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