agm battery drained to 1.5v

Yes, just check the terminal voltage regularly to make sure it doesn't go over 15.5v, you don't want to boil it dry, but you do need to stir up the acid water mix created by the recombiners in the cells. If you don't, all the over strength acid is eating the bottom of the plates away will the water on the top gives no capacity for start up/storage, because there is no sulphur from the sulphuric acid to react with the lead and create the voltage at useable amps across the terminals .....

T1 Terry