number one, 12.5 is not fully charged, maybe 75% or so
number two, your batteries will not take 50amps when they are fully charged
if they are you have the voltage set way to high,, at full charge i would expect
maybe 3-5 amps max.
your method of using a amp meter to determine state of charge is also flawed
it will tell you what the load is taking from the batteries or if connected correctly it will tell you how much power is being put back into the batteries but
that is all.
you can use a good voltmeter to get a rough idea of state of charge, but
you will need a couple of things, one being a good hydrometer and the other is
what the specific gravity is on a fully charged cell in your battery and you get that from your batteries manufacture.
while you are talking to the manufacture, ask him for a chart of state of charge
and its correlation to specific gravity,
it should read something like 100% state of charge = 1.270 (or possibly higher)
75% state of charge = 1.250
50% state of charge = 1.230
then use your batteries down to whatever the specific gravity should be according to the manufacture (at 50% = 1.230)and take a voltage reading right there,, this will be approx where 50% state of charge will be as measured in volts.
that will get you close,,, but temperature compensation comes into play as does
whether or not you let the cells rest for 24 hours or not before you measure.
i think it will get you close enough to learn how your batteries act, keep a good chart of specific gravity, volts, and time resting and after a while you can tailor
your voltage to mean closer to your goal.
just remember specific gravity on a rested cell at 77F is the most accurate method of determining state of charge.
hope that helps, i am sure others will chime in and maybe point out what i have missed or misstated.
bob g