freelance devs:

do you experiment with new technology on your client projects (and do you bill (some of) the extra time?) or on your own time (meant for learning)?

Good question. I'm waiting for the answers too.

For me it always depends on my agreement with the client. If they really wanted the technology that I never used, then of course I would bill learning time (most of it).

I would never experiment on my own just for the sake of it. That's what personal projects are for.

But if I thought certain technology could greatly improve performance (for example), there are chances I would use it. In that case I'd probably spent a lot of "free time" learning it and bill trial-and-error code at least 90%.

As long as you work on the project, and there is a progress, you should bill it.

If there's a bug because I didn't know how to use technology properly (and it was my own decision to use it), I would probably fix it without billing it. Learning is, in the end, something that you do for yourself and results stay with you much longer than whatever you get paid.

I think you should give part of the decision to your client. Especially if different technology will take longer and cost more.