The diode and battery trick is possible but there is a problem. Since the diode lowers the voltage to the sealed battery by a volt or so it lowers the charging voltage to the sealed battery. If your alternator cranks out 14.5 volts when running and the diode sucks a volt then the sealed battery will only see 13-13.5 volts.
This is barely enough to charge the battery in my experience. I did however have the sealed battery setup to charge only when the car was running so if I drove the car only a few minutes a day it wouldn't have enough time to charge.
I figured that if I set the sealed battery to charge even if the car was off then I would be constantly draining my battery because of the voltage difference. Bottom line it can be done but takes some work.