Solar Power Manager 5v v1.1 DFR0559 Lipo Battery taking out USB5Vdc output

USB 5vdc input connected to 2.5A rated supply

USB 5vdc output powering ESP8266 ok

As soon as lipo battery 900mA (3.7-3.8 vdc) connected 

The USB5vdc output drops out, taking ESP8266 off line why?

When Battery only connected

No USB 5vdc In charging connected

Only approx 1.8vdc on measured on USB output

Any ideas / help appreciated.