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.
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.
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.
©2015 maven0.blogspot.com
©2015 maven0.blogspot.com
or...divide RC by 2.4
ReplyDeleteIt will give us capacity @ 25A discharge, so way lower than C/20 amp-hours. That can cause confusion...
ReplyDeleteMy battery (NAPA 8302) is spec'd at RC of 130 minutes and 75 Amp Hrs. If I use the conversion above, I get only 54 Ah for a 130 RC. What's up?
ReplyDeleteat higher current you lose power as heat so that is expected thanks for the ratio I am looking at a 170 RC battery and I wanted to know the AH as I am using it as a deep cycle and so I know to expect close to 100 AH from mine this makes my life easier as far as chosing a charger and how many lights to expect for how long
DeleteI am at wit's end. My battery says: RC 128 @ 56amps. What is the Ah of this thing? So, it will deliver 54amps for 116 minutes? So, 128/60=1.9*56 (@amp)=119Ah. is this right?
ReplyDeleteYes, 119Ah would be right for RC 128 @ 56amps. Real world should be higher because you probably won't be discharging at 56A the whole time, slower discharge leads to longer times before the battery dies.
Delete