High Current v. High Capacity Cells - Updated

[Ginseng]

This post was returned to the thread on 2/8/06.

I've been doing some battery exploration for my high-powered incan mods and have been interested in the differences between high capacity and high current cells. Here are the results of some initial testing of representative cells of each variety.

High capacity cell:
MAHA Powerex 2000mAh AA. These are popular cells and regarded as "premium" grade high capacity nimh. 8,078 cubic mm volume. Button tops stacked end-to-end.

High current cell:
KAN 1800mAh 4/5A. These are economical small nimh cells used by the RC community. 9,760 cubic mm volume. Flat tops in a soldered stick.

Test Load:
12V, 55W automotive light bulb.

DMM:
Greenlee DM-55

Test Methodology:
1. Cells are fast charged. The MAHA on the MH-C401FS. The KAN on the CGR-340 at 0.9A. This is approximately 0.5C.
2. Cells are hooked up to load through a low resistance industrial switch with <20 milli-ohm resistance.
3. The no-load pack voltage is noted.
4. Switch is thrown and the t=0 voltage is determined as the peak displayed voltage before it begins dropping.
5. Voltage is recorded as a function of time until it falls so quickly that it cannot be manually recorded.

Findings:
1. The key difference between Hcap and Hcur cells is the ability to deliver voltage at high currents with minimal sag.
The KAN pack maintains a 1.2V advantage over the MAHA for the entire duration of the test. This works out to about 0.11V per cell or just over 9% of the rated nominal voltage. The KAN pack drops from a no-load voltage of 15.26V to an instantaneous t=0 value of 14.5V. The MAHA goes from 15.41V to 13.14V.

2. At high current demand, the Hcur cells deliver better than 95% of their capacity while the Hcap cells only deliver about 80%.
Note that MAHA does their capacity testing at 0.2C or 400mA in this case. Clearly, as noted in another recent thread, there is advantage to testing at lower current as this inflates the measured capacity. It also means that unless your application is at or below the test current, you will not achieve the rated Ah capacity of the cells.

3. The much lower internal resistance of the Hcur cells (6 mO) versus the Hcap cells (30 mO) results in more efficient power delivery.
The KAN cells were very slightly warm to the touch after the test while the MAHA were hot, over 145F. Clearly, resistance is bad and gets worse fast as current demand escalates.

4. Choose the right cell for your application.
While I thought the MAHA 2Ah were doing pretty well in the MoMag (3.5A draw), they were probably approaching the limit of what they could comfortably deliver. Even with the lower Ah rating of the KAN cells, I would have gotten more runtime from the pack. Also, the MAHA-type Hcap cells are probably sufficient for any existing LED application. For the ultimate hot-wire applications though, specialty cells are required. And in this case, the $3 KAN outperformed the $4 MAHA.

Wilkey

Edit: The KAN cells are 4/5A-size, new photo links, calculated cell volumes correctly

 

[raggie33]

Re: Head to Head: High Current v. High Capacity Cells

i got some energizer 2300 mah, aa dont have no way to test em but they seem to last a log time in my camera and pocket florecnt light

 

[pedalinbob]

Re: Head to Head: High Current v. High Capacity Cells

great info, thanks Ginseng!

i just recently got into rechargeables, not really knowing what to expect--the nimh work VERY well in my digital cam--and they arent even "broken in" yet.

what really floored me is that on fresh alkalines, my BB400's would run 75-85 minutes before falling out of regulation.
the rechargeables (not hot off the charger) ran one for 2:45, and the other for ~2:35.

of course, you are using yours for HIGH amp applications, but it does illustrate the differences in a battery being able to deliver current (in my case alkie vs nimh), and some of the inflated capacity ratings based upon very low current draw.

im learnin' something new every day around here!

Bob

 

[SilverFox]

Re: Head to Head: High Current v. High Capacity Cells

Hello Wilkey,

Very interesting.

I was wondering if the KAN cells are rated at 0.2C as well. They seem to have a bit more volume but that should not make that much difference.

The difference in internal resistance is very interesting. Do the battery specification charts from the manufacturer address this? I wonder how other batteries stack up.

By the way, 145 degrees is hot enough to do damage to batteries. Battery chargers try to keep the cell exterior temperature below 113 degrees.

Tom

 

[AilSnail]

Re: Head to Head: High Current v. High Capacity Cells

Very good Ginseng.

Just to nitpick, in addition to less volume, and their internal resistance, the MAHA's were not soldered together like the KAN's were.
No doubt internal resistance is a big issue, as I have noted with the AAA cells. They drop pretty much and heats up fast.

 

[Ginseng]

Re: Head to Head: High Current v. High Capacity Ce

pedalin,
Good nimh outperform alkies in almost any application save ones which require long storage. My Canon camera flash took 7 sceonds to cycle on alkies but only 4 seconds with nimh.

Silverfox,
I do not have extensive info on the KAN. The industry standard for capacity measurement seems to be charging at 0.1C for 16 hours followed by discharge at 0.2C. I imagine KAN use that as well. I would think the incrementally greater volume of the KAN is of almost no significance. The internal resistance is simply listed as a value after standard charging plus a 60 minute rest. Test frequency of 1 kHz. I recall that some of the Duracell alkies have IntImp of 80 and 120 mO. But much of that must be due to their internal structure which is very different from nimhs. Yes, I agree that 145 is very hot. Conicidentally, that is the temperature the MAHA cells often achieve when quick charging on the 401. I typically run a small fan over the cells/charger when I quick charge.

Ail,
You are absolutely correct that the MAHA cells were not soldered. I could tell that the voltage would drop maybe 0.2-0.4V if I relaxed my finger momentarily implying that button top contact and resistance is critical. Yes, I held the battery stack together with one hand and my thumb for the entire 20 minutes.

Wilkey

 

[Steelwolf]

Re: Head to Head: High Current v. High Capacity Ce

May be nitpicking here too... /ubbthreads/images/graemlins/tongue.gif

I would say that if you are going to test one specific aspect of performance, conditions must be held as constant as possible. Short of soldering your MAHA cells together, perhaps you could use a spring loaded tube, or hold them together with a clamp. And use soldered connections as far as possible (which would save you having to maintain a deathgrip on the cells and DMM probes for 20mins /ubbthreads/images/graemlins/grin.gif ). Perhaps even just device a test that uses only 1 cell?

Other than that, great info! /ubbthreads/images/graemlins/smile.gif

 

[Jarhead]

Re: Head to Head: High Current v. High Capacity Ce

linkage for KAN cells?

 

[js]

Re: Head to Head: High Current v. High Capacity Ce

Ginseng,

You and I seem to be in some kind of synchronicity! I was JUST researching this very thing, and lots of other stuff about batteries. Thanks for this GREAT info.

And as long as you're giving out info, I was wondering where in GOD"S NAME can a person find a decent selection of rechargeable Ni-Cads and NiMH batteries on the web? Links? And specifically, does anyone sell PowerStream batteries retail? I hear about people using PS 1/2D's in their MagChargers (Phaserburn, to be specific). Which 1/2D's? The ones built for robot combat that they will sell retail (no $200 minimum)? or what? Do group buys of PS batteries happen? And also, how on earth does a person charge a 1/2 D? Are they always put together as a pack and charged in toto, or can you buy/mod a 1/2 D size charger?

Thanks

 

[PaulW]

Re: Head to Head: High Current v. High Capacity Ce

Wilkey,

This is a long-needed comparative review of two kinds of NiMHs. A month ago “high-capacity” was just a phrase to me. Then you introduced these cells. And now we have substantive data that verify that the phrase is not just a sales gimmick. A difference in voltage of 0.11 volts per cell throughout the run is substantial.

You mention that this has been initial testing. Thanks for giving us an early peek. I look forward to seeing any future data that you might generate. The difference in run time will be of great interest, although I see you have already determined an important bottom-line number – the percentage of capacity delivered.

Good work. /ubbthreads/images/graemlins/thumbsup.gif

Paul

 

[cheesehead]

Re: Head to Head: High Current v. High Capacity Ce

Brilliant. Need those KAN cells. However, again, not to /ubbthreads/images/graemlins/dedhorse.gif, if you have the voltage drop .4 with pressure holding the cells together, wouldn't that create enough heat (a few watts just from the connections) to partially explain the heating up of the cells. Simply the contact points alone are creating heat. Eh, I think your conclusions are still very interesting and correct.

cheese

 

[Ginseng]

Re: Head to Head: High Current v. High Capacity Ce

Hi guys, thanks for your replies and interest.

Steelwolf,
I tried to keep things as robust as possible. As you guessed, I was not interested in soldering my MAHAs. I have though about using a spring loaded (instead of a thumb-loaded) tube for the next go around. You are right that a pure apples-to-apples analysis would involve much more closely matched and controlled conditions.

Jarhead,
I get my cells from www.dynamoelectrics.com/Batteries.htm I have not yet found their corporate site. Much of my specific technical information on nimh comes from PowerStream and GoldPeak.

JS,
You may check out the DynamoElectrics link above. Chuck Gant is a most upright person to deal with. Good selection, good prices, excellent top notch support. Email responsive. Beyond that, try also http://www.battlepack.com/ run by the Hills. Also, good prices, selection, support and responsiveness. Get started there and then once you become more comfortable with them, there are several others worth investigating. Nobody, at present, sells PS cells at retail. The 1/2D nicad battlebot cells are the ones you want. They charge in-torch in the MagCharger or they can be charged out-of-torch using any one of a number of RC pack chargers. Most folks use them loose in the MC but I have been building and selling soldered 6V 5-cell sticks.

PaulW,
Thanks. I'm glad to have been able to help out. There are plenty of discharge curves out there for this formidable class of cells, but unless flashlighters (besides me /ubbthreads/images/graemlins/wink.gif) intend to run at 10-30+ amps, they'll be interesting but not useful. I suspect I'll continue to study these cells. The next set up for test are the classic Sanyo CP1300-SCR nicad 1/2 sub-Cs. These small cells are the C-size analogs to the PowerStream 1/2Ds. Nuff said. /ubbthreads/images/graemlins/wink.gif

Cheesy,
There are probably several sources of heating. The primary source will still be the bulb draw. A few watts at the contacts are but a small percentage of the total 55W.

Wilkey

 

[SilverFox]

Re: Head to Head: High Current v. High Capacity Ce

I was just wondering...

Is there such a thing as a highly conductive gel or grease that you could dab on the battery connections to get improved contact. I would have to be cleaned up after each battery change to keep things from shorting out inside, so it might not be the most practical.

Just a thought.

Tom

 

[Ginseng]

Re: Head to Head: High Current v. High Capacity Ce

There is conductive greases, but I doubt they are meant to provide primary contact pathway. Low temperature solder tape is another possibility.

Wilkey

 

[PaulW]

Re: Head to Head: High Current v. High Capacity Ce

[ QUOTE ]
Ginseng said:

. . . Nuff said. /ubbthreads/images/graemlins/wink.gif . . .

Wilkey

[/ QUOTE ]

Now Wilkey. You know that is going to have us all speculating and gossiping. /ubbthreads/images/graemlins/grin.gif

Paul

 

[SilverFox]

Re: Head to Head: High Current v. High Capacity Ce

Hello Wilkey,

If you were able to improve the battery to battery contact, do you have a wild guess how much that would effect the internal resistance values?

Would it be possible to go from the 30 mO you measured down to 10 mO or so?

Also, is there any way to practically test this?

I guess my question boils down to this. If the internal resistance of both batteries were close to the same, would you expect similar results? I am trying to figure out if this is a battery construction issue or a battery chemistry issue.

Tom

 

[js]

Re: Head to Head: High Current v. High Capacity Ce

Ginseng,

Thanks for the links!

Everyone,

A stack of batteries pushed together with any reasonable pressure will have very little "extra" resistance vs. soldered cells. Now, if the pressure is minimal, well then that could be a problem. I am willing to bet that soldering them together will change nothing over having them pressed together. Pressure connections are EVERYWHERE in electronics. As long as the surfaces are clean and well matched, no problem.

Feel free to correct me if I'm wrong on this. I have tons to learn about these things, but from my experience so far, pressure is a fine way to ensure electrical connections are good. Think of all the pressure connections out there, from bannana plugs, to lugs, to terminal strips, to wire wrapped electrical boards, to connectors of all kinds, BNC, Elco, etc.

 

[EvilLithiumMan]

Re: Head to Head: High Current v. High Capacity Ce

This is a 'battery construction' issue, and nothing more. It's a matter of physics, changes in materials not withstanding. When a designer is told to design an AA cell, they are constrained by the physical size of the cell. To make a high capacity cell, you need to make the positive and negative electrodes as thick as possible. The thickness determines how long the cell will last. But current carrying capability relys on surface area. Make your electrodes as thin as possible to increase how fast current can be delivered on demand. As you can see, they are diametrically opposed to each other. You can design for one or the other, not both at the same time. Or you can compromise, which is often the case when designing any electronic/electric compoment.

It's no surprise that the KAN cell does better at delivering a 4.6 amp load. It was designed to, and at the expense of capacity, 1800 vs. 2000mah.

 

[Ginseng]

Re: Head to Head: High Current v. High Capacity Ce

Silver,
JS's post is an excellent commentary on the situation. Pressure contacts are indeed everywhere. They're in SNIIs, SF L4s and almost any other flashlight you'd care to mention. I wholeheartedly agree that contacts must be cleaned and kept free of oxidation for best conduction. For me, that means regularly stripping with 2000 grit sandpaper, alcohol and then DeOxit. I have personally experienced the much more injurious effects of oxidation (in my Lambda Illuminator and any device with a Kroll) and know that it can make a world of difference in brightness when removed. ELM is correct that the internal impedance is a measurement across a single cell and not across a contact pair. PaulW also documented some very useful resistance and resistance reduction information for the Elektrolumens Gen1 3-Ds.

JS,
Thanks for your comments.

ELM,
No surprise, sure but where else have we seen such a relevant comparison made? I think the KAN engineers have done well to "sacrifice" only 200mAh and yet provide stellar high current output.

PaulW,
I like your new avatar. I think that finding variants of benchmark cells for other than their classic application is a worthwhile endeavor. Not everyone wants to drag a 3D-sized light around even if it's sporting the potent WA01160. But how about a slimmer 3C body? Or maybe the WA01183 on 4 cells in a 2C? Now that would be a very interesting 500-lumen torch.

Wilkey

 

[Doug S]

Re: Head to Head: High Current v. High Capacity Ce

Ginseng, you need to recalculate your volumes, they are way off. There, nit picked.