The unit has a small internal battery that needs to be replaced every 3 years.
Ours lasted 6 years then failed without warning a long way from home
Skyforce can replace the battery but it ain't cheap (@ £130)
Alternatively thay will supply the battery and a wiring diagram(not easy)
The manner that the unit fails, i.e. no warning , is not condusive to safe flying so we intend to replace our internal battery regularly