Not because I need two, but back in 2004 I bought our trike, it had a Comp-U-Fire alternator in it, the seller put a brand new Odyssey PC680 AGM battery in it in August 2004, I know because I picked the battery up at Batteries Plus on the way to load the trike to haul home. That same PC680 was still doing duties in the trike in late 2015, I decided it was time to replace and bought two on sale, the second one was for a '85 GL1200A I had. Long story, I got sick in later 2016, so bike & trike sat. I took the new Odysseys out, put on a bench, and then in 2017 I was doing better and had a relapse. I also sold my '74 Triumph and decided to let the 1200 go as well. Forward to 2018 and I had a long recovery from my stem cell transplant … & shingles. In Spring of 2019, I got the 1200 out of basement, but could not find the new AGM (nor my electric chainsaw), so I put the 2004 vintage Odyssey in the 1200, it started it up OK. I rode it up through the yard to carport, but I could tell my balance was a mess. I pumped old treated fuel out of the 1200 and replaced with a couple gallons 2019 gas and then ran an ad for the 1200 on Craig's List. After selling the 1200 in June, I was browsing in the basement, and found my "now extra" new Odyssey (and my electric chainsaw I had stashed earlier).
Anyway, I don't need another unused battery sitting on a shelf. I have portable power in a wheeled cart I built housing two Group 65 auto batteries, together they'll start a Cummins. Tonight I made my "holder" to go in the lower trunk of my Lehman to secure the extra battery. It's painted & drying as I type. My plan is to wire it using suitable cabling and a screw type battery disconnect on the negative side between it and "ground". I figure the Comp-U-Fire can keep up. These disconnects are small than those used in cars or trucks, more suited to use with 1/4" bolts.
Not because I need it, I just hate to see a good battery sit unused. This way, I'd at least have access to it if needed on the road. Anyway, just thought might be of interest. I have a couple of these disconnects, thinking I might put one to each battery. Such would maybe be handy when leaving it parked for long periods, like in winter if harsh. Batteries don't push current, but two will supply a plenty if needed.