Thursday, May 31, 2018

A "floaty thingie" for keeping NiHM cells topped off

A charge-state maintenance device for NiMH cells

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.

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
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.

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 compared to alkaline cells.  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 than NiCd and LiIon cells.  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.  While NiMH cells can take a bit of abuse, LiIon cells can not, which is why they should always be connected using "protection" circuitry to guard against overcharge and overdischarge.

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.

Dealing with self-discharge:

As wonderful as NiMH cells are, the higher-capacity types and older, heavily-used cells do have a drawback:  Self discharge.

Referring to Table 1you'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.  Remember that the lower-capacity, "low discharge" types lose their charge slower than this, but I have yet to find specific information on these devices.

The data in Table 1 also assumes something else:  Typical, new cells.  As they age they tend to self-discharge even faster.

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!

At this point in the article I would normally provide a link to the sites of several cell manufacturers - but I've observed that these links are constantly changing, so I'll forgo doing this:  I will leave it up to you to find the technical data for larger manufacturers such as Eveready, Ray-O-Vac, Duracell, etc. that give recommendations for long-term float charging.

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!

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.

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!

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
2 AA
1 AA


  • 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!


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!

This blog posting was adapted from an earlier article on my web site.


This page stolen from