I use batting tape I got from Nancy's Notion site but I'm sure other sites have it too. There are 2 widths so be careful which one you order. I got the wider one and use it all the time to piece my batting leftovers. Also make sure that the pieces are turned on the correct side before adding the tape. After I've added a piece, I pin a scrap paper with the size of the pieced batting so I know if it will work on my next project or if I need to add another piece to make it fit. This keeps the cutoffs at a minimum laying around. I keep the smaller pieces under my cutting table for when I'm making embroidered items that need a piece of batting. So not much goes to waste.

I have been zig-zag with the pieces of batting left. I make the zig zag big and put them on the sewing machine and put the pieces together. They work beautifully!!!

Exactly what I do. If they are butted together, you can't even feel the join afterwards. It doesn't take long at all, I know it's secure everywhere even if I use an open quilting design, and it's never going to come unglued with time.

Batting pieces
 

I recently discovered the tape that I see a cpl of others have mentioned. It is wonderful. I make sure pcs of batting have straight edges. Butt two edges together. Lay the tale at the beginning of the butted edge. Iron the tape down. Works great. Sorry but I can’t remember where I bought it online. It’s called “Heat Press”. Comes in 1/2 & 1 inch rolls. I love it & can save a lot of batting scraps.