The battery will only charge if any of the following occur:
a running gasoline engine (via low throttle, need to heat the cabin, need to get the engine up to operating temperature, etc.)
EV mode is generally only good for about a mile on battery by itself. Mileage will vary depending on terrain and driving style. There is no need to fill the battery. Think of it as give-and-take. I drive a hilly route to work. If I ascend a big hill, I'll use battery and gas. Once I hit the top, I get some of that back to battery via the downhill and regenerative charging. I can go from three bars to full in about a half mile - yes, it's a big hill. If you live in a flat-ish area, don't expect the battery to fully charge. Since there are thermal/parasite losses with charging the battery via an ICE, you'd want that power going to the wheels straight away instead of to the battery. I believe, for most of us, the battery will hang out somewhere in the middle of the gauge. The exception to this is when you engage sport mode. This keeps the battery fuller in order to draw on it for better performance (at a cost of MPG).
Hope this helps.