Standard C ratings vs. Claimed C ratings

[swbluto]

It seems there's a discrepancy between C ratings between cells from different manufacturers, so I would think a standard C rating would be more definitive.

One discrepancy is Ping's 2.0 cells and Headway's cells. The standardized internal resistance of the Headways are only about 2 times of the Ping cells suggesting that if ping is correct, Headway's cells are only 4C, however, if Headway's C rating is correct, then ping's cells are actually 2.5 C. This seems to require some kind of standard.

So, which cell's rating shall be the gold standard? Once we define one cell's C rating as being essentially "correct", then you can apply the standard resistance formula / Shortest-time-to-discharge to determine another cell's C rating in comparison.

But.. there seems to be complications. Is a cylindrical cell able to withstand a greater power loss / dissipate heat better per unit weight than prismatic cells(They are typically spaced out instead of crammed next to each other)? If so, then if you're calibrating C-ratings by a stable temperature level(Say 150 degree F), then a cylindrical cell might have an inherently higher C-rating due to normal pack configurations and exposed surface area.

 

[julesa]

I would think it depends on what you're using the rating for. We may be more interested in the cell's continuous rating, but some people are going to be more interested in peak output. Seems to me a series of graphs of both voltage and temperature over time at various current levels, like Doc's been putting together, gives a much better picture. A single number can always be 'fudged' by salespeople who will... creatively interpret... what that number means.

 

[Doctorbass]

I would go with A123 standard here.

C rating is determined by the current a cell can do without reaching the critical temperature where the cell risk having permanent dammage.

Ex: M1 from A123 say 60C for 10 sec and at 2.3Ah it's 138A. one of the most powerfull cells on the market.. and Saft also have C rating like 250C.. but it's for military applications...

the M1 cell is alsi rated 30C continuous = 70A.. so it only mean that this cell can dissipate all the produced heat just as fast as it need to get the equilibrium where the max temp top just before the critical point.

it is easy to calculate since we know that the cell RI is around 10miliohm.. so at 70A continuous the cell will dissipate 49W of heat!

and at 70A the cell will deliver 2.3V so the continuous output power is 2.3*70=161W out.. if you add the lost power in heat, it's 49W + 161W = 210W total delivered by the cell and with 2.3Ah and 3.3V =7.6Wh..

so 210W rate with 7.6Wh of energy will take 210/7.6 = 1/27hour = 133second to empty to 0%soc...

133sec x 49W =6517 Joules of energy dissipated by the cell over one coplete discharge.

Usually the basic condition for determinating C rating is with the cell placed right on a test bench with one surface that touch the cell. That mean that they did not calculate max C rating based on a max temp when the cell is inside a battery pack. the cel in a battery pack condition is far worst thermal condition than a cell alone on a table!
this basic data allow thermal management calculation to have a basis without any interference from others parameters and conditions...

As for Headway and Pig.. the C rating must be secified for a given period of time.. like 10 sec burst.. or continuous.. otherwise it may intruduce some confusions...

Doc

 

[dogman dan]

I find it interesting that people may not realise that the max c rating is not always sustainable out on the road in all conditions. So the m1 cell can do 30 amps continuous, but that test was not done out in the hot streets, with the cell surrounded by 200 others, sealed in a watertight box, etc.

I just think that c ratings, even for continuous, need to be lowered some for real world use, and bike builders need to realise that because a cell can do such and such, it doesn't mean maximum lifespan used that way. The specs may say 3 c or whatever, but usually cycle life tests are at only 1c.

I think people need to build their bikes for 2c discharge rates (or less if they need range) to maximize reliability if the bike is for daily transportation. The ideal of a really light ebike is a great goal, but not if you toast batteries when 5 pounds more battery would have made all the difference. As batteries improve, as lipoly has, we will be able to stop worrying about c rates and just carry enough battery to get there.

Different attitude of course for recreational high performance use, or folks that have a freind in the reycling place.

One thing for sure, c rates that don't specify for time are meaninless. I think a c rate that doesn't specify cycle life is pretty vague too.

 

[bearing]

I wonder if the chemical reaction at discharge is exothermic or endothermic. Cells seem to heat up from internal resistance, but the temperature rise seems quite low to me, compared to the heat produced in the resistance. The heat capacity of the cells can't be that high, I reason.

I have read charging of NiMH is endothermic, so they cool during charge (if heating in resistance is not bigger). Maybe Lithium based batteries have endothermic discharge? or is the heat capacity LiFePO4 really so high that a 70g A123 can take all the heat during a 70A discharge?

Now I tried to make some rough calculations of heat capacity, but couldn't find the heat capacity of FePO4. If an A123 got the same heat capacity as a o26.5x65mm lithium cylinder then it will heat about 0.015°C per Joule. If that cylinder is heated by 50W in 133 seconds (as in dr.'s) example the cell temperature will rise 50*133*0.015 °C = 100°C (if cooling from air is neglected) which is too high of course. But then that lithium cylinder is much less than 70g. Other materials in the cell will increase capacity. Hm, maybe it's possible after all.