NiMH AA's. Why so many different capacities?

Looking at NiMH AA's and you can get them with quite a range of different capacities. Is there any reason why I would want to buy 2300mAH instead of 2700mAH?

Why do manufacturers continue to make lower capacities or is it because they have huge stocks and are trying to get rid of them?

Do some of these capacities need to be taken with a pinch of salt?

I await to be enlightened.
Thanks.

Hello Helius,

Welcome to CPF.

We have found that the lower capacity cells seem to be more robust. Once you get to 2500 mAh and above, there seems to be problems with high self discharge rates. I have some Sanyo 2500 mAh cells that originally give me 90 minutes of run time in a light. When they sit for 2 weeks, my run time is reduced to 3 minutes.

Another observation we have made is that the lower capacity cells have a longer cycle life. The higher capacity cells die in around 150 cycles, whereas the lower capacity cells seem to be able to go for around 500 cycles.

The lower capacity cells also seem to have lower internal resistance, allowing them to handle higher currents better than the higher capacity cells.

I believe high capacity cells have there place, but they seem to be a bit fragile.

Battery capacities are dependent on current load. They are rated at a 5 hour discharge, but unfortunately we use them at higher loads. This means that, for us, the capacities are lower than advertised.

Tom

Do they have different chemestry? Or different sized particles? What factor gives the measured results?

... and I think manufacturers have recognized these problems, that's why they decided to keep their lower capacity cells in production. Both Sanyo and GP (most other major brands stock the cells from them) have low capacity NiMH AA cells still in production. I just bought a set of 1700 mAh Sanyo AA cells to replace a set of IG Sanyo 2500 mAh, which developed high self-discharge

Hello Greenlight,

This seems to be a closely guarded secret. I don't know for sure, but there is a theory that has to do with separator thickness. A thinner separator allows you to get more material in the can, and this gives you greater capacity, however it seems to also be more fragile.

Tom

SilverFox hit the nail right on the head...............i use different flash units on my cameras and noticed that the higher capacity cells give me a better refresh rate for the next flash shot, but only after they have been topped off....................out of 2 dozen Maha 2500's i bought just over a year ago, 5 have already given up the ghost..............i have 2 sets of older MonsterCable Nimh's that in the 1600-1800 MAH (not sure which) and are almost 5 years old now and hold their charge much longer, and of course are still working fine.
i'm going with the Sanyo Eneloops for awhile and see how things work out.

SilverFox said:

Hello Helius,

Welcome to CPF.

We have found that the lower capacity cells seem to be more robust. Once you get to 2500 mAh and above, there seems to be problems with high self discharge rates. I have some Sanyo 2500 mAh cells that originally give me 90 minutes of run time in a light. When they sit for 2 weeks, my run time is reduced to 3 minutes.

Another observation we have made is that the lower capacity cells have a longer cycle life. The higher capacity cells die in around 150 cycles, whereas the lower capacity cells seem to be able to go for around 500 cycles.

The lower capacity cells also seem to have lower internal resistance, allowing them to handle higher currents better than the higher capacity cells.

I believe high capacity cells have there place, but they seem to be a bit fragile.

Battery capacities are dependent on current load. They are rated at a 5 hour discharge, but unfortunately we use them at higher loads. This means that, for us, the capacities are lower than advertised.

Tom

Click to expand...

Hi Tom,

Is the same true with NiCd batteries? I use a NiCd AA 1200 mAh battery daily. The battery will last almost 2 weeks before I need to recharge. You mentioned that lower capacity cells tend to have a longer cycle life. So, would I be better off going with a Nicd that has less mAh capacity? I understand that I would need to recharge the battery more frequently if I went with a lower capacity, but if I can get nearly 3 times more cycles out of the cells, I think that would be the way to go.

Please let me know what you think.


Thanks!

Hello Ifoxbox,

They don't make high capacity NiCd cells...

I think, in general, NiCd cells are more robust and can tolerate abuse better than NiMh cells. I have not heard of high self discharge rates with the AA 1200 and 1400 mAh cells, but I also don't know of a lot of people that use them.

I should also mention that I have not done any "formal" cycle testing on NiCd cells.

The testing that I have read about generally was run to around 2000 cycles and then abandoned because the cells were still going strong. I don't have that much time to spend cycle testing. It is hard to get 150 cycles done, I can't imagine doing 2000 cycles.

Tom

Thanks! I did not know. So, anything below 2500 mAh is not considered high capacity?

Helius said:

I'm hoodoo from STW

Click to expand...

single track world ??
another mountain biker??
........... welcome to CPF mate

SilverFox said:

I think, in general, NiCd cells are more robust and can tolerate abuse better than NiMh cells.

Click to expand...

Question:

Is this the reason why the bulk of portable tool batteries (ie: drills) are still NiCad?

Or is it because the technology for NiMh had not matured enough for tool applications?

I notice now that more upper end tools now carry Li-Ion rechargeable battery packs as well.

Hello Ifoxbox,

In the AA size, I don't think you can find anything much above 1200 - 1400 mAh in NiCd.

Tom

Hello Bill n opus,

Up until recently, NiCd had the ability to work in high current demands. NiMh is catching up, but NiCd is still though of as having lower internal resistance. Add to that the fact that it can handle many cycles of high current draw, and can tolerate being left on the charger for months at a time, and you can see why the tool industry uses NiCd.

Tom

SilverFox said:

Hello Bill n opus,

Up until recently, NiCd had the ability to work in high current demands. NiMh is catching up, but NiCd is still though of as having lower internal resistance. Add to that the fact that it can handle many cycles of high current draw, and can tolerate being left on the charger for months at a time, and you can see why the tool industry uses NiCd.

Tom

Click to expand...

:thanks: for the info Tom.

A significant factor in the variation in capacities is the creativity of the marketing departments and their propensity to exaggerate.

c_c

SilverFox said:

They don't make high capacity NiCd cells...

Click to expand...

Since NiMh is about double the capacity of NiCd wouldn't a 1200 mAh NiCd AA be as high capacity as a 2500 mAh NiMh? Or am I missing something here?

SilverFox said:

Hello Greenlight,

This seems to be a closely guarded secret. I don't know for sure, but there is a theory that has to do with separator thickness. A thinner separator allows you to get more material in the can, and this gives you greater capacity, however it seems to also be more fragile.

Tom

Click to expand...

Many also suggest that the outer skins may be thinner as well on hi-cap NiMH cells, which would seem to fit right in with the increased fragility theory.

While I've never bought any of the high-end, hi-cap NiMH cells, I can report that the lower-end, lower-cap cells that I've always purchased seem to perform much better (self discharge wise) than (A) the self discharge tables would indicate, and (B) the many reports I've read about hi-cap cell's self discharge 'in the wild' in threads here at CPF. In my reality map good chargers should help coax the best performance out of cells of any quality over the long haul.

The Eneloop clones (Enduro?) are low enough in price that I'll probably make that jump for my next AA cell order.

Pure speculation but I have noticed that the 2500 and up cells have a high self-discharge rate, low current capacity and higher voltage sag under high loads VS lower capacity cells.

Knowing that to make a higher capacity cell involves making the seperator thinner, there could be (probably is) higher current leakage through the seperator. This would explain why they self-discharge much faster and have a lower current capacity than the lower total capacity cells.

Another oddity I have noted is a huge self-discharge rate on lithium-ion cells. I have been using a Milwaukee 28V lithium-ion powered drill for the last year and the Molicell high current (10C+) batteries will self discharge 50% in two weeks.

Tested a year-old PowerEx 2500 with the Sanyo HR stamp on the cell at a 2.5 amp charge/discharge rate (1C) and for three cycles it tested 8%, 8% and 7% capacity down to 1.00V. At a discharge limit of 1.08V, it gave a capacity of 2% Dropped the charge/discharge rate to 1.25 amps (0.5C) and the first cycle gave 78% so I let the Cadex continue as it will constantly cycle until the batteries return a 50% capacity.

I have tested 12V 1 amp hour defibrillator batteries at a 1C charge/discharge rate and they sit at well over 100% of their capacity...even after several years of use. Spiked a defib battery (blasted it with 30V and 5 amps for a second) to "wake it up" as it was stored dead for 5 years and the battery was 7 years old. Cycled it a few times and it was back over 100% capacity at 1C charge/discharge rate.

Since I have been stuck testing medical batteries for the last 20 years, be it lead acid 144V mega packs, SLA, AGM lead acid, Gel Cell, NiMH, NiCd, Lithium-ion and Lithium-Polymer... the most rugged by far is NiCad. Lead Acid is #2 for ruggedness, amp capacity and longevity. AGM and SLA is in the middle with NiMH & Gel Cell underperforming. Lithium based batteries have low self discharge but only last a few years and have a low amp discharge capacity (Except the new Molicell and A123 System drill batteries)

The A123 System nano-technology LiFePO4 batteries hold promise and we are checking them out with DeWalt 36V drills, circular saws, sawzalls and yes... the flashlight (POS that it is) They do self-discharge about the same rate as NiMH but can throw 30 amps from a 2300mAH cell. They should have a 5+ year life as long as you don't let them sit on a DeWalt charge too long. The DeWalt charger will overcharge the cells since most DeWalt chargers tend to do that. Thanks DeWalt... $159 batteries Oh yeah, those "36 volt" batteries are NOT true 36 volts... more like 32 volts nominal since their are ten of them in series and LiFePO4 is nominally 32 volts under load.

It has been my experience that NiCad, normal lead-acid and lower capacity NiMH hold up better than the other battery types. The LiFePO4 batteries hold promise to finally bring lithium based rechargables above the low ampacity, couple of years use and dumpster time chemistry. As far as defibrillators go, NiCad is the most common battery followed by sealed lead-acid to handle the high current loads. The AED type defibs use lithium-primary cells so rechargables are not in that equation.

For normal flashlight, camera, MP3 player and wireless computer accessories... I am switching to Eneloop 2000mAH batteries. I'll gladly give up some fresh off the charger runtime for more durability, better voltage stability under high current discharge and much improved self-discharge rates.

Jeez, where else on the internet would we get this kind of info?

You guys are awesome. :rock:

jtr1962 said:

Since NiMh is about double the capacity of NiCd wouldn't a 1200 mAh NiCd AA be as high capacity as a 2500 mAh NiMh? Or am I missing something here?

Click to expand...

A 2500 mAh cell has 2500/1200 ~ twice the capacity of a 1200 mAh cell. This is true regardless of what the battery chemistry is. A 1200 mAh NiMH cell has the same capacity (namely, 1200 mAh) as a 1200 mAh NiCd cell.

This is a good place to note that capacity is measured and valid for a particular set of conditions, which might be quite different than the conditions of actual use. So the single capacity number doesn't tell the whole story about how a battery will perform. But NiCd and NiMH cells behave similarly, so you can usually expect about twice the run time from a 2400 mAh NiMH or NiCd cell as a 1200 mAh NiMH or NiCd cell in the same application.

c_c