BangleApps/apps/bootbthomebatt/boot.js

var btHomeBatterySequence = 0;

function advertiseBTHomeBattery() {
  var advert = [ 0x40, /* BTHome Device Information
              bit 0: "Encryption flag"
              bit 1-4: "Reserved for future use"
              bit 5-7: "BTHome Version" */

              0x00, // Sequence number
              btHomeBatterySequence,

              0x01, // Battery, 8 bit
              E.getBattery()
  ];

  if(Array.isArray(Bangle.bleAdvert)){
    var found = false;
    for(var ad in Bangle.bleAdvert){
      if(ad[0xFCD2]){
        ad[0xFCD2] = advert;
        found = true;
        break;
      }
    }
    if(!found)
      Bangle.bleAdvert.push({ 0xFCD2: advert });
  } else {
    Bangle.bleAdvert[0xFCD2] = advert;
  }
  NRF.setAdvertising(Bangle.bleAdvert);
  btHomeBatterySequence = (btHomeBatterySequence + 1) & 255;
}

if (!Bangle.bleAdvert) Bangle.bleAdvert = {};
setInterval(function() {
  advertiseBTHomeBattery();
}, 300000); // update every 5 min

advertiseBTHomeBattery();