When are my batteries charged??
A good "cheap" solution is an onboard voltage monitor!
They come in several different guises and are easy to install. Typically there is a row of LEDs green , yellow and red - you wiggle the sticks (ie put a load on the battery pack) and watch the LEDs - if you are in the green go fly !!
The down side is that they do not look very nice on a model and have to be installed where you can see them - but IMO worth having. I have seen some that have the LED monitor built into the on/off switch, which is OK if your switch is visible !
You can also buy a voltmeter from Hitec or similar that plugs into your charge socket and loads your battery to give a reading so you can check your rx battery between each flight. These will also help when you are charging but are not the most elegant solutions!
Get a good charger with an automatic cutoff - in the long run it is the best solution because it takes all the guess work and calculations out of the process and will save your model.