My method for testing batteries falls back to the tried and true old-school method. If the ambient temperature is not 20°C to 25°C, then adjust the measured results to the manufacturer's specifications. (I prefer to just test in the specified temperature range.)
Step 1 - Properly and fully charge the battery to 100% for 24 hours.
Step 2 - Fully disconnect and allow the battery to remain idle for 24 hours.
Step 3 - Connect the test equipment that will permit measuring the battery voltage, measuring the output current and permit application of a C/20 Hour load. Note that C is the battery capacity, such as 100AH, Ampere-Hour. In that example 100AH / 20H = 5A load
Step 4 - Start the test, noting time zero, voltage measured at time zero, apply the C/20 Hour load and current measured at time zero.
Step 5 - Log the voltage and current throughout the test.
Step 6 - Stop the test, remove the current load when the voltage reaches the manufacturer's specified end value, such as 10.5V.
Step 7 - Properly and fully recharge the battery to 100% for 24 hours.
The battery meets the new nameplate rating when the measured ampere-hour capacity is met or exceeded.
The battery is typically considered end-of-life when the measured ampere-hour capacity is under 50% of the nameplate rating, in this example if the battery cannot supply 50AH during the test.