I did spring maintenance yesterday on the Predator 3500 (oil change, clean air filter, clean spark arrestor, cleaned the spark plug). I did this since it's been six months, not because of hours of run time. There is 72+ hours on the clock now. Its provided power during outages at home because of weather, no a lot of run time while camping, though it does a great job.
I had the first issue with the 3500 a few days earlier. When I tried to start it the battery was dead after not being used since February. It started on the second pull when I tried the pull cord. Not bad for almost 2 years old!
I pulled the battery to find it did not have a manufacture's name on it or a model number, so I had some difficulty finding a replacement. I looked in the owners manual, but the schematic did not give a part number. I went online to HF parts and found a part number (not available in the store) and price at $69+shipping. So I sent an email to HF. I'm still waiting on that response.
The battery had a label that said "sealed lead acid - 12v
6ah". I measured the battery case and started searching online with the info I had. After looking at specs for many batteries found one that would fit at Battery's Plus. ~$70+tax, with 12 mo warranty. I would have to drive ~40 miles so I kept looking.
I could not find another by searching, so I sent an email to BatteryShark.com. They responded quickly and suggested a SigmasTek STX7L-BS Motorcycle Battery - 12V
6AH (100 cca) for ~$20 + shipping, about $30 total delivered to my door. 6 month warranty. Measurements were perfect fit. It's on the way.
Hope this saves someone from having to search for hours like I did!