Trackpad or mouse are frequently disconnected

When I work with my iMac with OS X 10.11.4 and a Magic Trackpad 2 and/or a Magic Mouse the connection between them is very unstable. Regurly I get the message that trackpad or mouse is disconnected and in a short time is reconnected . This is started to occur after installing OS X 10.11.4.

What can be the problem?