Switching between backup battery and main power supply
It is possible, that GPS did not went to lower power mode when you power off MCU. It seems that there's a pin '24' for this. If MCU is still driving TX pin to Vcc then your MCU have phantom power going through this connection. You should disconnect communication lines when you turn off eighter device to keep off from such effects.







simulate this circuit &ndash; Schematic created using CircuitLab

It is always good to avoid phantom power issues. There are some methods for it.


There could be specific interface chips. Unfortunately I do not remember single one.
There are several logic level shifters (like TXS0102) that implement disconnection (Hi-Z) when power on ether side goes low.
Tristate buffers.
Open-L (open-collector/open-drain) signalling with pullup at receiver side.
As a low man's solution a resistor limiting current through signal wire to reasonable value.