I have 4 tubular C20 batteries, 225 Ah each (2 series-parallel-2 series) for a residential solar system.
I suppose the below is true:
C20 gives us the recommended current to obtain the advertised cycle life (900 cycles at 80% DoD) using the below equation:
My battery bank is 450 Ah divided by 20 hours; 450/20 = 22.5 A.
If I need to discharge for 15 to 20 minutes (less than 20 hours), can I use more than 22.5 A and still maintain the same cycle life?
What is the maximum current in this case?
There is no such thing as a "maximum current" for a cell (the way there is for a fuse, for example). If you read that a cell has a "maximum current of 1 A" it doesn't mean that it's perfectly fine at 0.99 A and that at 1.01 A it's destroyed. Instead, it's a continuum. The higher the current, the more the degradation. 0.1 A is better than 1 A which is better than 10 A, all the way to the short-circuit current of a cell. When they say "maximum current of 1 A", that's just a compromise value that they picked. They could have picked 0.5 A and increased the expected life, or 2 A and decreased the expected life.
So, when you ask "What is a safe max. discharge current for a tubular lead acid battery?", there is no specific answer. You want long life? Then either reduce the current or add more cells in parallel. You want high current? Then be prepared to have to replace the cell sooner.
If I need to discharge for 15 to 20 minutes (less than 20 hours) can I use more than 22.5 A and still maintain the same cycle life?
Do you mean "fully discharge from 100 % to 0 % in 15 minutes"? And you're asking if you would get the same life as if you fully discharged over 20 hours? Then, no, you would not.
