C20 is the ah capacity when discharged over 20 hrs, I20 would be the constant current during this 20 hr discharge.
C20 is AH capacity not amps
From the same manual
BULK CHARGE
The first phase of the charging process is the bulk charge. This is when the
maximum amount of current flows into the battery bank until a desired voltage
is reached. The recommended charge current is 10%-20% of the AH capacity
of the battery bank, based on the 20 Hr AH rate (C20). Higher current levels
may cause the battery bank to overheat. A lower current may be used; however,
this will prolong charge time and increase the potential for sulfation build-up.
Using the 20 Hr AH rating of the
battery bank (C20)
C = 20 hr RATED CAPACITY (total battery bank)
This setting is typically 2%-3% of the 20 Hr AH rating (C20)
of the battery bank
3. Charge at a low DC current (5-10% of C20 battery capacity).
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