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. 

[Jam Alan]

A Li-ion cell typically varies from about 4.2 V (fully charged) down to 3.0–3.3 V (near depletion). If the BLE radio's PA is powered directly from the battery (or from a regulator that does not tightly regulate the PA supply), the maximum RF output power often decreases as the supply voltage falls. speed stars