Why are NiMH 1.2v instead of 1.5v

[hopkins]

jaygrant - I've also been a bit steamed over the 1.2 & 1.5 volt package labels.

Somehow I'd be more satisfied if the news came out that our Shadowy Overlords were messing with our heads just to be mean rather that something as boring and ordinary as 'Electrochemistry'. Thats what we'd
like to hear I think.

Your question jaygrant is the same vein as 'why are there only 15 calories in a level teaspoon of white granulated sugar? Why can't someone make there be more?

I'd be quite happy to buy NimH AA cells in a package labeled 1.4volts. Why not? In the charger at full charge with a small trickle they're 1.4volts or more.
Could the Consumer Product Safety people file an injunction?

 

[LuxLuthor]

Yes, it is indeed a dumb question. Why is the sky blue and not red? Why are plants green and not blue? Why aren't humans all the same size? It would be a lot easier for clothing manufacturers if all humans were standardized, wouldn't it?

LOL, yeah that was pretty brutal. Oh well.

 

[lctorana]

A fresh zinc-carbon cell delivers exactly 1.56 volts into a near-open circuit. You can, and people did, calibrate your voltmeter on that.

But under load, they deliver a falling voltage, and bulbs have always been rated in multiples of 1.2 volts.

A fresh zinc-chloride cell delivers about 1.60 volts into a near-open circuit. But under load, they too deliver a falling voltage, and bulbs designed for these cells are also rated in multiples of 1.2 volts.

A fresh alkaline cell also delivers over 1.5 volts into a near-open circuit. Under load, they too fall off, but maintain an initially higher voltage, and bulbs with a design voltage of 1.4 volts per cell are available to take advantage of this. However, if you subject these cells to long, continuous service, then 1.2V per cell is what you're gonna get.

Charged and rested NiCad cellsdeliver about 1.3V per cell into a near open circuit, but will initally hold at 1.25V no matter how heavy the load, within reason. This figure drops to a constant, reliable 1.2V for the vast majority of the discharge cycle. Hence the standard 1.2V-per-cell bulbs work beautifully with NiCads.

Charged and rested NiMH cells deliver about 1.4V per cell into a near open circuit, but, under load, will deliver about 1.25V for the vast majority of the discharge cycle. So the standard 1.2V-per-cell bulbs also work well with NiMHs.

There are also Lithium AA and AAA cells, but I've never used them. Heard tell the open-cicuit voltage starts at around 1.7V, but that's all I know.

The point is this. The above six different chemistries all have widely varying open-circuit voltages, but if you put two of any of them in, say, a 2AA Minimag, all six will deliver, give or take, 1.2 volts per cell to the bulb. A little more at the start, a bit less at the end. The main difference is that of useful runtime.

And snakyjake, thanks for the "I've wondered if the limitation was chemical or technology." That joke works on three different levels all at once. Very, very clever indeed.

 

[Sandman_Bravo]

Obviously you guys have an extensive background in chemistry and electronics. I'm a simple man and all I know is my digital camera only works with rechargeable NiMH AA batteries; alkaline won't work. Somehow the circuit board won't accept them. It turns on then says, "low battery" and shuts off. :sick2:

 

[cave dave]

Too top it all off the small variation in voltage from different chemistries is meaningless in a high quality flashlight or other high quality devices. What matters is the ability to deliver power and capacity under load.

A good example is the Alk vs. Nimh runtime graph for the Fenix L2D:
L2D Runtime Chart

From the following review: (Dial up beware!)
http://www.cpfreviews.com/Fenix-L2DCE-Premium-Q5.php

**EDIT**
Actually maybe that wasn't the best example. The L2D has extremely flat regulation on high. The majority of decent lights and the L2D on MAX show a declining output on Alks and a flatter output for Nimh.
http://www.cpfreviews.com/LiteFlux-LF/RT_LiteFlux-LF2-Cree.gif

 

[SilverFox]

Hello Cave Dave,

Your link to the Green Batteries page is interesting. They seem to have run their discharge test with a used Alkaline cell. You will notice that both the Alkaline and NiMh cell started out at the same voltage.

I looked back through my archives and came across this graph that shows a comparison of an Alkaline and NiMh cell. The Titanium cell is rated at 2000 mAh. This discharge is also at 500 mA.

You will notice that the slope of the Alkaline voltage during discharge is similar in both graphs, but in my graph the voltage starts off at 1.5 volts. Also note that the Alkaline cell, at this discharge rate, has a voltage advantage during around the first 30 minutes of discharge. Then the NiMh cell holds a higher voltage.

You may want to inform the people over at Green Batteries that they may want to run another test with a fresh Alkaline cell this time...

This graph may also help explain the differences in the way NiMh and Alkaline cells are labeled. As you can see, the Alkaline cell starts out at around 1.5 volts, then gradually drops off in voltage from there. The NiMh cell quickly drops to around 1.2 volts, and spends most of its discharge time at 1.2 volts.

Looking at the discharge graphs, I think it becomes apparent why Alkaline cells are called 1.5 volt cells, and why NiMh cells are called 1.2 volt cells.

Tom

 

[hopkins]

Silverfox - well stated. I was thinking of why this subject comes up every so often. Is it a bit like the often debated question is the glass half empty or half full?


The psychological affect on ordinary peoples buying choices of packages labeled 1.5volts or 1.2volts is probably a factor.

cheers
hopkins

 

[Mr Happy]

There are some other interesting things about alkaline batteries that are described, for example, in the Energizer application manual: http://data.energizer.com/PDFs/alkaline_appman.pdf

One is that 500 mA continuous discharge is considered heavy drain for an AA cell. For instance, the capacity down to 0.8 V is only 1400 mAh at 500 mA, but is nearly 2500 mAh at 100 mA and 2800 mAh at 25 mA. Another is that alkaline batteries perform much better with intermittent discharge than with continuous discharge. At a 500 mA continuous discharge the capacity is only 1400 mAh but with a 10 second on/90 second off duty cycle, the capacity increases to 1800 mAh.

Therefore it is important to choose the right battery for the application. For low drain and intermittent loads, the nearly 3000 mAh capacity of a good alkaline AA cell is hard to beat for economy of operation in a primary cell.

Choosing the right battery for the duty is one reason why really huge zinc carbon batteries are still manufactured. Due to the size of the battery the relative discharge rate can be very low, and therefore the battery can operate in its most efficient discharge region and be very economical.

 

[Nubo]

Ok, so it's another dumb question, but I haven't seen the answer yet. I'm sure it's probably here somewhere, but I can't find it.

Why are NiMH batteries 1.2v instead of 1.5v? If they are supposed to be a replacement for Alkaline, why not 1.5v? I can't believe it's a technology issue. Maybe a marketing issue, to keep Alkaline alive, so why 1.2v?

As people have said, it depends on the chemicals in use. Look at it this way -- the electricity is the result of a chemical reaction. If you took the molecules involved, just enough to do ONE reaction, and let them react, you can consider that to be one very very small battery. It produces an infinitessimally small current for a infinitessimally brief time, but it has the same voltage as the battery. The voltage potential of those particular chemicals is always the same. When you put billions of these molecules in a can and let them react gradually over time, you then have billions of these tiny batteries turning on over time. They are all wired in parallel, so to speak. So the voltage stays the same but the current increases. This is why a small 2-inch lead-acid battery that you may use as a battery backup in an alarm system can have the same voltage as a huge lead-acid battery that you use to start a diesel truck.

 

[Sandman_Bravo]

^That's a very cool concept, when you put it that way.

 

[Mike Painter]

Thanks.

So the chemistry couldn't have been manipulated for a .3v difference in the result. Hmmmmm.:thinking:

The chemistry pretty much decides what the voltage of a cell will be.

But teh voltage of an alkaline will drop below 1.2 volts in fairly short order and because of the internal resistance of an NiMH most LEDs will be brighter under the lower voltage.

Back in the "Old Days" two or three years ago people often let the magic smoke out of their lights when they went from alkalines at some multiple of 1.5 to NiMH at a multiple of 1.2

 

[kusitb]

You also have to realize how those numbers are calculated. Alkaline 1.5v is peak voltage. Under load, they almost immediately sag below 1.2-1.3v.

NiMH start out around 1.38v, and maintain >1.2v under heavy load. Compared to alkaline, they will maintain a higher voltage under load over the majority of a discharge.


And yes, it's simply because of the chemistry. Each electrochemical battery type has it's own unique nominal voltage produced. The chemical cocktail used is what decided the voltage. It's much easier to make good cells and adapt to their voltage than to standardize on a single voltage and suffer crappy cells which are negatively altered to produce ONLY that voltage.

Hi,

A couple of months lurker, first time poster...

I've used NiMH cells in several devices, but only up to 4 AAs in a single device. I've had no problems whatsoever.

But now, I am shopping around for an emergency lamp/radio device and the one I am considering uses 8 AAs.

http://www.batterysavers.com/best_amfm_portable_noaa_weather_radio.html

Question: Is it safe to say that 8 NiMH AA cells will be able to power this device with no problem? My main concern is that the voltage difference will be multiplied 8 times to a point that the total voltage is too low for the device?

TIA,

-kusitb

 

[HKJ]

Question: Is it safe to say that 8 NiMH AA cells will be able to power this device with no problem? My main concern is that the voltage difference will be multiplied 8 times to a point that the total voltage is too low for the device?

Alkaline is supposed to be drained below 1 volt before they are empty, i.e. if the radio cannot use NiMH it can only use a fraction of the alkaline capacity.

You can see curves for Energized AA here: http://data.energizer.com/PDFs/E91.pdf
They go down to 0.8 volt.

 

[kusitb]

Thank you HKJ, that is very informative. I didn't know about the 0.8 volt before, and knowing that now makes me comfortable in buying this device.

 

[Etsu]

The only device I've noticed a slight problem running NiMH cells is one of my (infra red) remote controls for a TV box. It works, but the range on it is more limited than when I run it with fresh alkalines. I think the lower voltage causes the IR emitters to be less bright, so that affects range.

It's a crappy remote on either battery chemistry, though. So I think it's just these borderline cases where alkalines win out.

 

[Nubo]

I find NiMH work fine for the great majority of devices. But there are some devices that can be very picky about voltage. For example, my wife has an expensive device that doesn't run very long on (good eneloop) NiMH. It will run only for a few days, vs. weeks on alkalines. This seems counter-intuitive. What I suspect is this:

It's obviously a low-drain device. So the alkaline voltage will be higher than NiMH. While the circuitry might function fine on 1.2V per cell I suspect the manufacturer very much wanted to guard against damage from cell leakage due to the cost of the device. The best way to do that would to be very conservative in terms of depth of discharge and so does a voltage test based on the assumption of an alkaline cell discharge curve.

And so you can occasional find a device that has a design bias towards one type of cell, in this case alkaline. Or in the other poster's case, their camera prefers NiMH -- perhaps measuring the voltage while pulling multiple amps to charge the flash. An alkaline AA voltage will sag pitifully under such a load.

The differences accumulate the more batteries you are running in series.

When I was flying I had a portable aviation radio that wanted 12 volts. It had 10 AA slots. You could use 8 alkalines and 2 dummy metal cells, or 10 rechargeable cells (NiCD at the time). Not sure if that was necessarily a voltage issue. It could have been to increase the energy payload. Those NiCD cells were pretty low capacity; about 800mAh, if I recall.