PLEASE NOTE: Messing about with batteries/cells
can be
hazardous:
Most cells contain hazardous materials and injury and/or damage can
result
from mishandling them.
Cells that are shorted,
improperly
charged or otherwise maltreated can pose an explosion/burn/chemical or
other hazard. It is entirely up to you to do research and
provide the appropriate precautions to prevent damage and/or
injury.
You have been warned!
The problem:
Table 1: Comparison of
self-discharge of various types of cells.
Comparison of self-discharge rates of various
types
of cells
The table below shows the approximate amount of time that it takes to
lose 10% of the cell's current charge capacity at different
temperatures.
Cell
Type
0C
(32F)
20C
(68F)
40C
(104F)
60C
(140F)
Alkaline
>15 yrs.
4 yrs.
18 mo.
3 mo.
NiCd
3 mo.
1 mo.
14 days
5 days (A)
NiMH
1 mo.
10 days
5 days
1-2 days
Zinc
6 yrs.
2 yrs.
10-12 mo.
2-3 mo. (A)
These are typical values for new cells, published by various
manufacturers. Note that aging/mistreated cells will probably
exhibit
much higher self-discharge rates.
Notes: - Storage or use of NiCd or Zinc-type cells at 60C
violate the manufacturers recommendation for consumer-type cells and
one may expect poor lifetime. It is not recommended that any
cell be exposed to such high temperatures for an extended period of
time.
- "Zinc" cells are those in the category of "General Purpose" or "Heavy
Duty" - in other words, those non-rechargeable cells that are NOT
alkaline!
NiMH cells are ubiquitous these days - and for good reason:
They have usable capacity comparable to that of an
Alkaline cell of the same size. A typical AA alkaline cell
has 2.4-2.8 amp-hours of capacity whereas modern NiMH cells range in
capacity from 1.8 to 2.8 amp-hours.
They are relatively inexpensive. If you shop around
you can easily find AA NiMH cells for $2 each - often much
less! This means that if they are used just a half-dozen times,
they may pay for themselves.
They have low internal resistance. When you pull
power from a battery, the output voltage sags - something that can make
many devices such as digital cameras shut down before the battery is
drained: Alkaline cells typically have higher internal resistance
than NiMH (or NiCd) cells which means that many devices cannot
fully-utilize the energy of the cells - particularly when partially
discharged.
NiMH cells are more forgiving. NiCd battery packs
suffer from a problem called "cell reversal" in which when just one of
the cells runs down before the others - an inevitability when several
cells are connected together - the weakest cell ends up being charged
backwards as the others pull power through it. This causes an
irreversible chemistry change that robs the NiCd cell of its power -
making it more likely to run down first next time and become even more
damaged than before! NiMH cells are more tolerant of such abuse.
"Ready-to-use" types. There are some types of NiMH
cells that are marketed as being
"ready-to-use" that have significantly lower self-discharge rate than
the standard cells. It would seem that these cells - at least
when new - do, live up to the claim, but I've yet to see information as
to how much the self-discharge rate increases as they age. I've
also noted that these types of NiMH cells tend to have lower rated
capacities than some other NiMH cells, ranging between 1500 and 1800mAh
for these types versus 2100-2800 mAh for "normal" NiMH AA-size
cells.
Such cells shouldn't be damaged if they are put in the "floaty-thingie."
As wonderful as NiMH cells are, they do have a drawback: Self
discharge.
Referring to Table 1 (to the right) you'll notice
something: At ordinary room temperature, a good NiMH cell will
lose 10% of its power after just 10 days - which means that after 6-8
weeks it's already half dead - and that's just from sitting there,
doing nothing! At higher temperatures things get far worse.
If you have a device with NiMH cells in it in a car on a hot, summer
day you can expect it to be mostly dead in just a week or two. The data in Table 1 also assumes something else:
Typical, new cells. As they age they tend to self-discharge even
faster.
An important note about
rechargeable "C" and "D" cells:
Before we go on, a few words about "C" and "D" cells that you
might
find at retail outlets:
Most "C" and "D"
rechargeable NiCD and NiMH cells sold at stores are really
larger
cases containing a single AA cell. You can often verify
this by
comparing the Milliamp-Hour (mAH) rating of these "larger" cells with
those of the AA cells often found on the same store shelf! A
"real"
C-sized NiMH cell would have over 4 amp-hours capacity while a "real"
D-sized NiMH would have well over 8 amp-hours. If you see a "C"
or "D"
cell with just 1.8-2.8 amp-hours of capacity you can be pretty sure
that inside that plastic is just a normal "AA" cell!
Another way to determine if the "C" or "D" cell is really
what it appears to be is by weight: A true "C" or "D" size cell
will
have quite a bit of heft to it for its size where a "fake" one will be
only a bit heavier than an AA cell by itself.
Is this cheating? It may be misleading, but if they state
anywhere on
the package what the amp-hour capacity really is, then they are being
honest about it - even if you didn't know what those numbers meant...
What does this mean, then?
Don't leave NiMH cells around for "later use." If
you charge up your NiMH cells and the just leave them around,
chances are they'll be mostly dead by the time you get around to using
them - unless you have a system of cycling through them very quickly.
Don't put NiMH cells away in your emergency box. You
should not rely on NiMH cells for emergency
purposes unless you have a system by which you can
guarantee that they are kept fully-charged. For those devices
that are put away for months at a time, Alkaline cells are a much
better choice as long as they are stored outside the device to
prevent possible damage from cell leakage and/or accidental discharge.
The challenge, then, is to have a system by which you can be reasonably
assured that any NiMH cell you pick up is likely to have a full charge
- but you don't want to do anything that is likely to damage them.
Maintenance charge:
In the case of NiMH cells (where the self-discharge rate is rather
high
- especially as the cell ages) it may be desirous to leave it on a
"maintenance" (or "trickle") charge for very long periods of
time. Recent recommendations by some battery manufacturers
suggest a
"C/300" current for this while other manufacturers recommend a charging
rate as high as C/40. Following the C/300
example, our
hypothetical 1
amp-hour cell above, this would be about 3.33 milliamps - that is, 1/300th
of the cell's rating. I have
not
seen any specific recommendations for such a maintenance charge for
NiCd cells, but I would expect that the same C/300 rate would
be
suitable.
It should go without saying that charging a "dead"
battery at the maintenance charge rate may take weeks to
accomplish!
A "Floaty Thingie" - A simple device to maintain NiMH
cell
charge during periods of non-use.
Because I extensively use NiMH cells - and because I'm aware of their
tendency to self-discharge - I have built a simple device that does a
maintenance charge for large numbers of cells. This device, which
I
have called a "Floaty-Thingie" (a highly technical term, I know...)
consists of several multi-cell battery holders with series resistors
and LEDs to both limit current and indicate that a maintenance charge
is
occurring. The battery holders are simply attached to a sheet of
wood
or plastic and powered by a 12 volt DC "Wall Wart" from my junk
box.
Note that while I use mostly 4-cell holders, there is also one 2-cell
and one single-cell holder so that I don't need exact multiples of 4
cells to fill a holder!
The circuitry is extremely simple: A resistor and cell(s) in
series with an LED - the latter being used to indicate current flow
which allows you to be sure
that the battery is connected. All of this is powered by a 12
volt (nominal) voltage source.
Using a 12 volt (unregulated) DC "wall wart" supply (which ranges from
12-15
volts, depending on total battery load) a resistance was calculated,
taking into account how many cells were used and what size. My
"Floaty-Thingie" handles only AA and AAA sizes as these are the most
common, but using the information here and a simple application of
Ohm's law,
other
values
can
be calculated.
For the maintenance charge I chose to follow the "C/300" float
rate
as this seemed to be adequately comparable to the self-discharge rate
of the cell itself. For typical AA NiMH cells, this would be
about 8
milliamps - assuming a cell capacity of 2.5 amp/hours - and for AAA
NiMH cells, this would be around 3 milliamps - assuming a cell capacity
of 1.0 amp/hours. These values are typical and are definitely
not critical! Do not worry if your AA cells have 1800 mAH
or 2800 mAH capacity, for example!
Figure 1: Top: The "Floaty-Thingie"
used to maintain
charged on NiMH cells. (This version only does AA cells in
groups of 4). Even though there can be up to 48 cells being
floated, a small 12 volt, 100mA wall-wart is all that it necessary. Bottom: The schematic of one section of the
"Floaty-Thingie." Click on either image for a larger version.
At this point, a few assumptions are made:
A supply of 13.5 volts. This is a reasonable voltage
to see
from
a "12 volt" unregulated "Wall Wart" under moderate load, but anything
from 11 to 15 volts would be OK.
About 1.5 volts per cell. (We are assuming that our
cells
are
already
fully-charged.)
Float currents: The float current is 8 mA for AA
cells and 3 mA for AAA
cells - values that roughly correlate with C/300 for typical NiMH cells
of those sizes.
The series resistance for various cell combination under the
above conditions is as follows:
Table 1: Typical values for
different types and numbers of cells using the circuit in figure
1 with a supply voltage of 12-15
Number
and type of cells
Resistance value (ohms)
with 2 volt LEDs
Resistance value (ohms)
with 3.6 volt LEDs
4 AA
680
470
2 AA
1000
820
1 AA
1200
1000
4 AAA
1800
1200
2 AAA
2700
2200
1 AAA
3300
2700
The above values are not critical and variations of +-25%
should
not be of any concern
1/4 watt resistors or larger are suitable.
In Figure 1 may be seen the schematic of the
"Floaty-Thingie."
As you
can see it is very simple and there's nothing critical about it -
except to say that any exposed wires should be insulated to prevent
accidental shorting of any components: Remember that NiMH cells
can put
out many amps under such conditions!
On the schematic, "R" is a
resistance from the table above, "D" is the LED, and "B" is the holder,
containing 1, 2 or 4 cells. When operating from a "12 volt"
supply (which can be anything from 11 to 15 volts)
it is not recommended that more than 4 cells be used as you need
several of volts of drop across resistor "R" in order to limit current
effectively and maintain fairly consistent current with minor voltage
fluctuations.
Note that Table 1 shows different resistance values for "2
volt" LEDs and "3.6 volt" LEDs. The older-style "normal
brightness" red, yellow and green LEDs (but not blue or white!)
are of the 2 volt variety while the newer "ultra bright" LEDs (most
notably green, blue and white) are of the "3.6" volt type. When
you by the LEDs, a quick look at the "forward voltage" specifications
will tell you what you wish to know - but don't be worried by slight
variations. For example, the "2-volt" types may vary from 1.7 to
2.2 volts while the "3.6 volt" types may say anything from 3.2 to 4.1
volts.
A note about the use of 3.6 volt LEDs:
These types are usually the "ultra bright" (green, blue,
white) LEDs. If you use these - and you have a lot of holders
- the total amount of light coming off the "floaty-thingie" may be
surprisingly bright - even at just 8 or 3 milliamps. If you build
one of these, expect that they may still be painful to look at and also
that at night, the entire assembly may be annoyingly bright!
Remember: We aren't aiming for ultra-precise results here -
just those that are "in the ballpark."
Using the "Floaty Thingie"
I've used this thing for several years now - as have several friends
who have seen it and made their own. Here are a few observations
and
comments:
Put ONLY fully-charged cells in the
Floaty-Thingie.
It
will take a very long time to charge a dead cell (several
weeks,
perhaps!)
at the above currents. Since the whole idea is to have
fully-charged
cells on hand for immediate use it would be a bad idea
to put anything
but fully-charged cells in it in the first place!
Completely fill up the cell holder. This should go
without saying: Unless every position in the cell holder
is filled, you won't complete the circuit and do charge
maintenance. Because of this, I recommend having one single-cell
holder and one two-cell holder - in addition to a larger number of
four-cell holders for each cell size (e.g. AA and/or AAA.) Doing
this allows you to "float" any number of cells that you may have
onhand. Some people who have built it have used two-cell holders
(and a single one-cell holder) instead of any four-cell holders, which
works, too, but remember that since each holder takes the same amount
of current, regardless of the number of cells, you'll be able to
maintain fewer cells overall if your wall-wart is rather small.
Make sure that you adequately size the wall-wart.
When you pick your "wall wart" supply to run this, consider how much
current you will pull from it if you load cells into every
holder. To play it safe, assume that each AA holder will pull 10
milliamps and each AAA holder will pull 5 milliamps and simply add the
total number of holders of each size - and make sure your supply can
handle this.
Note that a one-cell holder pulls the same current as a two or
four-cell holder of the same cell size: The
difference in power is "eaten" by the series resistor used to limit
current. Again, this means is that if you have a very small wall
wart - of if you have a limited power budget (say, from a small solar
panel) you can get better efficiency by using mostly four-cell holders
rather than mostly two-cell holders.
Yes, you can use a 12 volt solar panel for this.
Since
the
sun
only
shines
part
of
the
day,
don't
worry
if
the voltage
goes well above 12 volts (as high as 18-20 volts) during bright sun as
the "average" current
will be in the general range of what it should be...
This "maintenance" charge doesn't seem to have damaged the
NiMH
cells.
Over the past 5 years or so, neither I or others who have used a
Floaty-Thingie have seen any
evidence
that its use causes loss of electrolyte due to overcharging, "Lazy
Cell" syndrome (see below) or obviously shortens the life
Nevertheless, it would be a
good
idea to rotate through and use all of the cells: This would
prevent
the possibility of "Lazy Cell" syndrome (if it is likely to occur in
NiMH at this "maintenance" rate anyway) and it give you another chance
to spot those cells that are going bad! Even when treated
well, cells won't last forever!
The "Floaty-Thingie" doubles as a night light.
Since my Floaty-Thingie can hold over 30 cells, its LEDs give off a
surprising amount of light when all holders are populated! If you
happen to use a mixture of different colors you can get some pretty
cool effects! Remember, though: The modern "ultra bright"
LEDs put out a lot of light - enough to make looking at them painful
and keeping a room annoyingly bright at night. If you do
use these newer, modern LEDs be aware that many of them (such as the
blue, white and green) have higher voltages - between 3 and 4 volts as
opposed to around 2 volts for the old-fashioned, dim red and green LEDs.
I try group group "like" cells together. If you are
like me, you have been acquiring NiMH cells for years so you not only
have different brands, but different milliamp-hour capacities of cells
- even of the same brand! Grouping like-cells
together
will also assure that when you use them in a device that takes several
cells, you'll get optimal performance.
Note:
When I buy rechargeable cells, I always write the month and year of
purchase on them with an indelible marker as this also makes it easier
to group them together.
DO NOT put alkaline cells in the "Floaty-Thingie."
When one attempts to recharge alkaline cells, they can do unpredictable
things such as leak, so don't!
Come up with a system for "rotating" stock. It is
best if you make sure that all cells get as equal use as
possible. One way to do this would be to leave at least one empty
holder at
all times, knowing that the next holder contains the
cells to be used when previously-charged cells are to be installed in
the now-blank one. In this way one can help assure more even
usage of cells over time.
Can you put NiCds in the "floaty thingie"? Yeah,
probably... It probably won't hurt them to keep them in there for
short periods such as days, but I'm not sure that I'd leave them in the
device for weeks/months at a time!
Using "similar" cells:
As with other types of cells, it is recommended that you avoid, as much
as possible, mixing different brands/capacities of cells. While
the chemistry of NiMH cells makes it less likely that - unlike NiCds -
they will be damaged by cell reversal, it never hurts to play it safe.
This is fairly easy to do, actually: Simply group the same brand
and same-capacity cells together and use them as such.
Personally, I write the month and year of acquisition on cells when I
buy them with an indelible marker, making it even easier to match the
cells into groups - plus, it lets me readily identify the oldest of the
cells and keep track of how old they are and whether or not they
deserve further scrutiny as they age.
Detecting apoptosis (e.g. "cell death"):
The "floaty-thingie" has another use: To detect cells that are
near the end of their useful life.
Inevitably, cells will lose their capacity and die - but how do you
detect that fact before discovering that the device you put them in
quit working sooner than expected?
In using the "floaty-thingie" there are some signs that an individual
cell may be "sick" and might have lower-than-expected capacity.
To do this, you'll need a reasonably accurate digital
voltmeter: It needn't be expensive - I've found that even the
$3-on-sale digital multimeters from places like Harbor Freight have
more than adequate accuracy.
Here's the procedure:
Charge the cell normally using your normal charger.
Put it in the "floaty-thingie" and wait a week or so.
This
wait
time
is
required to allow the cell to equalize and "do its
thing" - that is, if it's really bad, it may take a few days for the
symptoms to show up.
While in the holder, measure the cell voltage. I
have found that a normal room temperature that typical NiMH cells
measure between 1.35 and 1.47 volts. I've noticed that same-brand
and same-vintage cells tend to stay very close to each other and that
this voltage seems to slowly decrease over time as the cells age and
self-discharge (leakage) currents increase.
If you find one cell that has radically different voltage from the
others - especially if it was made at the same time and is of
the same brand as the others - then be suspicious of that
cell! If the cell's voltage is unusually high after a week of
being in the "floaty-thingie" (a reading above 1.5 volts should
certainly set off alarm bells!) then it is very likely that there
is something seriously wrong with that cell!
If the cell voltage is lower than it should be - say below 1.3 volts -
mark it with a piece of tape (so you can tell it apart from the others)
and then try
charging it normally, re-install it in the "floaty-thingie" and wait
another week or so - just to make sure that it is really
sick. If it tests OK this second time, chalk up the first "bad"
results to, perhaps, accidentally putting a battery that was not
fully charged into the "floaty-thingie" - but if it tests bad again,
get rid of it!
Of course, it should go without saying that all batteries
should be
disposed of properly!
Disclaimer:
Again, messing about with batteries/cells
can be
hazardous:
Most cells contain hazardous materials and injury and/or damage can
result
from mishandling them.
Cells that are shorted,
improperly
charged or otherwise maltreated can pose an explosion/burn/chemical or
other hazard. It is entirely up to you to do research and
provide the appropriate precautions to prevent damage and/or
injury.
You have been warned!
Do you have any comments or
questions?
Send an email.
Please note that the information on this page is believed to be
accurate,
but there are no warranties, expressed or implied. The author
cannot
take responsibility for any damage or injury that might result from
actions
taken (or not taken) as a result of reading this page. Your
mileage
may vary. Do not taunt happy fun ball.
Other
battery-related
pages at this site:
The
NiCd/NiMH
page- This
page
describes
in some
detail the
care
and feeding of NiCd and NiMH cells and batteries. This explains
how
to keep NiCd cells going, and what that "memory" effect really
is!
(Hint:
It's not the "memory" effect at all!)
A few web sites with
info about various types of cells.
