Some confused info among the good so far! Beware! Eg do not draw them down to empty, just go half way down.
EDIT --This is assumed to be at 25C. If you run the test when it is colder, your bank capacity will already be less than rated. Allow for what AH the bank should be at that temp when doing the test.
Your 220AH total is based on the "20 hr rate" so 220/20 = 11 amps as your 20 hr rate.
To test this you need to run an 11 amp load until the batts are down to 50%. If you don't have an AH counter like a Trimetric, you have to use the hydrometer to note when your SG is at the 50% mark for your battery model. Say it is 1.180. You can tell when you are getting close to 50% when the loaded voltage at the 20 hr rate falls to 11.6. The SG will be close now, so spot check.
If they are at full rated AH you will also have been running this for 10 hours. If you reach 10, stop anyway!!! If your batts are a few years old you might hit 50% at 9 or 8.5 hours, indicating they have lost that much capacity (10 to 15% lost)
After you stop, the battery voltage will come back up from 11.6ish to 12.1 after an hour. If it only comes back to 12.0 and stops you overshot and at 10 hours you are at 40% so you can see your capacity is less than rated (you can calculate how much by doing some math)
Note that you can use an inverter to run a 100w lamp which will be 10 amps. As battery voltage drops that inverter draw will go up so by the end of the test it would be more like 12 amps so it comes out ok on average.
If you just run some DC lamps to make up the 11 amps, then as battery voltage drops, the lamps dim and draw fewer amps, so half way into the test (at 5 hours) you will need to turn on another lamp to get the amps back up to 11.
