NiMH AA and AAA batteries are 1.2V?

January 11, 2008 12:50 PM

What's the deal with NiMH AA and AAA batteries being 1.2V?

I'm frustrated that I bought into these things without knowing they put out a lower voltage than alkaline batteries. Two of my intended uses of the batteries are my LED flashlights and my LED bicycle headlight. Neither of these will power on when I use NiMH batteries (both Rayovac and Sanyo Eneloop).

How is it ok for these batteries to masquerade as AA or AAA, when they are not drop-in replacements for a standard AA or AAA battery? Is there any hope for future rechargeables actually being 1.5V? At this point, I have a dozen battery-shaped objects that are pretty much useless to me.

posted by knave to technology (32 comments total)

Cell voltage is a function of the electrochemistry. It isn't really something that can be adjusted.
posted by Steven C. Den Beste at 12:56 PM on January 11, 2008

Here's a previous question about rechareables that has some good info.

Also, this comment has this to say about the 1.2V issue:

I haven't found any 1.5V rechargeable AA batteries

And you won't. The chemistry of NiCD and NiMH rechargeable batteries is such that they are 1.2V, period.
posted by burnmp3s at 12:56 PM on January 11, 2008

If only there was a 2,500 year-old saying for this sort of situation.

Oh yeah: caveat emptor.

And what they said about battery chemistry.
posted by GuyZero at 1:02 PM on January 11, 2008 [1 favorite has favorites]

Are you sure the reason the flashlights/headlights aren't working is the voltage disparity? I use NIMH batteries for a zillion applications (including bike lights) and have had no problems. Maybe something is wrong w. your batteries and/or charger?
posted by ManInSuit at 1:03 PM on January 11, 2008

ManInSuit: After charging, I tested the batteries with a voltmeter. In the case of my flashlight which takes 3, the batteries were putting out about 3.7V. I'm speculating that this is the reason the lights won't work, but if someone can offer alternative theories, I'm all ears. I'd love to get these things working. (I hate the thought of buying and disposing of alkalines on a regular basis.)
posted by knave at 1:07 PM on January 11, 2008

Cell voltage is a function of the electrochemistry. It isn't really something that can be adjusted.

Yeah, you would have to stack up lots of constant voltage 'cells' to form some sort of 'battery' in order to do that!

And that would just be crazy.
posted by delmoi at 1:08 PM on January 11, 2008 [3 favorites has favorites]

there are plans for small cheap dc-dc powersupplies that could help (but might also be a hassle)

Google "Joule thief" for one example
posted by Good Brain at 1:20 PM on January 11, 2008

Yup, band gaps are band gaps and you're stuck with them.

What you aren't stuck with is resistor values. If you are enterprising, you can crack open your lights. Find the wires that go to and from your LEDs. Follow them onto whatever circuit board is in your light. In line with each LED you will find a little yellow rectangle (or if your light is old, little dumbell shaped cylinders). These are the current limiting resistors. If you are enterprising, find a soldering iron and a digital resistance meter (ask your friends...). Pull out the resistor and measure the resistance.

You will need to buy new little yellow rectangles or dumbell shaped cylinders, aka surface mount chip resistors or axial lead carbon film resistors. You can get these from Digikey or perhaps even still Radio Shack. Buy them with this resistor value:

R_new = R_old * 1.2/1.5

Solder the new ones in place and you are good to go!
posted by noble_rot at 1:29 PM on January 11, 2008

Nice hack suggestion noble_rot, but most likely the poster will encounter surface mount components, which are non-trivial to replace. Desoldering alone can be risky, so be prepared to replace the item before attempting this.

Can I just condense the standard disclaimer into Caveat haxxor?
posted by butterstick at 1:39 PM on January 11, 2008

Like ManinSuit, I've used NiMH batteries in a few different headlamps without trouble.

To answer your question in a little more detail, battery voltages aren't as simple as a certain cell outputting a certain voltage. All batteries have a discharge curve: they start out at a certain voltage that decreases as they discharge. Alkaline batteries start out around 1.6V and then drop quite quickly to about 1.35V, and provide most of their power between 1.35 and 1.15V. NiMH batteries start out around 1.3V, but provide most of their power between 1.2 and 1.1V. Here are a few graphs of discharge curves for alkaline and NiMH batteries (you probably want to look at the lower discharge rates graphs for the alkalines, rather than the first few). In reality, despite the different nominal voltages (1.5V versus 1.2V) alkaline and NiMH batteries provide most of their power at fairly similar voltages.

Using NiMH batteries in LED lights isn't absolutely ideal, because the devices are usually designed to work with the slightly higher voltages of alkalines, but it is usually close enough.
posted by ssg at 1:41 PM on January 11, 2008

I had this problem with a couple of small fans I like to use. It turned out the rechargeable batteries were slightly shorter than alkalines. I had to build some tinfoil shims so they would reach the contacts on the positive terminal.
posted by RussHy at 1:47 PM on January 11, 2008

Yeah, you would have to stack up lots of constant voltage 'cells' to form some sort of 'battery' in order to do that!

Yeah, but you would no longer have a battery the shape and size of a AA or AAA using NiMH chemistry! Which would be a bit of a design problem!

!
posted by Blazecock Pileon at 1:49 PM on January 11, 2008

There's an interesting wrinkle to this problem. Alkalines are rated at 1.5 volts, which is their voltage when they're new and fresh. NiMH's 1.2 volt rating is actually their average nominal voltage, which you can see on the discharge curves that ssg linked to. Adding to this is that alkalines dip below 1.2V very early on while being used, while NiMHs stay around their rated 1.2V much longer. So it's likely that a NiMH will provide a higher voltage much longer than an alkaline.
posted by zsazsa at 1:56 PM on January 11, 2008

Note that the above holds true only in medium-to-high drain applications. For something like your remote control, the alkaline's going to have good staying power.
posted by zsazsa at 2:00 PM on January 11, 2008

Okay, I'm a tool. I basically just said what ssg already said. And I even referred to his comment!
posted by zsazsa at 2:02 PM on January 11, 2008

I'd just like to agree with everyone else that it's probably not the batteries. Even if your lights had a boost or regulation circuit (probably not, since they're using *three* batteries), they would output light of some sort at low voltages.

Does the flashlight work with alkaline batteries?

It's also possible that one or more of the rechargeable batteries is damaged, and doesn't work under any serious load. Using a voltmeter is sometimes problematic for batteries because it doesn't measure what the voltage is when there's a real load applied to the circuit. You might want to individually swap known-good batteries in with each of the questionable batteries to narrow down if they're all a problem or just one.

(and on preview, what RussHy says is also very likely)
posted by meowzilla at 2:04 PM on January 11, 2008

Thanks for all the input guys.

Yes, both the flashlight and the bike headlight work with alkalines. I can swap back and forth between alkaline and NiMH and see them both work, then not work. It appears that they fit snugly, but I'm going to double check that it isn't something like what RussHy mentioned.

Yeah, but you would no longer have a battery the shape and size of a AA or AAA using NiMH chemistry! Which would be a bit of a design problem!

That doesn't exactly make sense, because AA and AAA are two different sizes, yet they both have the same voltage. However, if I take the assumption that everyone is right, 1.2V is the base voltage for NiMH chemistry, then I'm guessing you can only achieve integer multiples of that voltage by stacking them. (Whether stacking multiple batteries, or multiple cells within a battery.) Is that basically the issue?

It's also interesting to hear that alkaline batteries aren't truly 1.5V either. I'm really curious why I'm having these issues now.
posted by knave at 2:08 PM on January 11, 2008

Yeah, you would have to stack up lots of constant voltage 'cells' to form some sort of 'battery' in order to do that!

And that would just be crazy.

OK, then we've got either 1.2V batteries, or 2.4V batteries, or 3.6V batteries, etc. In fact, 7.2V and 12V NiMH batteries are made this way.

delmoi, perhaps you'd like to show us how to make a 1.5V battery by connecting 1.2V cells in series? All you need to do, after all, is solve 1.2x=1.5, with the restriction that x is an integer.

----

It is possible to make batteries where the voltage can be adjusted as you like--where some or all of the chemicals involved are in aqueous solution, the voltage can be varied by varying the concentrations of the chemicals in solution, in a well-understood way. (Google "Nernst equation" if you want the gory details.) The problem with that type of battery is that as the battery is used, some chemicals are used up and others are created, and the concentration of the chemicals in solution changes, and thus the voltage chagnes, decreasing at a fairly steady rate over the life of the battery, which is undesirable. Batteries which have all reactants and products in the solid phase (which is pretty much all types of consumer batteries, whether alkaline, NiCd, NiMH, or Li-ion), keep a relatively steady voltage over most of their usage (see RussHy's link to discharge curves above), dropping off only at the very end of their usage. But then you have no aqueous chemicals whose concentration you can manipulate to alter the voltage; you have only a fixed voltage available to you.

Compared to the discharge curves RussHy linked, the discharge curve of a battery with one or more chemicals in solution looks a lot closer to a straight line going from the battery's initial voltage to zero.

Is there any hope for future rechargeables actually being 1.5V?

Not with the electrochemistry of NiMH or NiCD batteries, but that doesn't rule out batteries with different chemistries being developed in the future. It's far from trivial, however.
posted by DevilsAdvocate at 2:15 PM on January 11, 2008

Chart for rechargeable batteries giving the nominal cell voltage as a function of the chemistry.

Producing 1.5V with NiMH technology in a AA or AAA form factor would be problematic. Just to do some speculating about it:

You might be able to fit two cells in series and include a voltage regulator (or DC-to-DC converter) to bring the voltage down from 2.4V to 1.5V, but then the problem would be how you'd recharge them. The regulator would be in the way of the back-voltage flow. Maybe you could toss in a bypass diode around the regulator?

Even if you did, the regulator would prevent the charger from measuring the cell voltage, to determine when the cells were near full, the point where the charger has to go into "topping off" mode (pulsed recharging). Missing that is particularly dangerous for NiMH because the cell resistance doesn't rise when near-full the way lithium batteries do. If you keep pouring in continuous power at that point, nearly all of it gets converted into heat, and the battery can melt or explode. At the very least it's going to get really damned hot.

The charger would have to do all of that by dead reckoning, simply by timing, but that would be dangerous. If the battery isn't fully discharged when you start recharging, you'd be late going into topping off mode, leading to a fire hazard. I guess the charger could do a full discharge before doing a recharge, just to be safe, but that's getting pretty elaborate. It would significantly increase the length of the recharge cycle. And I wonder if cell capacity and recharge cycle changes as the battery ages; I bet it does. If so, a dead-reckoning recharge cycle that worked when the cell was new would not work when it got older.

What you'd really want would be a third lead into the battery that bypassed the regulator to permit clean charging -- but then it wouldn't be AA or AAA.

I think there's a pretty good reason why no one has offered such a thing for sale: it would be tough, and there really isn't any commercial demand for it. Someone who insists on an honest 1.5V out of their rechargeables probably ought to be looking at rechargeable alkalines.
posted by Steven C. Den Beste at 2:28 PM on January 11, 2008

Are the batteries held in place in such a way that you could measure the voltage across the terminal contacts while the flashlight or whatever is on? That might help you figure out if the problem is the cells themselves or something else.
posted by ssg at 2:29 PM on January 11, 2008

That doesn't exactly make sense, because AA and AAA are two different sizes, yet they both have the same voltage.

The physical size of the cell affects the amount of energy it contains, but doesn't significantly affect the voltage. The voltage is a function of the electrochemistry.
posted by Steven C. Den Beste at 2:30 PM on January 11, 2008

How is it ok for these batteries to masquerade as AA or AAA, when they are not drop-in replacements for a standard AA or AAA battery?

They are AA or AAA cells, or at least as much as the alkalines are. That designation effectively refers to the physical size of the cell. (It used to specifically mean a carbon-zinc cell of that size, but who uses carbon-zinc batteries any more?) What the cells are not is LR6 cells.
posted by hattifattener at 2:38 PM on January 11, 2008

Just a bit more speculation: a dead-reckoning recharger might be able to control the recharge cycle using a thermocouple to measure battery temperature, killing the recharge current when the battery heated up, and turning it back on when it cooled.

Heat propagation latency from the cell to the battery skin would be the big concern. You'd have to choose a temperature threshold for the skin measurement which was low enough to make sure the temperature inside the cell wasn't dangerously high.

In practice this would mean the charger would go into topping-off mode much too soon, and would stop too soon. The recharge cycle would be longer, and it wouldn't use the full capacity of the cells. Not very satisfactory, but at least it would be safer.
posted by Steven C. Den Beste at 2:44 PM on January 11, 2008

knave writes "That doesn't exactly make sense, because AA and AAA are two different sizes, yet they both have the same voltage. However, if I take the assumption that everyone is right, 1.2V is the base voltage for NiMH chemistry, then I'm guessing you can only achieve integer multiples of that voltage by stacking them. (Whether stacking multiple batteries, or multiple cells within a battery.) Is that basically the issue?"

Essentially yes. See How to make a B Battery for an example of rolling your own voltage.
posted by Mitheral at 2:46 PM on January 11, 2008

I have a theory!

The + terminal on Eneloops (and other slow-self-discharge NiMH batteries) are not quite the same as the nubs on standard Alkaline batteries. The battery case I am using for a small electronics project (similar to this)had a plastic piece that I had to shave off with a knife before the electrical connection was made.

Look around in there to make sure that your + terminals are actually touching metal rather than plastic.
posted by aubilenon at 3:19 PM on January 11, 2008

Any decent LED lamp has a converter that provides a fixed current regardless of the supply voltage. There is no current limiting resistor like the one noble_rot talks about. The converters are more efficient for higher supply voltages, but accept a wide range of input voltages while giving the same light intensity. Many (most?) of the switching LED drivers commercially available do upconversion from low voltage, so a single NiMH cell can drive even a high-voltage white LED.

This makes me think the 20% voltage difference isn't your problem. It could be the internal resistance, but that's high for alkalines too, so probably not the most likely cause. The best way I can think of to check it is to connect four batteries in series between the terminal where you'd usually put three. If the lamp lights up then, you need the higher voltage. If it's still dark, check your charger and make sure the batteries are really charged to 1.2V (you know they're likely to be discharged when you buy them, right?).
posted by springload at 3:33 PM on January 11, 2008

White LEDs have a forward voltage somewhere between 3 and 4 volts. If your LED flashlight and headlight have simple-minded driving circuitry consisting only of a current-limiting resistor (which seems likely to me, given the design choice of three cells) then they probably really do need 4.5V input, or close to that, instead of 3.6V. If that's the case, you will probably notice that they run OK for a while on alkalkines, then suddenly poop out; and you'll also find that alkalines that won't run these things at all any more will be fine in a clock or a radio for quite some while afterwards.

Changing the value of the current-limiting resistor isn't going to help much, if at all. What you really need is a small constant-current switching power supply to drive the LED. Maxim makes these.
posted by flabdablet at 1:34 AM on January 12, 2008

To answer your "how is it OK" question: NiCD and NiMH battery chemistry gives these cells a very low internal resistance, so if you're pulling large amounts of current from them (e.g. driving a motor in a cordless drill, or a motor-drive and flash and assorted computing electronics in a camera) they will still be supplying something very close to their rated 1.2V without causing excessive internal heating. Drive the same kind of circuit from a nominally 1.5V alkaline battery, and you will probably find that the alkaline's internal resistance causes the voltage available at the battery terminals to drop to 1.2V or below in any case. So any high-current device designed to run OK from alkalines will probably also run OK, or maybe even better, from NiMH.

LED flashlights would typically draw less than 50mA current, which is not a huge amount. They're therefore not an ideal match for the grunty but low-voltage NiMH cell chemistry. The rechargeable alkalines that SCDB linked to would be a better fit. I've used them, and they work very well for low-current-drain applications like wireless mice. Their advantage over NiMH and NiCD cells is their much lower self-discharge rate - you can quite happily run a wall clock of one of these things for two years, just like you could with a standard alkaline cell, where a NiMH would flatten itself in a month; their main disadvantage is that they do die after fewer recharge cycles than NiMH.
posted by flabdablet at 1:46 AM on January 12, 2008

I'm with ManInSuit -- it sounds like a different issue. The only way I can see 1.2V not working is if they use a specific linear voltage regulator that doesn't accept anything below, say, 1.4V. I don't think that's the case, though, since the reason they sell these is because they know that most consumer electronics are tolerant to 1.2V vs 1.5V.

Just as another data point, I also use rechargeable NiMH batteries to power my bike's headlights with no noticeable change in light intensity.
posted by spiderskull at 2:47 AM on January 12, 2008

1.5v*3=4.5v , 1.1v*4=4.4v

Maybe you could just add another rechargable?
posted by Orb2069 at 8:29 AM on January 12, 2008

Orb, there's no room for more.

However, RussHy's aluminum foil shim worked! Let there be light!
posted by knave at 9:27 AM on January 12, 2008

Also, thanks for all the replies in this thread. There is a lot of good information here, even if the voltage wasn't my real problem.
posted by knave at 9:28 AM on January 12, 2008