Why are nimh batts 1.2 volts, and lithiums 3.6?

[woodrow]

I know that this has to be the dumbest question ever, but why are AA nimh rechargables 1.2 volts instead of 1.5 like alkalines? Are there any 1.5v nimh AA's? Also, why arn't 18650's 6 volts like the 123a primaries they replace? Thanks for your help!

 

[blackdragonx1186]

the nimh question has been asked and answered already on here, somewhere. as for the 18650's, its prolly 3.7 because if it was 7.4 nom., that would cut the capacity severely. thats prolly not the real reason, but it makes sense to me

 

[mdocod]

You have to turn around the thinking for it to make sense.

when designing a new type of cell, the designers set out with some goals in mind. Voltage is rarely one of those goals because you can always design the device around whatever voltage becomes of the design, or multiple cells can be arranged to achieve higher voltages when needed.

When designing a li-ion cell, main goals were probably energy density, cycle life, and low self discharge. They achieved those goals when they put a Cobalt cathode in a lithium chemistry cell, perfected it, and discovered that they had created a 3.6V cell. It's the result if the chemistry of the cell and there is no avoiding it. There's no way to make cell of the same chemistry have some other voltage, it's just not possible.

Alkaline happens to be 1.5V, because that's what alkaline is. heh...

In reality, alkaline cells discharge under a load from ~1.5V quickly diminishing to ~1.2V then finally finishing at ~0.9V give or take. When compared to the discharge graph of a freshly charged NIMH cell, the NIMH cell actually maintains higher voltage through the MAJORITY of the discharge, and often has much longer runtime in high drain devices. The only part of the discharge that the alkaline beats the NIMH is in the first few minutes.

 

[woodrow]

Thanks!

 

[Ninjaz7]

Chemical composition :thinking::tinfoil:

 

[2xTrinity]

I know that this has to be the dumbest question ever, but why are AA nimh rechargables 1.2 volts instead of 1.5 like alkalines? Are there any 1.5v nimh AA's?

This has already been covered, but there's a difference in notation here. Primary batteries (eg, disposable batteries) usually report their starting voltage, which is 1.5 in the case of Alkalines. Rechargeables usually indicate their AVERAGE voltage throughout the runtime, this leads to confusion and a lot of misinformation that NiMH batteries won't work as alkalines replacements because ther voltage is too low -- simply not true.

In either case though, the voltage of the batteries has to do with the chemical reaction that is used to store the energy.

Coincidentally, 3 NiMH in series just happens to have a voltage curve VERY similar to a single LiIon cell. (ie, LiIon is 4.2V peak, 3.6V nominal... NiMH is 1.4V peak, 1.2V nominal)

Also, why arn't 18650's 6 volts like the 123a primaries they replace? Thanks for your help!

One point worth noting is that using 18650s in flashlights (eg, to replace 2x CR123s) is an afterthought. 18650s are basically the most commonly used to build laptop computer battery packs, and hundreds of millions are manufactured for that purpose. The number of 18650s used in flashlights is probably only in the tens of thousands.

 

[Zerius]

The voltage of a cell is determined by the difference in the redox reaction occuring in the cell. Reduction - Oxidation. Every different element and compound has a different Eo (E not) value, and the a cell is primarily an anode as well as a cathode (2 different metals) the voltage difference in the metals determines the starting voltage. As the cell is used more and more the internal resistance builds up and according to emf the internal resistance acts as a resistor and lowers the potential (V) in the battery so therefore cells are dead when the potential difference becomes equal to the internal resistance in the cell.

 

[yellow]

Also, why arn't 18650's 6 volts like the 123a primaries they replace?

they are!
ONE 18650 and ONE CR123a are the same voltage.
But You are replacing TWO CRs in series with just one 18650, so ...


the rest is, was has been typed already, chemistry.
f.e. You forgot lead batteries with some 2.1 V / cell
(even if You put some wires into a potatoe, You would get a voltage reading --> again: chemistry)

 

[Mr Happy]

As the cell is used more and more the internal resistance builds up and according to emf the internal resistance acts as a resistor and lowers the potential (V) in the battery so therefore cells are dead when the potential difference becomes equal to the internal resistance in the cell.

I think it is rather more complicated than this though, as in most cases* the open circuit voltage tends to drop as the cell becomes discharged. Internal resistance cannot account for that since no current is flowing when measuring the OC voltage.

(*) Exceptions include cells like mercury oxide that were used as a constant voltage reference. Unfortunately mercury oxide cells have been banned and there is no convenient replacement for them.

 

[Fallingwater]

Coincidentally, 3 NiMH in series just happens to have a voltage curve VERY similar to a single LiIon cell. (ie, LiIon is 4.2V peak, 3.6V nominal... NiMH is 1.4V peak, 1.2V nominal)

The curve may well be similar, but my experience is that gadgets that normally run on LiIon and that aren't made to run explicitly from both chemistries do not particularly like to run from 3xNiMH. They do work for a short while, then they start flashing the low battery warning and shut down shortly afterwards.

If the gadget's converter can handle it (it often can), 4xNiMH will ensure it works properly.

 

[12Johnny]

I also wanted to know about that, so thanks for the information! :thumbsup:

 

[Fallingwater]

Note that, due to the higher voltage, the gadget will lose all ability to detect a low battery situation; when the batteries run out of steam, it'll shut down instantly and with no warning.

This can give rise to situations such as lost or broken files (digicam and/or mp3 player suddenly shutting down while writing data), or simply finding that your gadget fllatly refuses to stay on for more than 10 seconds without apparent reason.

I've done plenty of such mods, but I always carry spare batteries for mission-critical stuff, so the instant-off problem doesn't bother me too much.

 

[kurni]

Thanks for the explanation, everybody; I wondered about this but haven't gotten around to look for the answer, but the answer presented itself.

Eric, you've been busy, haven't you

Regards,
Kurni