Shields are a pain in BC and any Sci-Fi series in general. Mainly because it's a made up concept that has no practical working out. I found this a pain balancing the Stargate ships. In that series there were occasions where the shield seemed directly proportional to the power gong into them, essentially once the batteries ran flat the shields went down.
The best explanation I have is that 'energy shielding' in general works along the principal of creating an energy bubble around a ship with the aid of emitters installed along various points on the hull. Shields therefore absorb energy opposed to simply forming a solid barrier. In essence, shield have an energy absorption level. When it comes to recharging, its all to do with emitters. The more power you feed into them, the higher the energy absorption level rises, but that in itself has limits. An example would be trying to charge an iPod with a car battery, it would just explode if you put that much power into it.
Same analogy with that TNG ep. When the Shields drop, they have been overwhelmed, but they constantly recharge. The thing to also take into consideration is BC can't manipulate ship systems the way they should, ie you cant divert power to forward emitters to recharge them faster. In theory it's also possible to move the power from the aft portion to the front, so drop the rear to raise the front.
As for Voyager, yeh the tech on that show was never consistent. I will go far as to say in its defence that over the year it accumulated so much Borg/ 29th Century Borg tech that there was no way to guess what kind of power it had.