I have an ASUS K50IN laptop which requires an input of 19 V at 3.42 A. Its charger has stopped working (wires cut by rats!). My brother have a charger that is 19 V, 7.1 A. Their polarity is the same and the tip fits correctly.. Can I use this charger on my laptop?
The current rating on the charger indicates the maximum current it can supply. So it is OK to use a charger that can supply more current, but not the other way around. That is, if you went to a lower-current charger and the laptop tried to draw more current than the low-current charger can supply, you would likely damage the charger (e.g., burn it out) and possibly the laptop. But using a more powerful charger (one that can supply more current) should work fine. The laptop will draw the current it needs and the charger will be able to supply it.
This is from the Apple Support Site, but the principles should apply to any charger. As an additional consideration, using an higher rated adapter may use more electricity because it will not be operating as efficiently at lower power levels.
Power adapters for Intel-based Apple portables are available in 45W, 60W, and 85W varieties. Although you should always use the proper wattage adapter for your Apple portable, you can use an adapter of a higher wattage without issue.
For instance If you have a MacBook (13-inch Late 2009) that normally uses a 60W adapter, you can also use an 85W adapter with that computer. You would not use a 45W adapter with that computer; it would not provide enough power for that MacBook. Using an adapter of higher wattage than the adapter that came with the computer will not cause the computer to charge more quickly or otherwise operate any differently than using the adapter that came with the computer.
The laptop should have systems capable of handling the increased power. There is the power the charger is rated to handle, and then there is the amount drawn by the peripheral, the laptop, in this case.
The laptop shouldn't charge any faster that it does normally, but it is most likely not to be harmed by it either.