Problems faced while billing the client

I am facing some problems regarding the transparency levels and sharing backlogs related information with the client. We charge our clients on the basis of time spent, and it would be difficult to convince the client to pay for each and every activity we carry out in the sprint, since some of the tasks we have to carry out include R&D and learning new aspects of the technology, and it would be difficult to convince the customer to pay for such research oriented activities. I have the following problems: 1. We bill our client on the basis of time spent (working hours), but not everything we do in the Sprint is billable to the client. 2. We have to spend time in fixing bugs and issues, but we cannot bill the client for these activities. 3. Our team velocity is less since most of our developers are relatively new and need more time to develop features compared to experienced ones. As far as requirements are concerned, suppose we estimate 45 working hours to develop a feature, the client insists it should not take more than 20 or 25 hours. What is the best way to tackle these problems?

Posted By - JJG