Is there a limit to the number of kilo-watt hours an AAA (or AA) battery can hold?

Question

Or in other words, is there a limit to total battery capacity? Is the limit a physical one or is it an economical one?

Answer 1

Yes, batteries act like fuel tanks: the bigger the tank, the greater the total energy stored. Also note that different types of AA cells can store different amounts of energy: Lithium cells can store more energy than NiCd cells, etc.

Why does battery size determine the total energy available? Simple answer is that batteries are a type of "Fuel Cell" but with all the "fuel" contained on board. A battery is a pump for electric charges, and when its internal fuel is exhausted, the "electricity pump" stops running.

In a standard fuel cell you'd inject a certain amount of hydrogen and oxygen into the device, which then would send a certain amount of electrical energy out into the connected circuit. But what if you had small tanks of H2 and O2 on board the fuel cell? In that case the cell would only operate until the fuel was used up (and turned into water.) If you ran it at higher power, the fuel would be consumed faster. And a bigger device would provide more H2-O2 fuel in its larger tanks.

Batteries are only a bit different: they typically use solid metals as fuel, turning them into solid metal compounds. The battery weight doesn't change as the fuel gets used up. The fuel turns into solid waste. (This also lets us explain rechargeable batteries: whenever we force electricity backwards through the cell, it converts the waste products back into the metal "fuel" again, and energy is stored, because work is required to force the "pump" backwards.)

And as always, electric circuits become much more understandable once you eliminate typical physics misconceptions:

http://amasci.com/miscon/eleca.html#batt

Answer 2

The measures of energy density are mass energy density and volume energy density. When a fixed size package is used (eg AA / 14500 cell) the Watt hour capacity or for a given chemistry the mAh capacity are of interest for comparison purposes.

Figures of around 400 Watt-hours per litre and around 150 Wh/kg are "doing well". Consumer AA NimH cells are approaching 400 Wh/l capacity now. This is above the target figures quoted for LiIon less than 10 years ago.

Higher to much higher figures are claimed for experimental cells using new chemistries but none are yet market ready.

NimH - Nickel metal hydride:

In the past few years I've been involved in the use of hundreds of thousands of AA NimH batteries.

A 2500 mAh range NimH AA battery will weigh 26 grams up and some are over 30 grams. Batteries substantially lighter are fraudulent. The amount of Ni metal that can be packed in the space is one limit. The loss of trickle charge capability (see below) appears to be due in part to the available room for the Hydrogen recombination chemicals being taken over by extra active battery material. This points to the makers being near the upper limit of their capabilities.

No current commercially available AA NimH battery achieves 3000 mAh - despite the claims from the junk battery makers - but the quality battery makers are getting perhaps 2500 mAh and may get as far as 3000 mAh in due course. The big 3 are BYD, BPI and GP (ever heard of any of them? :-) ) and BPI has recently been proudly touting their 2500 mAh as an industry first. Really. (GP = Gold Peak = Hong Kong company, mainland Chinese manufacture. BPI (Better Power) and BYD are mainland Chinese.)(I understand that Warren Buffet owned about 30% of BYD at one stage - whether still so I know not). Most of the better brand batteries that are sold are made by these 3 manufacturers.

As capacities have edged up beyond 2000 mAh in an AA some features have been lost. Up to about 2000 mAh AA NimH could be trickle charged at about the C/10 rate. Chemicals and appropriate electrode design allowed recombination of hydrogen formed by electrolysis. That capability has now been removed. Reputable makers of 2000+ mAh AA NimH will say use zero trickle charge, or low trickle chaarge rates for short periods only, or very very very low rates (C/100 or less).

ANY manufacturer claiming 2600 mAh or more is a fraud and most claiming 2400+ mAh are fraudulent.

Any manufacturer claiming say 2200 mAh+ and trickle charging at C/10 is a fraud.

Other

More later maybe.

New battery technologies with much greater energy densities than LiIon are under active investigation, but, so far liIon is about the leader in practical use, and NimH has more or less caught up.

Lithium Ion gives modestly more volume energy density than NimH - used to be much more. LiIon is capable of much better than it achieves but practical aspects which affect longevity affect capacity. Variants such as Lithium Polymer are not faring too much better. Long cycle life variants such as liFePO4 (Lithium Ferro Phosphate) have lower energy densities (often lower than NimH) at the gain of cycle life - 2000 cycles is easily enough achieved against LiIon 300-500 cycles.

Capacities:

Lithium Ion

• 300 - 360 WH/l (Watt-hour/litre)

• 150+ WH/kg claimed

Matsushita NiMH 1996!!! 300 watt-hr/litre

Dropping 20 x AA cells into water and measuring their volume indicates that current consumer AA NimH at 2500 mAh x 1.1V delivered yield about 350 Wh/litre ! And that includes the cell packaging which is not technically part of what would usually be measured when such stats are calculated, so
NimH AA is approaching say 400 Wh/litre!.

Lithium/Sulfur-dioxide 250 WH/Kg 400 Wh/l

Peroxide-Aluminium fuel cell. XL space systems suggest potentials of 280 wh/lb = 600+ Wh/kg

These all seem to compare vv poorly with the energy content of known fuels. eg LPG & air at around 14 kWh/kg (adding air mass still gives a very superior result.) (Overall thermal to electric efficiencies of well under (10% would be needed to get down to the BEST of these batteries.)

For comparison: LPG provides 14 kWH/litre (!) - Efficiency of conversion to electricity will reduce this but it is still >> batteries. Also sg of LPG is probably << that of batteries. Even at 10% conversion efficiency this is 1400 watt-hour/litre. Mass of air used needs to be considered even though it is "free".

Cells which use air as one "electrode" fare better at the expense of "very very severe difficulties" with recharging. eg Zinc-Air - well known in hearing aid use. And eg Al-air proposed for electric vehicle use with mechanical recharging and Al2O3 residue being essentially resmelted to produce new metallic Al.

Answer 3

Yes there is a limit (will vary a bit depending on the chemistry used) but ~3000mAh would be a good figure for an AA battery made using current chemistries. If you make the battery using the same chemistry bigger then it will have more capacity, just like if you make your fuel tank bigger your car will go further.
There are many forms of energy storage (fuel cells, atomic cells) that can supply much more power per gram than e.g. a standard alkaline battery, but can be amazingly expensive so are generally limited to things like spacecraft, research labs, pacemakers, etc. This paper seems quite interesting and informative, comparing different methods.
I would say the answer to the questions is capacity is limited by both size and economical (and safety/practical operation) reasons.