Seeed c3 battery and solar combo

Hi, new to esp and lovings the seeed c3, however im struggling with a strange issue. i have a soil moisture sensor connected and working fine, a lithium battery on the battery pads and a 5v solar panel connected to usb. on battery power all is good consistantly, however when the solar panel is plugged in it still works but when the sun starts to drop the volatage of the system seems to get dragged down with the panel until its totaly dark and only then does it switch over to battery. so the battery can be 3.8v but when solar drops it can go down to 2.5v and seems not to switch over to battery soon enough if that makes sense. what am i doing wrong.

The voltage supplied by the USB connector must be 5 V. To switch to battery, the 5 V supply must be completely disconnected. How many volts it takes to switch depends on the characteristics of PMOS used in this switch, which can vary widely. In your case it would have happened at 2.5V.

What is the maximum voltage supplied to the USB connector?

the usb is fed with a 5v solar panel so its up and down, i was kinda expecting the switchover to battery to be more seamless. is there a way of making it switchover sooner or a better way ?

the usb is fed with a 5v solar panel so its up and down

Note that the on-board 3.3 V regulator may be damaged if more than 6.5 V is applied to the USB connector.

The breakout boards in the link below may work; EN pin can be used to cut off the output.
Adafruit MiniBoost 5V @ 1A - TPS61023 : ID 4654 : Adafruit Industries, Unique & fun DIY electronics and kits

MPM3610 5V Buck Converter Breakout - 21V In 5V Out at 1.2A : ID 4739 : Adafruit Industries, Unique & fun DIY electronics and kits

VERTER 5V USB Buck-Boost - 500mA from 3V-5V / 1000ma from 5V-12V [TPS63060] : ID 2190 : Adafruit Industries, Unique & fun DIY electronics and kits