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. The NiMH information above is for "standard" cells, not the so-called "low-self-discharge" variety.
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.
About "Ready-to-use" low self-discharge 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 the average consumer doesn't know what the numbers mean!
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 VDC
Number
and
type of cells
Resistance value
(ohms) with 2 volt LEDs(standard-brightness red/yellow/green)
Resistance value
(ohms) with 3.6 volt LEDs(high-brightness green/blue/white)
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 (over a decade!)
- 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 10
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 as this would reduce 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 and 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, yellow and green "indicator" type LEDs, so be sure to take that into account when selecting the resistor values.
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 than with
NiCds that 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.
Duracell
OEM Battery Data - Another useful page with data,
specifications, and ratings on various battery technologies.
Rayovac
OEM Product Guides - This page has some OEM product
guides with some technical information on Alkaline as well as
other types of cells. Unfortunately, this site has less
technical information than that of the other two
manufacturers.
Please note: The links above tend to change
frequently - please let me know if one (or more) cease to
work.
