What "losing quality" actually means
Quality loss almost never comes from the "batch" part itself. Converting 50 files at once doesn't damage them any more than converting one does. What actually causes it is re-encoding an already-lossy file over and over, or a tool quietly resizing or re-compressing images to a lower setting behind your back to save its own processing time or storage — which happens more than you'd think with free tools trying to keep their server costs down.
Common batch-conversion mistakes
The most common mistake is generational loss — take a JPG, convert it to another lossy format, then convert that result again, and each pass throws away a bit more. It compounds fast. Another one is inconsistent settings across a batch, where some files end up compressed harder than others because the tool just applies one blanket quality setting regardless of what's actually in the image.
There's a sneakier third issue too: uploading to a server-based converter. Aside from the privacy angle, a lot of free web tools quietly downscale your images before processing, purely to save their own bandwidth and compute.
How to batch convert properly
Convert from your original files, not a copy that's already been compressed once, whenever you can. Keep the resolution as-is going in — let the output format's encoding do the size work instead of pre-shrinking things yourself. And lean toward tools that process everything locally on your device, since there's no upload step where something could be quietly re-compressing your files without telling you.
Checking your output
After converting a batch, actually zoom into a few pages at 100%, especially ones with text or fine detail, instead of trusting the thumbnail. If something looks soft or blocky, nine times out of ten the source image was already compressed before you even touched it — not something the conversion introduced.