Battery cells; Current capacity vs discharge rate

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videobruce

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Using these two examples;

http://www.onlybatterypacks.com/showitem.asp?ItemID=10231.22
http://www.onlybatterypacks.com/showitem.asp?ItemID=10715.5

The 1st contains 2400mAh cells w/ a 30A discharge rate.
The 2nd contains 5000mAh cells w/ a 20A discharge rate.

How can a cell with a higher capacity have a lower discharge rate and vice-a-versa? Also, I'm told that the one with the higher discharge rate is better with a lower temperature durning use.
Confused. :confused:

Separate subject; I know this has been beaten to death, but I was under the impression NiCAD packs do NOT have to be cycled every time before a recharge. Now I'm told they do. I thought only a decade ago that was true.
 
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gmclam

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Battery chemistries

Separate subject; I know this has been beaten to death, but I was under the impression NiCAD packs do NOT have to be cycled every time before a recharge. Now I'm told they do. I thought only a decade ago that was true.
Nickel Cadmium batteries have something called a "memory effect". In order to mitigate that effect, they should be fully discharged before being (re)charged.

We've been told that Nickel Metal Hydride batteries (NiMH), which is what most of us use now, do not need to be fully discharged as they do not have this same characteristic that NiCds have. But some chatter here recently makes me think otherwise. It appears that the memory effect of NiMH batteries is LESS than that of NiCd, but there still is some.
 

kb2vxa

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No matter how we try the deep cycle myth persists. Maybe you would believe Adam and Jamie? (;->) There is no need to completely discharge NiCd batteries, nobody does, just pop them (or the radio) in the charger. The memory develops if the battery is left discharged or partially discharged for long periods, then it becomes impossible to charge beyond that point. Think logically, the battery remembers that state of charge thus the "memory effect". That's why they're supplied fully charged off the shelf, maybe so they don't go flat ON the shelf?

As for the storage capacity and maximum discharge rate, they both depend on the internal construction of the battery. You can only draw so much current through a resistance before a lot of heat is developed and a battery like anything else has internal resistance. Draw too much current or short the terminals and POW, you have a bit of a mess to clean up if you're lucky enough not to have been hit with flying fragments and hot chemicals. They usually have an internal fuse to prevent surprises should the terminals get shorted by say a bit of metal in your pocket.

Speaking of Adam and Jamie, how much dynamite would it take to blow up a stubborn battery pack? Mostly they just sit there melting and smoking, that's no fun!
 

slicerwizard

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The 1st contains 2400mAh cells w/ a 30A discharge rate.
The 2nd contains 5000mAh cells w/ a 20A discharge rate.

How can a cell with a higher capacity have a lower discharge rate
Smaller internal conductors.


Also, I'm told that the one with the higher discharge rate is better with a lower temperature durning use.
Larger internal conductors.


Separate subject; I know this has been beaten to death, but I was under the impression NiCAD packs do NOT have to be cycled every time before a recharge. Now I'm told they do. I thought only a decade ago that was true.
They don't have to be fully cycled every time. Variety is the spice of life. Just make sure they get run down periodically (like every 10th cycle, for example).
 

key2_altfire

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How can a cell with a higher capacity have a lower discharge rate and vice-a-versa? Also, I'm told that the one with the higher discharge rate is better with a lower temperature durning use.
The capacity and discharge rate usually have little to do with each other. In some cases you will actually sacrifice one for the other. However, it is generally correct that the cooler-running cells will have a higher discharge rate, this is due to lower internal resistance.

Also, the discharge rate may not be purely rated on a temperature basis; the cell may simply have a significant voltage sag at higher discharge rates. Also, capacity get smaller at higher discharge rates. I learned this on a West Mountain Radio "CBA" (Computerized Battery Analyzer). You take a NiMH cell rated at 2,000 mAh and discharge it at 500 mA, it actually gets a rated capacity of about 2,200 mAh.

Discharge the same cell at 3 A and suddenly the capacity plummets to around 1,450 mAh. Obviously, the manufacturer's rated capacity of this cell was not at 3 amps.

Of course, the CBA is probably not going to give you the same capacity rating as what the manufacturer lists, but you can easily see that higher discharge rates mean lower capacity, probably due to voltage sag.

Don't forget other cell chemistries like Li-ion. The general discharge safety limit is 2C. A "C" is 1x the cell capacity. So if you have an 800 mAh capacity cell, discharging at 2C is 1600 mA of current. For a typical 2,200 mAh industrial Li-ion, the max safe discharge rate is considered 4,400 mA. You can go a lot higher, and I've done it, but those cells get HOT. I had one rupture once, and that was it, I was satisfied that 2C really is the sane limit hahaha.

Right now my fave rechargeable is the Sanyo Eneloop. It is only a 2,000 mAh rated capacity, but you can discharge them at about 20C, and they hold their charge on the shelf for a looooong time compared to any other NiMH I've used. (Had a set of Eneloops rolling around in my trunk for over a year and they still had plenty of power for the HT.)
 

videobruce

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A higher discharge rate is only good for devices that have a high current draw, correct?
That would do me no good for devices that draw less than 1 amp?
 

rescuecomm

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I usually think of memory as a layer of chemicals in the cell that isolates the chemicals below it. So it effectively cuts off part of the battery capacity. It used to be said that if you used a nicad battery pack to 50% of its capacity, then charged it up from there constantly, the cell would "remember' the stop point. I used to cut apart the Motorola battery packs for Motorola GP-300 radios to see why they developed memory and would not hold a charge. What I found was shorted cells probably from too much rapid charging causing whiskers to grow between the anode/cathode parts. A typical pack of 8 cells would have two or three shorted ones in it. Memory was not a problem in these cases. All of the shorted ones also showed signs of venting (corrosion on the anode). When the squad chief bought a battery rejuvenator to save money on batteries, I told him he was wasting his time and our money. Alas, only one battery was ever recalled from battery heaven "bless his heart."

Bob
 
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videobruce

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Makes it run faster. :D

The reason for that question, I was told it had to do with pack life. Highre was better. It was the ame guy that said that pulsed charging was required for NICADS.
 

key2_altfire

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The reason for that question, I was told it had to do with pack life. Highre was better. It was the ame guy that said that pulsed charging was required for NICADS.
I noticed (in personal CBA testing) that a lot of cells with the same mAh rating vary widely in actual load testing. Some run better than others in low-draw, some run better than others in high-draw, some don't run well at all.

I think the best advice is: Try several brands of cell. They're not expensive unless we're talking a specialized pack or large quantities.

I probably went through 8-10 brands of NiMH cells before deciding I like the Eneloops best.
 
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