Author Topic: 18650 current ratings  (Read 2083 times)

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Offline cnt

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18650 current ratings
« on: January 17, 2022, 06:46:16 am »
besides the obvious, what makes a 30A 18650 cell different from a 10A 18650 cell. 

I assume the higher current rated cells have lower internal resistance.   Are there other relationships between rated current and other properties like cycle life?  will a 30A cell last longer (more charge cycles) running at 5A it's whole life vs a 10A rated cell?
 

Offline ConKbot

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Re: 18650 current ratings
« Reply #1 on: January 21, 2022, 01:29:41 pm »
There is very much not a correlation between current capacity and service life in cycles or years. Ultra high current LiPo packs used in drones and by RC cars etc, good for 30-50C or more, can have quite frankly a garbage service life. The ones my coworkers use are allegedly pretty high end batteries in that area, but they still frantically discharge them when done to store them at a storage voltage because they shit the bed and die from being stored full for even a few weeks. They still end up with puffy cells contained by the heatshrink pretty often. I would normally scrap a lipo cell with any hint of puff, but that would probably limit those to <5 cycles  :P That and I don't feel like arguing any more with the pig-headed ME whose pet project it is, and they aren't keeping their box of fire hazards in my building.

So the batteries there are extremely optimized for high current output, and being low cost. Everything else is very much secondary. And apparently that is a good approach for the market they serve.

Now obviously not all high power cells are like that. Power tool 18650s very much last longer than a few dozen cycles. So it all comes down to the secret sauce added during cell construction, and what the batteries were optimized for.

For your hypothetical 30A vs 10A cell, since nether will be particularly near their rating at 5A, and presumably the 10A cell is higher capacity, at a fixed mAH discharge amount, the 10A cell may last longer, with the depth of discharge percentage being lower.
 

Offline f4eru

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Re: 18650 current ratings
« Reply #2 on: January 21, 2022, 02:02:47 pm »
One big difference :
solid shell cells (like 18650) contain the pressure naturally, while pouch cells have to either be used more gently, or be constrained to hold pressure.

Offline Wolfram

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Re: 18650 current ratings
« Reply #3 on: February 17, 2022, 10:58:18 am »
In cell design, power density can be improved at the cost of energy density, by making the current collectors thicker. This lowers the internal resistance, but leaves less space for the actual anode and cathode materials that store energy. Different cathode materials also have different tradeoffs in energy density, power density, cycle life and safety.
 

Offline Siwastaja

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Re: 18650 current ratings
« Reply #4 on: February 20, 2022, 07:27:15 am »
Current collectors (copper and aluminum sheets) do not dominate in the internal resistance, it's the ion transfer from/to anode/cathode lattices. This can be seen from the fact that DCR is so much dependent on the cell temperature, the hotter the lower; no such effect from the metal foils. But the chemical activity speeds up with temperature.

So on microscopic level, the optimization of high power vs. high energy means optimization between high surface area vs. high volume of material, really. How that is exactly achieved is manufacturer's secret. It's even possible that a high-power cell has thinner current collector sheets, if the anode/cathode coatings are thinner, too. Then the roll would be longer. Slightly inferior volume of active ingredients, but much more surface area.
 
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Offline Wolfram

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Re: 18650 current ratings
« Reply #5 on: February 28, 2022, 03:44:24 pm »
Current collectors (copper and aluminum sheets) do not dominate in the internal resistance, it's the ion transfer from/to anode/cathode lattices. This can be seen from the fact that DCR is so much dependent on the cell temperature, the hotter the lower; no such effect from the metal foils. But the chemical activity speeds up with temperature.

So on microscopic level, the optimization of high power vs. high energy means optimization between high surface area vs. high volume of material, really. How that is exactly achieved is manufacturer's secret. It's even possible that a high-power cell has thinner current collector sheets, if the anode/cathode coatings are thinner, too. Then the roll would be longer. Slightly inferior volume of active ingredients, but much more surface area.

I stand corrected, thanks for the info!
 


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