Reserve Capacity To Ampere-hours conversion

 http://maven0.blogspot.com/2015/02/reserve-capacity-to-ampere-hours.html ©2015 maven0.blogspot.com

 Automotive batteries in the United States use the Reserve Capacity rating while in Europe the Ampere-hours rating is required by law. This difference leads to a lot of confusion.

These two ratings are defined as follows [1]:

Firstly,

Reserve capacity minutes (RCM), also referred to as reserve capacity (RC), is a battery's ability to sustain a minimum stated electrical load; it is defined as the time (in minutes) that a lead-acid battery at 80 °F (27 °C) will continuously deliver 25 amperes before its voltage drops below 10.5 volts.

Secondly,

Ampere-hours (A·h) is a measure of electrical charge that a battery can deliver. Nominal capacity(A·h) by EN 60095-1 is rated at a fixed discharge current of I/20, within 20 hours until final discharge voltage of 10.5 V at 25 °C is reached.

These ratings are not very similar so there is no way to make a precise conversion from one unit to another.

However there is a way to make a rough approximation for informational purposes only.

Consider a battery rated at 12 RCM. According to the RC definition this means that it can deliver 25 Amperes for 12 minutes. In order to get the estimate for the Ampere-hours we need to convert this value to hours by dividing by 60 minutes then multiply by the 25 Amperes.

So 

(12/60)*25=5 Ampere-hours

more conveniently we can combine 60 and 25 in one constant by dividing 25/60 and getting the 0.4167 value that keeps appearing at battery sites.

So the formula for converting Reserve Capacity (RC) to Ampere-hours is the following:

Ampere-hours = 0.4167*(Reserve Capacity)

Here is a handy table displaying approximate Ampere-hours for Reserve Capacity from 1 to 10 minutes

RC	1	2	3	4	5	6	7	8	9	10
A-h	0.42	0.83	1.25	1.67	2.08	2.50	2.92	3.33	3.75	4.17

Of course the proper way to find an equivalent battery would be to examine the datasheets of each battery and particularly the discharge characteristics curves.

References

[1]  Wikipedia contributors. "Automotive battery." Wikipedia, The Free Encyclopedia. Wikipedia, The Free Encyclopedia, 7 Feb. 2015. Web. 10 Feb. 2015.

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