Battery efficiency questions in the context of running

I've followed Gordon's advice, uploaded "gps only" on a fully charged banglejs2. I was able to race 2 hours with the run+ app and the wrist heart rate monitor running. Yeees ! A few questions still.

• Would it draw more power to use the btrhm instead of the internal heart rate monitor of the bjs2 ?
  
• I've asked in a different thread if it's possible to get more data out of a bthrm (ground contact time, stride length, vertical oscillation,...). Would it draw noticeably more power to get a bit more data out of the bthrm ?
  -I have "Wake on Tap" and "Wake on button" on on my watch ("face up" and "twist" are off). I noticed during my runs, that the screen lights up without me touching it. Is it related to "wake on tap" being on ? (actually going to answer myself to this question).
  Now I know the battery can last a half marathon, very happy with that. Next year, I will probably up the game to the marathon. Any chance the bjs2 can make it ? Are there some enhancements, tweaks that would help (considering there's a lot of time to think about it) ?
  

I noticed during my runs, that the screen lights up without me touching it. Is it related to "wake on tap" being on ?

Check this out: https://forum.espruino.com/conversations­/392603/#comment17230115

I don't remember if we already went through this before. But what settings for runplus and recorder do you use? E.g. interval for logging data, etc.

Would it draw more power to use the btrhm instead of the internal heart rate monitor of the bjs2 ?

'it depends' on the connection that gets negotiated with the HRM, but it's quite possible it'd be less power (as long as the on-watch HRM is turned off).

Compared to the GPS most should be negligible though.

Would it draw noticeably more power to get a bit more data out of the bthrm ?

Probably not noticeably, if there was no calculation involved.

I noticed during my runs, that the screen lights up without me touching it.

Yes, it's wake on tap. You can turn on 'wake on double tap' but even that does turn on sometimes, so if you really want to eke out your battery I'd turn it off.

Are there some enhancements, tweaks that would help

GPS is really the biggest power hog (assuming your backlight is off) but really that should last for maybe 7 hours.

It's possible the GPS could be configured differently to draw less power (eg by only receiving signals 20% of the time), but unfortunately the commands you can send to the GPS aren't well documented. Maybe if someone did a bit more digging they might be able to find something

Thanks !
I switched off wake on tap and voila! It doesn't turn on on its own.
To answer @Ganblejs, my settings in Runplus are:

• 4 boxes : Distance, time, pace avg, HR.
  
• notifications : buzz every 1km (shouldn't be too bad)
  my settings in Recorder:
  
• Time period 1second.
  
• Log gps, steps, bthr
  Maybe, for really long runs I should use the run app instead of the fancier runplus (I ran runplus on race day, but didn't use the second screen).
  
  

GPS is really the biggest power hog (assuming your backlight is off) but really that should last for maybe 7 hours.

Maybe it's because the screen was sometimes on, but after 2 hours, the battery gauge was red and very thin. I'll do more real life testing in the coming weeks.

the battery gauge was red and very thin

Well, at least in testing here with the GPS on (in GPS only mode) and with a lock, it draws 20mA, so the 175mAh battery should be able to do at least 7 hours.

However, I guess I'm not factoring in the battery usage from the Run app updating every second, so that could definitely be a bit of a hit to battery life.

Also, the increased power draw will actually drop the battery voltage which will make the battery appear more empty than it is. It'd be interesting to see what the battery level ends up showing 1 minute after you've turned off the GPS and run app - it might be a little higher (although maybe that only accounts for around 20% capacity).

Yes @Gordon, you mentioned that in another thread : the battery level incrementally increases in the 10 minutes following the end of the run. Possibly by up to 20%.
I suppose by the end of next month I will have done a more than 2 hours test. I'd be amazed if I can run a marathon with a bjs2.

However, I guess I'm not factoring in the battery usage from the Run app updating every second, so that could definitely be a bit of a hit to battery life.

I don't remember how redraws are done in run but maybe we add a setting for the redraw interval? I'm thinking 1 (or 'on new data'...), 2, 3, 5, 10 seconds - as choices.

Maybe, for really long runs I should use the run app instead of the fancier runplus (I ran runplus on race day, but didn't use the second screen).

The karvonen screens code should not run, i.e. not add extra battery load, if it's not visible. See https://github.com/espruino/BangleApps/b­lob/master/apps%2Frunplus%2Fapp.js#L201, where its refresh interval is cleared when moving away. So I don't think there should be any difference really if you don't use that screen. But doesn't hurt to try run to compare.