[echo-francis]

I will describe my setup:
I have a lifepo4 20ah battery. I choose 20ah because with my p&g I draw about 3ah (all accessories off no DRL) that give me about 6 hour range. but if I do not use p&g I can go about 2 hour if I p&g with lights on my range is about 1 hour. add blower or other accessories and bye bye range
the other part of my setup is my ultracapacitor. the primary function of my ultracapacitor is to help my battery to start my car because it can do it alone.
the secondary function is when I have long trip I remove my lifepo4 and put my alt belt back on and drive with the ultracapacitor only. I did my ultracapacitor is 16v because my lifepo4 charger and my alt charge at around 14v and 5 ultracapacitor would have been too tight

if your allways using your alt a ultracapacitor is a good idea because its lightweight and waste less energy to be charged but can be drain in one day not using your car. you can add a small solar panel to fix that.