Cut some strips of fusible interfacing and butt them together and iron the interfacing to them. I do this all the time.
Mary in TX

I bought batting seam tape to put batting pieces together, I haven't used it yet, as my projects lately go to the long armer. I got it at Keepsake Quilting. It is MADE IN USA, in good ole Limerick Maine. Try to use MADE IN USA as much as possible.

I trim the two pieces and butt the edges together, then use a very large whip stitch by hand, and don't pull tightly. My goal is to get the pieces together without compressing the batting, so that the seam is not detected after the quilting is finished. This has worked well for me for the last 30 years.

I have joined pieces of batting to use in coasters, used a lot of them up. I was surprised how well it worked and could not feel the seam. Great way to use up batting, which takes up even more room than fabric. Thanks for bringing this topic up!

I do like Mary Booth......cut 2 or 2 1/2 inch strips of fusible interfacing, butt the pieces together and join them. I have better luck if I use a press cloth and a little steam. The fusible just sinks right in to the batting. I almost always use W and N.

I use a very lightweight fusible interfacing (cut into strips) making sure that the edges are straight so the edges butt up next to one another before pressing. No one can tell because it's so much smoother then a zig zag stitch. I always have used Hobbs Cotton batting.

Me too, (except the 30 years) and I really enjoy the process and the use of all my materials that way.

I haven't tried it yet, but plan to do so soon. Thanks for all the advise.

This is what I've done, too. I figure if they are quilted, they aren't going anywhere. But reading all these posts just now made me quake until I got to yours. I figured I had been making a big boo-boo, even though I've used this patched together batting only in wall hangings or other small pieces.

The last quilt I made, I laid the back on the floor and put batting pieces over it to cover. (Lots of pieces.) Then I moved each piece, put Elmer's School Glue onto the back, and put the piece back down. They were all overlapped about 1/2 inch. After everything was down, I used regular fabric glue, the kind that does not wash out, and put a drop every few inches under each overlap. That way I knew the batt would not move and shift if I hadn't quilted closely enough. The quilt had to stay on the floor for a day (bummer) so that the fabric glue could dry in place.
When I FMQed it, I could feel the layers with my fingers, especially where more than two layers came together on corners, but normally you would never notice.

Small, really scrap pieces of batt do not go into dog beds here. Only fabric scraps (the lint) go there. I have a small wastebasket lined with strong fabric sewn into a pillowcase-like shape, and all the batting scraps go into it. I try to pull the batt pieces apart so they are more fluffy. Then when the basket is full, it is sewn shut, and the resulting "pillow" can be donated to the food pantry or Salvation Army etc. I try to get them stuffed quite firmly. I've made dozens of these. Our food pantry lady loves them.