A few years ago, my brother and I told dad, not asked, told dad "time to go, they're under 15%".  He was skiddish because, "you can't dry beans without heat."  The wettest were close to 15%, I don't think anything went in the bin under 13%.  Well, a few edges and low spots were pretty cabbagey but you get the idea. I don't remember how we ran the fan, but it ran a lot.  1.9 maturity in central MN so not a late bean, probably started taking them in late Sept. so we had some decent weather to work with. The fan is no bigger than 1hp on a 27', 7 short ring bin.  Every load was sold at 12.9%, maybe 1 or 2 varied a bit lower but not much.  It's like the beans wanted to be at 13%.

In dad's defense, ~15 years ago he had to take beans in the high teens.  That was the year you couldn't buy a burner because everybody in the region had wet beans.  Guy were setting up shop heaters and blowing heat through the bin fans to dry beans.  Dad didn't wait to see if the bin fan would take the moisture out.  After harvest they went on the truck and he took the dockage.  Didn't dare risk the whole crop in the hopes they would dry without a proper drying setup.

Long story short, yes you can dry beans in the bin without heat, but I don't know how much.  I wouldn't worry at all about taking them around 15%, but much higher would keep me up at night.  All it would take is a couple weeks of on and off rainy weather to stop the fans and those beans would be producing their own heat.

Edit:  Here, taking beans early gives you better weather to run the fans