Reserve Capacity Testing On Lead-Acid Car Battery
The Battery Council International (BCI) created a group of standardized specifications for lead-acid batteries including Reserve Capacity.  This particular test gave consumers a useful comparison for a lead-acid battery.  Measuring the full discharge in minutes is the reserve time a consumer could expect with a new battery after an alternator failure and a standard  25A constant drain.

Although I was not aware of this standard test until this question, I agree it is quicker, reliable and more accurate than leaving all your vehicle lights on and hoping it will start in the morning.

Although it may not be the criteria used for warranty purposes or factory acceptance testing, I agree it is an important consumer test for State of Health (SOH). The ratio to the original battery spec or similar replacements is a valid metric.

Reason:     


It demands both a good short term 30 second crank (CA or CCA) and long term C20 max capacity (20hr CC) performance which uses all the storage capacitance and not just the short term charge without needing a precision piece of specialized test equipment or knowledge of the 20h battery capacity to choose the right CC current level.
It can be used as a ratio of the Mfg spec for a new battery.


Reserve Capacity, RC is not a test you want to do often, nor is it practical for manufacturers to spend this much time but it is relatively easy to perform. 

Here are two different priced batteries comparing their specs from one supplier for Absorbed Glass Mat (AGM) 12V batteries.

RC: RESERVE CAPACITY in minutes for a fully-charged battery to supply 25 amps at 80°F (28'C), until the battery is discharged down to 10.5 volts (1.75V/cell)

AGM compare:  Batt A (D27F) Batt B (D51)
-----------   ----------    -----------
RC: (25A)*    140 minutes.  66 minutes

*RC equiv.*   58Ah          27.5Ah  *calculated

C20 Capacity: 66 Ah         38 Ah
CCA:(30s)     830 Amp       450 Amp 
CA: (30s)     1025 Amp      575 Amp 

*CA equiv.*   8.5 Ah        4.8 Ah  *calculated

charge cycles 300+          300+ 
Weight:       53.2 lb       26 lb 
Ri:           0.0025 Ohms   0.0046 Ohms
Warranty:     12 mo.        12 mo.
Series w/o BMS 1 mo.        1 mo. 


RC: how long a fully-charged battery can deliver 25 amps of current in an 80°F environment before the battery is discharged down to 10.5 volts (1.75V/cell)

Reserve capacity, RC is the number of minutes a battery can maintain a useful voltage with a constant 25A drain from a full charge.

AMP-HOUR Rating. This is battery capacity and is measured by the 20-hour constant current product from 100% to 0% in 20 Hrs.

CCA is the maximum amperes sustained for 30 sec. when 12.5V battery with Sp. Gr.= 1.255 - 1.260 is loaded down to 7.5V at 0°F. This applies to starter batteries.

(Thanks for the question, I learnt something new)