# Converting c/20 rate ah.



## frodus (Apr 12, 2008)

get the datasheet and find out the capacity at 1C.


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## Dink (Jun 3, 2010)

frodus said:


> get the datasheet and find out the capacity at 1C.


 I tried that. I was looking at Optima yellow tops. They don't give that figure on thier web site. All they give is cold cranking @ 0 and 32 degrees, reserve capicity, capicity(c/20 rate), and internal resistance. No 1c rate. So then can I assume then that dividing the c/20 rate by 20 will get a 1c rate capicity? With the 1c=ah?


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## frodus (Apr 12, 2008)

nope

Calculating c/20 isn't as simple as dividing by 20. you have peukert working against you.

I can't finda curve for them at 1C either. Keep looking, its got to be out there.


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## Dink (Jun 3, 2010)

frodus said:


> nope
> 
> Calculating c/20 isn't as simple as dividing by 20. you have peukert working against you.
> 
> I can't finda curve for them at 1C either. Keep looking, its got to be out there.


 Looking at reserve capacity definition, it says that is the number of minutes a battery can be discharged at 25 amps before droping voltage below acceptable level. Would that mean a reserve capacity of 120 min would be equal to a 50 amp hr battery, or would the peukert effect come into play again?


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## dexion (Aug 22, 2009)

I have found 1c=50% of the 20 hour rate.


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## samborambo (Aug 27, 2008)

The equation you need is Peukert's Law http://en.wikipedia.org/wiki/Peukert's_law

In order to find the Peukert constant for a particular cell, you need the discharge current and time for two datums. It looks like you have that - the reserve capacity (120 minutes or 2C at 25A) and the 20C rating. Divide the 20C capacity in Ah by 20 to give you the current discharge. 

Once you have both datums you can solve for k, the Peukert constant. From there you can rate the capacity for any discharge time.

Sam.


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## frodus (Apr 12, 2008)

Thanks Sam, I wasn't sure how to do that.... great info!


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