Inquiry regarding the phenomenon where BLE TX power varies depending on the lithium-ion battery voltage

[sangyun.kim]

Device : wearable IOT device

Battery capacity : Max 4.2V (Typ. 3.7V)

BLE Tx Power : Tx power is highest when the battery capacity is 4.2V (measured in air, not conduction)

I wonder if this phenomenon is natural, and if so, what the reason is. 

[sangyun.kim]

I thought it was the nRF52832, but it turns out this phenomenon occured with the nRF5340 MCU. 
Please double-check based on the nRF5340
VDD input voltage is 3.3V(for nRF5340)
RF Tx power is +3dBm

[Ressa]

Hi,

Can you provide more details about power variation? how much it varies? and what is the voltage mode you are using ? For High voltage mode you need at least 2.5 V for reliable operation. IS  the voltage dropping below 2.5 V when you see lower TX power?