The battery in the GSC10 can easily be changed. It is a standard CR2032 type battery. See page 27 of the user manual for the Edge500 (available from the Garmin website).
Ditto for the heart rate monitor, same battery - see page 22 of the manual.
The display is infinitely changeable - you can easily change it to have both heart rate and cadence on the same page (max. of 3 pages, max. of 8 data fields on each). But note - if you use 8 data fields per page, the fields get a bit small. In my case I need my reading glasses - not very practical while going flat out.
Page 35 of the manual explains how to do this - press and hold MENU, select Settings> Bike Settings> Data Fields
2 years, 5 months ago