Dumb battery question

[oldtimer]

I'm new here, and being awake 3am in the morning doesn't help either. /ubbthreads/images/graemlins/blush.gif But I was wondering since Li123s run at 3.0V, why are the Pilas undercharged at 3.7V? And why are the rechargeables overcharged at 4.2V? Can't they just make one taht is exactly 3.0V? /ubbthreads/images/graemlins/icon23.gif

 

[3rd_shift]

I don't think anyone has been able to solve that riddle yet.
Many have tried already. /ubbthreads/images/graemlins/banghead.gif

 

[markdi]

the different chemistry involved in the construction of
the cells or battery's result in different voltages and capacity per cubic volume etc.

There is no such thing as a dumb question.

 

[oldtimer]

3rd_shift: The whole riddle? Or just parts of it?

 

[oldtimer]

markdi: So I guess that's because it's too new, and the phenomenon will remain until someone standardizes them? (I'm trying to make a comparison between this and the early NiMH cells.)

 

[Steelwolf]

I doubt that it is a matter of standardisation. It is a matter of whether they can find a chemistry that can satisfy the criteria for a good rechargeable cell, such as fast recharge times, good cycle life, high capacity and cost-effectiveness. And, of course, being close enough in nominal voltage to the primary cell it is meant to replace.

Lithium-ion and lithium polymer are pretty close to lithium thionyl chloride, though not as close as NiCd and NiMH to alkaline cells.

Improvements made to Ni-XX rechargeable cells over the years have been more in terms of capacity that nominal voltage. This is because, to get a voltage closer to the primary cell, the chemistry needs to change and the change may not result in a chemistry that would perform well as a rechargeable.

Capacity, OTOH, is more due to physical construction than chemistry. More reactant squeezed in to the same nominal volume by thinning the walls and the size of the electrodes, smaller particle sizes for greater reactive areas, maybe a couple of additives to ensure that the particle sizes remain small instead of conglomerating. Stuff like that.

One of the major changes in capacity did come from a chemistry change from cadmium to metal hydride and a fair amount of improvement since then has actually come from finding a better metal for the "metal" part of the metal hydride. But because the basic reaction has been nickel-hydride, the nominal voltage has not changed.

As development of the rechargeable lithium cells progresses, we will definitely get better capacities, but it is highly doubtful that they will get closer to the nominal 3.0V mark with the current chemistry. We will probably find them marketing another lithium-xxxx cell that may be closer or they may abandon the lithium altogether and find another combination that works better.

 

[markdi]

I dis agree with part of your capacity statement
some chemistrys such as lithium have far more energy per unit volume than others.

 

[Steelwolf]

markdi: Sorry... I should have qualified my statement. Yes, certain chemistries have more energy per unit volume. I was referring to the progress of the NiMH batteries. When they started out, they were immediately a big improvement from NiCd capacities, and that is evidence of the benefit of changing to a more energetic chemistry. But since then, the basic chemistry has not altered, yet they have at least doubled in capacity and also in peak current supplied.