While simple tasks like browsing web, a lightning bolt appears in the top right part of the screen. I'm using a 2A charger. Is this really dangerous, for the pi or the SD card or the peripherals connected?
The allowed voltage range is the normal one (for 5V logic) 5.0V ±5% so its a the range between 4.75V and 5.25V. Note that the lower limit of 4.75V is more a matter of many USB devices not working below that value than it is that the PI itself is that sensitive, especially as much of it (including the ethernet/hub chip, and the SD-card) is working from 3,3V through a regulator on board of the PI that will keep working even with say 4.0V.
Also note that many 5V logic devices will be damaged by voltages of 6.5V or higher! Therefore there is a protection device on the PI (D17) that will short the 5V supply if about 6.0V is supplied! Then the (poly)fuse will blow out to protect D17 from burning up! If you do not power the PI through the normal micro-USB port then you are actually bypassing that fuse!
there is a section of the PI Wiki at http://elinux.org/RaspberryPiBoard dedicated to boot problems like this.
The power logo means that the Pi isn't getting enough current. This is often the case with cheap chargers from China.
According to the Raspberry Pi Cookbook by Simon Monk, he says that:
...The only question, then, is if they can supply enough current.
If they can't, then a few bad things can happen:
- They may get hot and be a potential fire risk.
- They may just fail.
- At times of high load (say, when the Pi is using a WiFi dongle), the voltage may dip and the Raspberry Pi may reset itself.
Fire is never good in electronics. Resetting the Pi is never good, as it could lead to file corruption.
So your solution is to get a better charger.
While simple tasks like browsing web...
Browsing the web is not a simple task. It is resource-intensive and requires a lot of power.