SIM808 Battery Charge

Hi All

Just wondering if anybody else has come across this. I have an ESP8266 and SIM808 circuit all working fine generally but it was crashing every 12 hours or so while trying to connect to the server. Anyway I left that on the 'to come back to' list and started to plot the data was sending to the server. Now one stream of data it sends is the LiPo battery percentage as read by the SIM808 AT commands. It was then I noticed that even though this circuit is connected to a mains supply with the LiPo being charged constantly the SIM808 battery percentage dives down to 0% every 12 hours or so and then very quickly springs up.

I am using this SIM808 module ( with a micro USB cable attached which is connected to a 5v PSU (iPad Charger). The rest of the circuit (ESP8266 etc) is powered from a 3.3v breadboard PSU (grounds linked etc).

Any ideas on the odd battery readings? I have added a check to my code so that if the battery is below 5% it doesn't attempt to connect to GSM network because this seems to be whats causing the crashing.