Stupid question... Why 1.2v on rechargeables?

wildstar87

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I have wondered this for a long time, and initially just thought it was a technology limitation. Now though I'm almost positive they could make a 1.5v rechargeable, but don't. I guess part of it might be that they have done it this way for so long they just don't want to change.

I'm more curious as to the initial reason for going 1.2v and why (other than tradition) they don't just make 1.5v cells instead for the typical battery sizes (i.e. AAA, AA, C, D)

Seems like it would make things a lot simpler. I've searched for the answer, but couldn't find a wiki entry or anything on an urban legend or anything like that.
 

Marduke

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It's inherent to the NiMH chemistry. They actually start out at 1.42v off the charger, and maintain their voltage under load much better than "plain old alkalines"
 

Mr Happy

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It's because the voltage is not a free choice. When you try to find a battery chemistry with the features you are looking for (e.g. rechargeable, long lasting, inexpensive, safe, etc.) then the voltage chooses itself. You get whichever voltage the universe chooses to offer you and you must take it or leave it.
 

VidPro

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check out the CHARTS in the stickeys for ni-mhy and alkaline
for MOST devices that ~1.2v is very similar to the alkalines speced ~1.5v when used in a device.
i dont think you can get a better balance of voltages for replacing alkalines without having more problems.

meaning if the battery STAYED at 1.5V for the entire time many devices would have a problem with that, many more than have a problem because of not enough total voltage.

the charts that show the discharge curve of both shows the good reason why ~1.2v works so well as a replacement for alkaline.


https://www.candlepowerforums.com/threads/64660
https://www.candlepowerforums.com/threads/79302

these 2 charts, figure about how much power your device uses, then look at that discharge curve for each TYPE of battery.
that answers the question quickly.
 
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Darkpower

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Now though I'm almost positive they could make a 1.5v rechargeable, but don't. I guess part of it might be that they have done it this way for so long they just don't want to change.

They do make rechargeable alkalines known as RAMs (rechargeable manganese alkaline) that have a nominal voltage of 1.5.

Rayovac originally marketed these RAMs in the late 1990s under the trade name "Renewals". Rayovac discontinued them sometime around 2003 and went with NiMh batteries. Currently you can still buy RAMs made by GP, Pure Energy and Juice. The advantages that the RAMs have are they have literally no self-discharge and they are perfect replacements for regular alkaline batteries.

The disadvantage is they have limited recharging cycles like 25 for deep discharge upwards of a 100 recharges for shallow discharge. Also their voltage sags under load and the voltage discharge curve is similar to regular alkalines as indicated by the previous poster. If you have a heavy load, they will soon discharge well below the 1.2 volt of a NiMh.

Also you must use a charger made for them. Rayovac still makes a charger the PS3 that charges all chemistries, RAM, NiCd and NiMh.

I bought about 16 AA cells from Pure Energy which are a newer generation designed to be recharged up to 500 times (shallow discharge) and a dozen from 'Juice' to try them out. I also bought their charger.

I used the Rayovac when they came out with them in the late 1990's. Oddly I found many of those in one of my 'junk' drawer not to long ago and they all had a good charge on them (1.35 to 1.5 volt), more than 6 to 8 years after their last charge. I recharged them and they are still good.

I use them in devices like a small portable radio. That radio prefers fresh alkalines at around 1.5 volts. If the voltage is around 1.3 volts or lower, it has trouble tuning distance stations, and so the NiMh were always marginal, although they did work. The biggest problem is that the NiMh would self-discharge since I use that radio intermittently for a few hours a week. The MiNhs would literally self-discharge before I actually used the full charge. With the RAMs, I can use the radio for several months on a charge, and I can top off the batteries at any time.

The RAMs are ideal for flashlights that are reserve for occasional emergency use. The nice thing is they have little if no self-discharge and the starting voltage is around 1.55 to 1.60 volts. You can always top-off the batteries and it is recommended that the batteries get topped off frequently. Complete discharges ruin the battery and deep discharges to 1.0 volt will limit the life cycle to about 25 cycles.
 
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LuxLuthor

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All good information given.

I suspect the decision to have NiMH be a nominal 1.2V value is likely a very complex interplay of being useable by many 1.5V Alkaline applications (as others have said....making the NiMH perform in much of the same range), but also based upon electrolyte formulation, number of layers, ion/charge migration rate, cathode compositions, overall surface area of various components, heat tolerances, gas pressures, component prices, reliability, safety, etc.

Ultimately it was the result of Ovonic perfecting a workable manufacturing process to work at this voltage and licensing it to all the manufacturers (but based on work done by Dr. Masahiko Oshitani from Yuasa Company, Philips Laboratories and French CNRS). WiKi page information here.
 

Derek Dean

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BTW wildstar87, I didn't find it to be a stupid question, and I've wondered the same thing, so now we both know! Thanks to everybody for all the information.
 

wildstar87

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I guess I can understand why now, but I still think they could actually make a 1.5v rechargeable. Seems weird that both Ni-cd and Ni-mh are both exactly 1.2v

I think equipment would be ok with 1.5v all the time, they have power adapters for a lot of equipment.

In any case it was interesting info, thanks!
 

Mr Happy

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I think both NiCd and NiMH are 1.2 V because of the common nickel-based chemistry. If the cell used some metal other than nickel then the voltage could be different from 1.2, but then the cell would not perform the same way (would not be able to hold so much charge or withstand as many charge/discharge cycles). In other words it would not necessarily be a useful cell any more.

Of course, someone may discover a totally wonderful new rechargeable chemistry tomorrow that does have a nominal voltage of 1.5 V, has a greater charge density than lithium polymer, has no self-discharge, has no internal resistance, can deliver currents up to 100C, can withstand 1000s of cycles without deterioration, is quite safe and has no tendency to vent with flame, contains no hazardous heavy metals, and is cheap to manufacture. However, don't hold your breath while you wait...
 

alfablue

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I guess I can understand why now, but I still think they could actually make a 1.5v rechargeable. Seems weird that both Ni-cd and Ni-mh are both exactly 1.2v
except, given the above answers, it seems that they are not exactly 1.2v, they are just nominally rated at that.
 

Marduke

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I guess I can understand why now, but I still think they could actually make a 1.5v rechargeable. Seems weird that both Ni-cd and Ni-mh are both exactly 1.2v

I think equipment would be ok with 1.5v all the time, they have power adapters for a lot of equipment.

In any case it was interesting info, thanks!

Fully charged, they are actually around 1.42v,and under load will keep a much higher voltage than the 1.5v alkalines.
 

Anders

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Hello wildstar87.

This is a disharge test with a Varta Alkaline 1,5 Volt and MAHA Imedion Nimh 1,2 Volt.
The disharge rate is 0,5 A.

After 0:37:09 Varta showed 1,20 Volt
After 3:19:57 Imedion showed 1,20 Volt

It took 0:02:00 minutes to equal voltage, 1,42 Volt



Anders
 
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Illum

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Keep in mind that the batteries voltage is relative, not absolute, to its remaining capacity

Alkalines are rated 1.5V, new cells have sometimes measured 1.8V [OCV]
NIMH's as said are rated at 1.2V but is 1.45V off the charger
Li-ions are rated 3.6V, but their 4.2V off the charger...a 3.6V li-ions at most 10% remaining capacity and sags further under load
 

mdocod

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In my experience the 1.2V/1.5V difference is very misleading. It causes this question about 1-3 times a month to pop up here.

For some reason, that I have never understood. rechargeable cells have a label voltage that represents something in the ballpark of the output voltage that could be expected towards the END of a discharge into a typical load. It is kind-of like a conservative minimum expected rating. This holds true for cells of many chemistries. Just about any NIMH will hold above 1.2V into typical consumer loads, (0.5-1C) until it is almost drained. Li-Ion, being listed as either 3.6 or 3.7V is actually close to completely drained at that point into a typical load (0.1-0.5C is common in li-ion consumer devices). The New LiFeP04 cells and Nano-Phosphate (both alternative Li-Ion chemistries) are rated ~3.2-3.3V but tend to come off the charger around 3.8V and hold above those listed voltages into very harsh loads (1-10C)...

On the other hand, I have found, for a reason beyond my understanding, that the labeling of PRIMARY (single use cells) seems to be done much differently. They tend to list a nice sounding number that is very close to the initial full voltage of the cell instead. A 1.5V alkaline is only 1.5V for the first few moments, often it never even delivers 1.5V into a load, (immediately sagging to 1.45V and steadily dropping off from there is the norm), while they are commonly 1.6V open circuit fresh out of the package, only a very light load would ever get 1.6V from that cell. The same holds true for Primary CR123 photo lithium cells, labeled at 3.0V on the side of the cell, they will spend most of their life delivering 2.75-2.25V into various loads.

As has already been said before, As a result of this labeling variation, a typical "1.2V" NIMH cell will actually deliver higher voltage than a typical "1.5V" Alkaline cell through the MAJORITY of the discharge.

For conspiracy theorists, one might come to the conclusion that battery manufactures are doing everything in their power to hide the limitations of alkaline cells, by doing things like listing them at 1.5V they cause joe public to think that the cells are "superior" in some way.
 

hopkins

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Hello mdocod
thankyou very much for the concise and educational post. You've pointed
out what should be uppermost on a persons criteria when considering
alkaline or NimH. Good Job!
I may have to print it out and quote you during the battery arguments
between coworkers.

Think if I was making NimH batteries in China, I'd change the packaging to
read 'New and improved for 1.42Volts' output in a large font.
Probably my sales would go way up.
 

TMorita

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...
For some reason, that I have never understood. rechargeable cells have a label voltage that represents something in the ballpark of the output voltage that could be expected towards the END of a discharge into a typical load. It is kind-of like a conservative minimum expected rating. This holds true for cells of many chemistries. Just about any NIMH will hold above 1.2V into typical consumer loads, (0.5-1C) until it is almost drained. Li-Ion, being listed as either 3.6 or 3.7V is actually close to completely drained at that point into a typical load (0.1-0.5C is common in li-ion consumer devices
...

Erm...I have a bit of experience with Li-ion batteries, having used them with R/C helicopters.

The cutoff voltage recommended by Li-ion manufacturers varies between 2.8 volts and 3.2 volts per cell. The better quality batteries seem to tolerate a deeper discharge, and their manufacturers specify about 2.8 volts. The cheaper ones are usually either 3.0 volts or 3.2 volts. So the statement about a Li-ion cell at 3.7 volts at the end of the discharge is not accurate.

Lot of information on RCgroups about this:

http://www.rcgroups.com/forums/showthread.php?t=209187

Toshi
 

mdocod

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The cutoff voltage recommended by Li-ion manufacturers varies between 2.8 volts and 3.2 volts per cell. The better quality batteries seem to tolerate a deeper discharge, and their manufacturers specify about 2.8 volts. The cheaper ones are usually either 3.0 volts or 3.2 volts. So the statement about a Li-ion cell at 3.7 volts at the end of the discharge is not accurate

I must defend my statement, as I believe it to be accurate and can prove it.

please refer to this thread:
https://www.candlepowerforums.com/threads/117117

find a cell operating at around 0.5C (like a 2000+mAH cell operating at 1amp). which is the type of load I was talking about. The LG 2400 chart is by far the best example that proves my statement correct, you have a cell tested at 1 amp which is about a 0.44C discharge rate, that reaches 3.7V around 75% into the run.

A Li-Ion cell has depleted most all of it's capacity when it's dragged down to 3.5V under a load, eeking out that last bit to 2.8V isn't worth it it's hard on the cell.
 

SilverFox

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Hello Toshi,

Keep in mind that some of the discussion revolves around loaded voltage, and other parts revolve around open circuit resting voltage.

I believe if you have a Li-Ion, or Li-Po cell or pack with an open circuit resting voltage of around 3.7 volts, it wouldn't be recommended for "flight."

Tom
 

TMorita

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I must defend my statement, as I believe it to be accurate and can prove it.

please refer to this thread:
https://www.candlepowerforums.com/threads/117117

find a cell operating at around 0.5C (like a 2000+mAH cell operating at 1amp). which is the type of load I was talking about. The LG 2400 chart is by far the best example that proves my statement correct, you have a cell tested at 1 amp which is about a 0.44C discharge rate, that reaches 3.7V around 75% into the run.
...

Okay, looking at the charts that show a 0.5c discharge rate in that thread, I see:

Microfire 168A 2150maH at a 1.0 amp discharge rate to 3.7 volts:
0.7 amp-hours out of a total 2.0 amp-hours, so about 35% of rated capacity

Pila 600P 2200maH at a 1.0 amp discharge rate to 3.7 volts:
0.6 amp-hours out of a total 2.0 amp-hours, so about 30% of rated capacity

Wolf-eyes 168A 2000maH at a 1.0 amp discharge rate to 3.7 volts:
0.25 amp-hours out of a total 2.0 amp-hours, so about 12% of rated capacity

LG 18650 2400 maH at a 1.0 amp discharge rate to 3.7 volts:
0.75 amp-hours out of a total 2.4 amp hours, so about 30% of rated capacity

Tenergy 18650 2400 maH at a 1.0 amp discharge rate to 3.7 volts:
0.25 amp-hours out of a total 2.4 amp-hours, so about 10% of rated capacity

Emoli 26700 2900 maH at a 1.0 amp discharge rate to 3.7 volts:
1.5 amp-hours out of a total 2.9 amp-hours, so about 50% of rated capacity

...etc.

To me, it looks like the charts in the thread you cited do not support your assertion, and in fact, disprove it.

Toshi
 
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