very easy plug in hho hybrid

hi all i am very new and excited to be part of hho forums, how easy or hard would it be to make your very own plug in hho hybrid? and would it be any more efficient than using the alternator , meaning if you took say a deep cycle battery hooked up to a battery charger that you plugged in over night
and used that to run your hho. would it be any more efficient than running your hho off of the vehicles alternator ? keep in mind that electrical companys power is produced 2 to 4 times more efficient than an ice , the only question is is the extra battery extra weight 960 or 70 lbs) worth its weight i am thinking that you could always hook up a battery isolator and high powered toggle for longer trips . thanks for any help:)