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My current understanding is that the C20 capacity rating of a battery is the number of Ah that is supplied by a battery to a load that will fully discharge the battery over a period of 20 hours.

Is 20C the same as C20, or does a 20C rating mean that the battery is discharged in 1/20 hours and the current being drawn is ~20 times the rated Ah capacity of the battery?

Edit/clarification: I'm looking at battery specifications and this question is in the context of a battery having different nominal capacities at C20 and C100, rather than trying to determine the maximum allowable load that can be from the battery.

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  • \$\begingroup\$ where did you see this C20? \$\endgroup\$
    – Zy Gan
    Sep 26, 2018 at 3:00
  • \$\begingroup\$ A few places, eg solarhomestead.com/battery-amp-hour-ratings and electronics.stackexchange.com/questions/53660/… \$\endgroup\$
    – John Haire
    Sep 26, 2018 at 4:28
  • \$\begingroup\$ They are probably the same, I did a search and saw different vendors putting the C in front and behind the digit, but best to make sure it's not just a battery model or something else \$\endgroup\$
    – Zy Gan
    Sep 26, 2018 at 4:31
  • \$\begingroup\$ I've seen people suggest that 20C is 20x the capacity per this description qr.ae/TUGGxv. We asked vendors for the number of hours that a 60Ah battery can continuously supply 2.2A, and one supplier said 29 hours to 1.75 vpc at 20C. I've asked for clarification but I assume they mean C20 load or 20 degrees celsius. \$\endgroup\$
    – John Haire
    Sep 26, 2018 at 4:38

3 Answers 3

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C = Discharge rate.

20C for a 2Amp battery can be discharged with a 40Amp load.

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  • \$\begingroup\$ Apologies, I've updated the question. This is related to the way battery datasheets might specify differing capacities at different loads, rather than maximum/continuous load ratings of the battery. \$\endgroup\$
    – John Haire
    Sep 26, 2018 at 4:48
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enter image description here

(Image from https://batteryuniversity.com/learn/article/what_is_the_c_rate)

With this table i guess it should clarify your question.

In short C20≠20C

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  • \$\begingroup\$ There is no reference to C20 in that table. \$\endgroup\$
    – Transistor
    Sep 26, 2018 at 6:18
  • \$\begingroup\$ The C/20 refers to C20 based on the discharge time calculation, and this matches with the explanation in the link provided by John in the comment section of the question. \$\endgroup\$
    – Zy Gan
    Sep 26, 2018 at 6:27
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'C20' is not a well-known form of expression.

It might mean 'capacity when discharged over 20 hours', and will have units of Ah. You would normally expect to find the expression written as C/20h, or C(20), or just described in words.

It might mean 'current that is 20 times the nominal capacity', and will have units of A. You would normally expect to find the expression written as 20C.

Fortunately, the two possibilities have different units, so it should be clear from the context of the data sheet which is meant.

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