My brother is a Honda parts manager and when I told him my daughter's Fit battery had died and it was barely 3 years old, he told me "wow, it lasted THAT long?" Basically, he told me that those 100-month warranty batteries don't ever last anywhere near the warranty period (which is over 8 years!). Most of them die before even half the warranty period has gone by.I recommend the OEM Honda battery. It's a 100 month warranty and it's full replacement for the first 3 years, not pro rated. At my Honda dealer it was $175 installed but I found it at the dealer's online parts website for around $108. With the core return it was something like $95. It only takes a few minutes to install a battery and I saved $85 installing it myself.
You really need to first find out what's causing this battery drain before you buy more batteries. But get the Honda OEM battery and stop trying to save a buck or two on used batteries.