Ryobi Adapter

They sell these, but they're underpowered. I'm going to build mine at 20V, that's what the 18V battery measures when it's charged.

I'll start with a 24V transformer. I found a MC33167 that might get it down to 20V (rate 5A). LT1270ACT is rated at 10A

this guy used 20 amps. I found this on Amazon, which seems like it would work pretty well.

Deep Cycle

The other option I'm considering is a deep cycle battery. The catch is that I can't get one near 20v, so I need a boost converter. But I can just double the voltage. The deep cycle battery would probably be around $25. I need a charger and a boost circuit. I think I need a 4-throw switch to change modes.