced cycles.
I'm don't know just where it works out in terms of 'lifetime" watt-hours.
I suspect that NiMH are a tad more expensive per lifetime watt-hour.
*early* NiCad batteries did have a 'memory effect'. If you're dealing
with batteries manufactured in the last 15 years (at least), it is a
_non-issue_, due to changes in battery design.
Running NiCads MULTI-CELL BATTERIES _all_the_way_dead_ is a bad idea, for
the reasons mentioned in your posting. When dealing with SINGLE CELLS
(1.2V units) running them all the way down is not harmful, and can extend
NiCads have a fairly sharp 'knee' in the output curve -- when output starts
to fall off, they are "quite close" to being totally run-down.
This is also what I have heard. I have no real proof or reference, but
my understanding is the whole memory thing was a real problem back in the
'80's or when ever NiCad's first came out. The manufactures kind of quietly
fixed the problem years ago, but a most of us that can remember when Nissan
was Datsun just can't get it out of our heads, so the problem now lies in
the memory of the user, not the battery. As others have said, recharge both
NiMH and NiCad as soon as they start to slow down. - Or so I have been told.
Really. Then what does the NiCd battery rejuvinator, sold by
Physio-Control, actually do? Because before rejuvinating a NiCd
defibrillator battery, it gives you the capacity of it, then it does
it's cycles protocol, and measures the capacity after the rejuve, and
the capacity has improved.
I don't think it's snake oil, what with being certified by the FDA as a
piece of test equipment for calibration of medical devices, and all.
Maybe they know something about NiCd batteries that you do not?
Meh. Following up to my own post with an example URL showing this
(mind the wrap)
FYI - Physio-control has been making life support equipment for decades.
I don't buy that. They STILL have a memory effect, but perhaps over
many more cycles than once before.
The memory is due to crystals forming between the electrodes each time
the battery is discharges, recharged. They close the gap between the
electrods, thus reducing it's capacity.
I've renewed run down (and newer NiCad) batts for my cordless phone by
"shocking" them with 12V (they're 3.6v packs) for a second or two. The
crystals burn off.
Everything I've read in the last couple years said that the memory
effect was not a factor in any nicad batteries anymore. What is bad is
heat, and letting them discharge completey before recharging makes
them get hotter during recharging, thereby lowering their longevity.
I think that articles such as this serve to feed the raging debate over
batteries. I'll express an opinion based on personal opinion. I am not an
expert by any means. I am an electrical engineer who has had to work with
batteries at various points of my career. Battery chemistry and design is a
complex matter. Its not as simple as classifying batteries into Nicad and
Nimh and LiIon. The spectrum of design characteristics supports a broad
swath of appplication requirements. Though Nimh and LiIon have become
prominent in consumer electronics, you still find a lot of Nicad in heavy
industrial applications because it works better.
I used to do all kinds cycles and incantations and records on my batteries
based on my "expert" knowledge. Today I don't do any of that. I throw it in
the charger and swap when something gets low (cordless drill, hand held tape
recorder or whatever). I don't pay any attention to rules or times or
myths. People basically get really ill when they have to replace the first
batteries on their cordless drill because the price is a real shock. When
you buy a cordless drill with two batteries, about 60-75% of the cost is the
batteries. I'm waiting until the pair of batteries on my Dewalt drill are
completely useless. Than I will buy a whole new drill. In the meantime, I
just swap batteries everytime I start a new project in the shop - sometimes
daily. It works fine for me.
Since there are opinions being slung left and right, I'll give you mine.
First take a look at this web site http://www.rcbatteryclinic.com /
You can read the bio and there is a bunch of information there, so I won't
repeat it here.
Second, my opinions:
I've been using NiCad's for 15 years flying R/C airplanes. In that time, I
have never had a battery develop a memory nor have I ever had a battery fail
in flight. I have made, literally, hundreds of flights and put my batteries
through many charge-discharge cycles (but not to complete discharge). I
cycle my batteries once a year on a modestly priced battery cycler. It
discharges the batteries down to 1.1 volts per cell and then recharges them.
The ONLY reason I do this is to determine the capacity of the battery pack.
If my pack drops below 80% of stated capacity, it flies a bench from then
I believe the secret is do not leave a battery pack on constant trickle or
ever let it stay completely discharged for any length of time. I wrecked a
number of packs before I figured that out (using expensive battery
maintenance equipment I might add). I do, however, keep them charged by
charging them at least every 2 or 3 months. That is not a problem since
they usually get this through normal use.
I maintain my Dewalt cordless drill in a similar manner. I don't check for
capacity, but I do note how long the packs last in use (no crash possibility
here). When I graduated from a 9.6 volt drill to a 12 volt drill, I passed
the older Drill to my BIL. I had it 4 years and he is still using it 6
years later with the original packs. The 12 volt drill is, of course, 6
years old and still on the original packs.
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