Add note about trying not to runn the Bangle battery flat

Author: gfwilliams

apps/chargent/README.md

@@ -1,6 +1,8 @@
 # Charge Gently
 
-Charging Li-ion batteries to their full capacity has a significant impact on their lifespan. If possible, it is good practice to charge more often, but only to a certain lower capacity.
+Charging Li-ion batteries to their full capacity can lower their lifespan. If possible, it is good practice to charge more often, but only to 80% capacity.
+
+**Note:** continually running your Bangle.js battery flat will impact its life far more than charging it fully - only use this app if you're able to charge often enough that the Bangle.js battery isn't dropping below 20%.
 
 The first stage of charging Li-ion ends at ~80% capacity when the charge voltage reaches its peak*. When that happens, the watch will buzz twice every 30s to remind you to disconnect the watch.
 
@@ -12,6 +14,7 @@ New in v0.03: before the very first buzz, the average value after the peak is wr
 
 Side notes
 - Full capacity is reached after charge current drops to an insignificant level. This is quite some time after charge voltage reached its peak / `E.getBattery()` returns 100.
+- The battery is 175mAh and charging happens at 100mA, so a full charge from flat will take around 1.75 hours.
 - This app starts buzzing some time after `E.getBattery()` returns 100 (~15min on my watch), and at least 5min after the peak to account for noise.
 
 \* according to https://batteryuniversity.com/article/bu-409-charging-lithium-ion assuming similar characteristics and readouts from pin `D30` approximate charge voltage