As a side question, if 6v is right to charge it then do the ones you plug into a cigarette lighter in a truck(12 v) have some kind of limiter in them?
Without additional components added to them, solar panels work more as current sources, meaning they're designed to push a small amount of current into a battery. You might think of the battery in your phone as being a small can of compressed air, and the charger as a small compressor that adds air to replace what runs out of it to power the device. Small solar panels like those used with feeder batteries are also current sources rather than voltage sources, which is why the motor on your feeder doesn't turn if you were to connect it to one. It takes the voltage or pressure of a battery to make it run.
This also explains why the 12-volt supply from your car doesn't blow up your cell phone when you charge it, even though the phone's battery voltage is usually much lower (3.7 to 4.2 volts). You can think of the 12-volt car battery as just a larger tank of compressed air. While there might be a larger supply of air, it's still pushing a small stream into your cell phone battery. However, because it has 12-volts of pressure behind it, it charges your phone faster than if you used a 6-volt lantern battery. It can also charge faster than a USB charger that plugs into a wall because these convert AC power to typically 5-volts of DC pressure to create the small current stream. In fact, you may have noticed how your phone charges more quickly while on your way to your lease than it does when pulled into your charger at home. Once the battery is fully charged, the charger is designed to stop pushing current into it.
Here's something else to consider. I have a cap light that has a re-chargeable battery in it and it came with a cord for connecting to a USB charger. Don't have a clue what size battery it has in it but it re-charges from all the usual sources.