I will be getting back into this aspect of our home spa improvements. Am making some other 'devices' too. So have a bit of mad scientist going on at the moment. I am prone to look at copper again as it worked very, very well in this ultra simple 20 qt setup. Part of it is getting the salt/ Epsom Salt amounts right too.
But have to say, am thinking this worked pretty well, and we have had some very nice results too. Note that I am using a different 12v battery charger. I have a 2 amp and 10 amp selection switch. Using 2 amp at the moment and will keep it there.