The "Floaty-Thingie"

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.


>15 yrs.
4 yrs.
18 mo.
3 mo.
3 mo.
1 mo.
14 days
5 days (A)
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.

- 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:

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?

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.
Picture of the
Schematic of
                the "Floaty-Thingie"

At this point, a few assumptions are made:

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


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:

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:
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:
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!

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.

Please note:  The links above tend to change frequently - please let me know if one (or more) cease to work.

Go to the main web page.

Any comments or questions?  Send an email!

This page maintained by Clint Turner, KA7OEI and was last updated on 20150706.  (Copyright 2010-2015 by Clint Turner)

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