Timer to Monitor Transmitter Battery Life

I would like to set up a timer that runs continuously, whenever the transmitter is powered on, and retains the cumulative value, as a way to measure Transmitter Battery life per charge. Does any one have a suggestion as to how to do that?

Per the Manual (Page 39) a permanent timer should do the trick, but the problem is, it doesn't automatically start running on power up.

Also, it appears that the displayed value is "minutes: seconds". What happens when the timer exceeds one hour? Does it then display "hours:minutes:seconds" (Preferably), or still just "minutes: seconds"? (i.e. 248:52, for 4hours, 8 minutes, and 52 seconds).

If the time exceeds 59:59, the hours are shown too (up to 99:59:59, if my remembrance works correctly).

I follow that it makes sense for countdown and stopwatch timers to be model specific, since they are typically used to manage and monitor flight times, and every model could have different battery sizes / capacities / current draws etc, but shouldn't "Permanent" timers really be model independent? (I understand that they're not currently implemented that way, but I'm just asking the question from a philosophical standpoint. i.e. What would one use a permanent timer for, that would REQUIRE being on a per model basis?)

@henkerhaus: One example would be a rebuild schedule based on time rather than number of flights.