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My question is that does the energy efficiency for Li-Ion battery varies for different C rates ? For e.g if a battery is charged and discharged at a lower C rate, would it's energy efficiency would be higher than the one being charged with a higher C rate ? If yes, why ?

I have searched about it, and i know the fact that the battery's capacity depends on the current amplitude extracted from it. At higher discharge rates the capacity is lower, and so will be the output energy. Same is the case with charging, with higher charging current amplitude, the battery wont be charged to its full capacity and the maximum cut-off voltage is reached. But what about the energy efficiency ? Would it also differ for low and high charging and discharging case scenarios ? Or does it remain the same ? Because in high C rate case, we are extracting less energy (capacity x voltage), but we also provided it with less energy while charging it. So doesn't the energy efficiency need to be same for high and low C rates charging/discharging case ?

I hope i am able to make my question clear. Your helpful suggestions and comments would be appreciated. Thankyou

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Generally, the Coulomb efficiency of a Li-Ion cell is quite high (> 99 % when not overcharged), but the voltage efficiency isn't as good. This is basically due to internal resistance of the cell (and effective resistance because of the diffusion of ions inside the cell). So, charging and discharging at high currents basically creates higher I*R drops across these resistances, and this wastes energy, but most of the Coulombs you put in (or out) get stored.

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  • \$\begingroup\$ Okay. So it means that one can say without any doubt that the energy efficiency WILL BE lower for higher charging and discharging currents. Right ? \$\endgroup\$ – yiipmann Nov 22 '15 at 18:19
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    \$\begingroup\$ Yes, until you get to charge and discharge currents so low that self-discharge also begins to take effect -- This could be about 1-10 % per month. \$\endgroup\$ – jp314 Nov 23 '15 at 0:26
  • \$\begingroup\$ Just to make myself clear, you said that coulomb efficiency of Li-ion cell is quite high. It means that if i just charge & discharge the cell in constant current cycle ONLY (i.e not fully charging/discharging it), i will get a very high efficiency, right ? What if i have 2 cases of constant current cycle charging/discharging; in one i do it with 0.5C, and in the other i do it with 1C. Will the efficiency of the cell in these 2 cases be different from each other as well ? (high for 0.5C case, and lower for 1C case, right ?) \$\endgroup\$ – yiipmann Nov 28 '15 at 19:44
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    \$\begingroup\$ There won't be much (few %) difference between 0.5 C and 1 C -- basically about the amount of the internal resistance * current. Since the internal resistance is usually very small (you can get 20 - 50 A from an 18650 cell), the I*R drop at C (2.5 A) is about 0.2 V, or 5 % of the applied voltage. At 0.5 C, it's 2.5 %, so the difference is 2.5 %. \$\endgroup\$ – jp314 Nov 28 '15 at 20:01
  • \$\begingroup\$ So summing up; LiIon battery's energy efficiency is 1) the highest in constant current cycle. 2) it is a bit less for full CC-CV cycle because of the voltage drop in CV cycle (towards the end of charge), and 3) is different in case of CC, for different C rates, (though its a very minute change but still) it is a bit high for low C rate as compared to higher C rate. I hope i made my confusions clear here with the help of your answers. Please comment if i have said something wrong here. \$\endgroup\$ – yiipmann Nov 28 '15 at 20:12

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