Run the ten hour test at the 20AH rate to see how long it takes to get to 50% (50% measured by SG and confirmed by voltage after waiting an hour after removing the load)
240AH 20 hr rate is 12 amps ( four 3 amp lamps or whatever) Use the Trimetric AH counter for more confirmation.
Say it takes 9 hrs to get down to Trojan's 50% SG of 1.172 and after an hour of bounce back you are nearing 12.1v so you are at about 50%
9 out of 10 is 90% or 216AH of 240 (Trimetric AH count for comparison cross-check)
If you use lamps then they dim as voltage drops and your 12 amps gets less so turn on more lamps as test goes on. If you use an inverter and some load to get 12a then you get inverter creep up in amps as voltage drops so in that case start with 11 and end with 13 to get 12 average.
If you are lower in capacity then try a serious session of overcharging at 15.5v to see if you can get the SG up if some of the sulfation is not yet permanent.
