"Maine suggested a 20 hour capacity test. A "load test" is a very different thing, as it is meant to simulate the high current draw of a car/truck's starter battery."
Not at all. The common load test that you are talking about uses a high load for a short time. A 2-hour capacity test uses a lower load for a longer time, but it is still a LOAD TEST used to measure capacity.
That might be different jargon to a power engineer, but in plain English, they're both load tests, just using different sized loads for different times.
A carbon pile load test tells you nothing about the actual Ah capacity
of the battery. It is a short duration "impulse" or cranking amp test but does nothing to tell you how many Ah's your battery can deliver over long durations at lower loads. It is an utterly useless test in terms of knowing anything about the physical capacity of cycling banks. If you need to know the battery can start your Camaro then ok, but for cycling banks, a useless test unless looking for dead/alive but capacity unknown.
I have batteries that pass carbon pile tests with flying colors that have lost 30% + of actual Ah capacity. I have tested 100Ah batteries that pass a carbon pile test that come in at just 60Ah's of capacity...
The only accurate way to know if a battery has lost Ah capacity is to do a controlled 20 hour test or a reserve capacity (RC) test which is a 25A load for the batteries rated minutes.
All a typical "load test" tells you is if the battery can start an engine, which is easy.
A 20 hour capacity test is a 20% of "C" as a load until the battery hits 10.5V and you count Ah's delivered during this test or count time. Counting Ah's and using a relay to terminate the load at 10.5V is much easier.
25A for XXX rated minutes or until battery voltage hits 10.5V - count minutes before battery hits 10.5V. This requires human attention for the duration and with used batteries may not always translate as well to house bank loads, which are usually closer to the 20 hour rate...
If the battery is rated at 5 hours or 10 hours these can also be tested and the test is shorter but the amperage higher..
Controlled capacity tests are not easy and you need the equipment to do it, which most boaters don't have nor do typical battery shops........ Concord battery will happily sell you a tester for $3000.00 plus dollars but they are optimized for the aircraft tests which is a different rate.....
The Midtronics/impedance analyzers come closest on FLA batteries but do not work as well on GEL or AGM to translate to actual Ah capacity. They more closely correlate however than does a carbon pile test.
20 Hour Capacity Test - 100Ah Battery:
Battery allowed to warm to 75F to avoid temp compensation calculations and mimic factory testing. I set my testing room at 75F to make this easier.
Battery charged to full & current allowed to fall to 0.5% of "C" at 75F and rated absorption voltage. This is 0.5A at 14.XV for what ever the particular battery requires..
Ah/Coloumb counter reset to 0.
A load of 5A is applied to the battery and this 5A load is held constant (requires manual manipulation to maintain 5A as terminal voltage falls) or a custom built adjustable load cell.. The test load applied to the battery is determined by: Rated Ah capacity divided by 20 or 100Ah/20=5A. A 225Ah battery would be 225Ah/20=11.25A load.
Test load is terminated once battery terminal voltage hits 10.5V
Record Ah's delivered and compare to the factory rated 20 hour capacity. If you lack an Ah counter you count hours & minutes but this is very tedious....
Recharge battery at 20 hour rate or a charge rate of 5A for a 100Ah battery or 11A +/- for a 225Ah battery....