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A certain manufacturer sizes their chargers by 20 hour capacity but the data I have on my batteries is only 6 and 8 hour...

So, if I know that a specific battery has a 6 hour capacity of 166 ah and an 8 hour capacity of 170 ah how do I know what that battery's 20 hour capacity is?!... Is it possible to calculate the 20 hour capacity from the above data?!...

Don't know how your battery manufacturer rated its batteries. While most do it at the C20 rate, not all do. If you were to see a battery rated at 100AH, it would have been tested at a 20 hour rate, unless otherwise noted. This means that the manufacturer posted the 100AH rating on the battery after testing that battery for 20 hours with an actual amperage drain of 5 amps

I'd contact the battery maker and ask them for the C 20 rate. Unfortunately the rate is not linear because if you upped the load to 15amps, the battery wouldn't last 6.6 hours but a much smaller number.