Question: I have a Canon SX210 IS, which doesn't have the possibility to plug-in a external power source.

Because of that, I want to make a connection to the pins where the battery usually is. I'm a little confused about the voltage requirement because the battery says 3.7V and the base of the camera says 4.2V.

Since I have a car battery and a 7805 voltage regulator I thought I could power the camera directly with the 5V the regulator would give me. But I don't know if it's safe to do that.

You may ask why would I want to power the camera with 5V instead of using 4.3 from a DC regulator. Well, I'm outside the city right now so there is no way to get a DC power supply like that.

(don't ask where I took the 7805 from).

I've found out that my helicopter charger has a voltage of 4.2V and a maximum current of 850 mA, but it's a typical switched power supply (like the cell phone ones). Is it dangerous to feed my camera with that? What about a little paralell capacitor to filter noises?

Answer: 
I looked up an SX210 manual online, and it says that there *is* an external power port. Look at page 156 of your user's manual; the online guide I found recommended the ACK-DC30 kit. You plug a coupler (which has the same form factor as a battery) into the battery bay, and plug the AC cord into a slot in the battery door. The ACK-DC30 is $50 on amazon; you can probably get an off-brand version for less, too.
I wouldn't recommend doing this. You have no idea whether the 4.3v marking on the camera is some engineer being conservative (in which case the camera can probably take something a bit higher... maybe), or that there's a component in there that's gonna fry as soon as anything over 4.3v hits it.

Seriously, don't take chances unless you don't really care if the camera gets fried. If you can't afford to lose it then it's *way* better to be right than to be fast before you connect anything up. The correct regulator is going to cost *much* less than buying a new camera 'cause you fried it...