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How does one determine the C rate of an aged cell?

I would like to find the discharge rate just before any cliff edge of capacity loss. Can the C rate of a cell be estimated by the cells rated heat dissipation and measured internal resistance? Where I could find the current that will use up all the cells heat dissipation capacity.

I have a number of recycled laptop battery 18650 cells.

Are there any negative effects of the cells being below room temperature?

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To determine the C-rate, you should charge and discharge two cycles very slowly say at 0.1 mA, between say 1.5 V to 4.3 V for your Li-ion (unless it's a very old laptop battery it should be LiCoO2), and measure how long it'll take. The C-rate would be your capacity divided by one hour.

Don't know of a way to measure it using heat or internal resistance.

Batteries generally have an adverse effect when heating up, so unless it is much below freezing, they should be ok.

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  • $\begingroup$ 1.5-4.3v is pretty extreme isn't it, very little capacity is past 3-4.2v from what I have seen? I already have measured the capacity of my cells, but that doesn't help me choose a C rate, it only lets me know what current I would get for the C rate I do choose. $\endgroup$
    – Quwat
    Commented Mar 18, 2016 at 20:46
  • $\begingroup$ A fresh LiMnO2 battery will have max around 3.5 V. A new LiCoO2 battery around 4.2 V, so it'll depend on what type of Li-ion battery you have. Choose the voltage range to determine the capacity. Not sure what you mean, as the C-rate is just another way to express your discharge rate... If you know what current you want to choose, and you know your capacity, you can just divide to work out your C-rate. $\endgroup$
    – khoo
    Commented Mar 20, 2016 at 21:39

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