Quote:
Originally Posted by philso
Your 25/60 number is close to the 1/2 number I found for converting RCM to an approximate Ahr. Since the RCM is measured at a constant current, regardless of battery, it isn't the same as the Ahr rating though it is close.
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RCM is how many minutes it takes to drain a (fully charged) lead acid battery down to 1.75 volts per cell (10.5 volts for 12V battery) with a constant current of 25 amps.
Thus if you divide RCM by 60 you will get the number of hours it takes, under those test conditions.
If you multiply that number of hours by 25 amps you will get the Ahr rating of the battery, under those test conditions. That's where the (25/60) number came from.
You will get a higher Ahr number if you run a test with a lower test current.
To compare the capacity of an FRC battery to an automotive battery, you have to compare apples to apples. That means running the same test conditions on each battery.
Discharge curves for the NP-18-12 are available in the datasheet. From these curves you can get a rough estimate of the FRC battery RCM rating. It's very roughly 17 minutes. That number can be compared to the RCM of an auto battery to get a rough idea of the relative capacity of the two.