Function of the 100uF capacitor from VBAT to GND?

Question: What's the function of the 100uF capacitor from VBAT to GND?

 

Answer:

The 100uF cap (the bulk capacitor) is primarily to support the supply during temporary battery disconnection during physical shock (eg banging or dropping).

The 100uF should be close to the battery terminals. The 100uF cap should be placed between the power connector to the PCB and the supply line/plane that feed VBAT to the radio. This effectively bypasses the inductance of the battery and or power supply leads which may be very long. The resistance of the VBAT supply line between 100uF cap and the radio should be kept very low - milliohms - to ensure no extra droop voltage when the switching regulator is running. So, for a four layer board where VBAT is an internal plane, the 100uF cap can probably be anywhere as the resistance of the plane is extremely low. But, for two layer boards where VBAT is a trace, care should be taken to ensure that the trace is wide between the power connector to the cap and the cap to the radio so that the overall resistance is in the milli-ohm range.

Note:  The output impedance of near dead batteries will be around 1-2 ohms which causes a lot of ripple when the switching regulator is running due to the 400mA peak inductor charging current. You don't want the layout parasitic resistance to add to the battery resistance so that you can eek out all the energy from the battery that is possible before your system dies. This means that the battery wire resistance should be kept very low as well so that only the battery internal resistance is the limiting factor.