Determine Battery Percentage

I am in the process of building a wearable device and using a lithium polymer battery to power it. I'm trying to find a way of determining the battery's charge percentage and then relaying that information to an app that I plan on making which would display it. The app will support both iOS and Android and will be receiving the data from a bluetooth 4.0 module. Does anyone have any idea as to how I can accomplish this? Thanks

Battery monitors are available for yachts that work out the charge percentage by continuously monitoring the charge and discharge currents. For more information, read the description of the example here. Perhaps you could do a similar thing using an Arduino.

Well, not sure if this is still followed by anyone but I did try to do the same thing a while back. I have actually made some research into some battery chargers which can actually display the amount of power left within a battery and I must say that I was a quite impressed about the CTEK chargers. They also come in different sizes and purposes for all battery types so I would suggest you to try one of those.
I've got my CTEK m300 marine battery charger from a local chandlery here in Suffolk. You can check it out