Been through the whole battery rebuild with a rebuilder here in town and trying it myself. Waste of time. If this is an occasional use tool (which I would rate Ryobi as that) then I would do something like this

https://goo.gl/HhFj5D

I have used a few of these, and when you get them from a reputable dealer they are a great deal.

BTW, using a battery powered circular saw to cut up old pallets falls far beyond the range of that tool. I have two of the Ryobi circular saws and I get a lot of use out of them, but continued use of chewing up dirty hardwood isn't one of them. I can cut up a couple of sheets of plywood, a bunch of 2X4s and have no problems. When I do demo, the saw doesn't have any real wattage power to back up hard, continuous work.

If this is your first little battery powered circular saw, don't think this is in the league of those big 20V things they are selling now for site work. These are handy, lightweight saws made for convenience, not a day's work.. You have to keep a sharp, CLEAN blade on the saw, and cut straight lines.. More importantly, you can't get much more than few cuts with the little black batteries that ship with the saw (all the more reason for secondary market batteries)and really need more amp hours than they provide.

I found the Ryobi 4ah batteries two for $49 a couple of years ago and sprung for them. It was like a completely different saw.

Robert