In the past, I have taken precisely 45Ah from my 90AH battery at an average amp rate
double that of The 4.5 amps it achieved a 90Ah rating, at temps cooler than 77f.
Voltage rebounded, and will still rebound, at 400+ deep cycles, above 12.2v, and My battery manufacturer says 50% rested voltage is 12.11v.
I figure the 50% discharge test and then seeing if resting voltage is 50% according to manufacturer, is not accurate.
If it is, My AGM battery is more like 115AH capacity instead of the 90AH rating the manufacturer claims. Deliberately underrated? I think not.
One guy on a marine forum, tests used batteries for those going blue water sailing who need to know if they have to replace their bank before leaving. He has a water bath to keep the batteries at precisely 77f, and a constant programmable load which will stay the same as battery voltage drops.
He claims to draw the batteries to 80% DOD twice, then recharge fully, then equalize or condition charge them, then runs the 20 hour test at the precisely the 20 hour amp rate. Sometimes he runs it twice and after the full discharge to 0 and the full recharge and another EQ the capacity increases.
http://www.pbase.com/mainecruising/voltage_vs_soc_lifline_batteryhttp://www.pbase.com/mainecruising/effect_of_charge_rate_on_agm_battery
