Those heated grips are designed to turn off automatically when the battery voltage drops below a certain threshold, if I were you I'd check the following before assuming its a problem with the grips.
Check that the bike is charging correctly, running voltage should be around 13-15v while holding revs at around one third of maximum.
Make sure you're not over loading the bikes charging system, if you're charging voltage is fine without extras turned on try turning them on one at a time until everything is turned on. you may find you're trying to draw too much power at once and the charging system can't keep up.
Do you have any cold starting issues ? it may be an old battery with dead cells meaning its lost some of its capacity to hold onto a charge, this is pretty hard to test for and most people only notice when the battery can't start the bike anymore.
If none of that applies and your grips are within warranty I'd get in touch with the place you bought them from, there could be a fault with the controller which will need sending back to Oxford for inspection before they issue a replacement.
