Not sure this is the right sub-forum, but...
So the scariest thing happened.
I've been using a 4 battery rack with rechargeable batteries (Sony CycleEnergy) connected directly to the 5V pin on my Mega. These batteries have the specs written on them and it said 1.2 volts so 4.8 total, close enough, tried it, it worked, I never bothered to check the actual voltage. 
The other day it just occurred to me to actually check, and they are 1.37 volts... 5.48 total.
I know it works very well (I get close to 20 hours out of them running the Mega, an LCD and one big 1.2" 7-segment display) but I was wondering if it's wise to do so... As I remember reading (but can't find now no matter what I search), safe levels are 4.75 to 5.25 volts, but then again "I remember reading", who knows.
Part of me thinks "it has been working well and you have better things to do and these Internet forum guys better tell you it's ok :D" and other parts of me think "you are going to kill it someday".
Thoughts?
Cosme_Fulanito:
Not sure this is the right sub-forum, but...
I suspect it will migrate to General Electronics.
As I remember reading (but can't find now no matter what I search), safe levels are 4.75 to 5.25 volts
Perhaps you should check the datasheet from the manufacturer of the processor. Such documents have been known to contain those kinds of details.
In my copy of the datasheet the VCC range is snuggled in between the section title (31.1 DC Characteristics) and a table of values.
Add a series Schottky diode to take .3 volts off.
Are you talking about the Arduino Mega 2560 with ATmega2560 microcontroller ?
It will run at 16MHz at 4.5 to 5.5V. That means that all the specification and lifetime will still be according to the datasheet when running at 5.5V.
Newer and more flexible ATmega and ATtiny chips can sometimes survive a short 8V. The ATmega2560 is older and more critial for voltage and speed. But perhaps it will still be okay at 6V.
Therefor 5.48V is no problem at all.
Koepel:
But perhaps it will still be okay at 6V.
What information does the datasheet have about 6 V?
Yeah sorry, it's the 2560 model.
The data sheet (page 355) says "Maximum Operating Voltage 6.0V" but then again I'm a complete newbie so who knows if it means what I think it means.
Also most of the listings there go to 5.5V so I guess that's the limit they care to show (thus, the limit you might want to go to and no further).
Cosme_Fulanito:
The data sheet (page 355) says "Maximum Operating Voltage 6.0V" but then again I'm a complete newbie so who knows if it means what I think it means.
It does. In a section with this title...
Absolute Maximum Ratings*
The asterisk indicates there is a footnote. The footnote is just to the right. And the footnote explains the reference.
*NOTICE: Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. This is a stress rating only and functional operation of the device at these or other conditions beyond those indicated in the operational sections of this specification is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability.
Also most of the listings there go to 5.5V so I guess that's the limit they care to show (thus, the limit you might want to go to and no further).
I told you precisely where to look for the value...
TA = -40°C to 85°C, VCC = 1.8V to 5.5V (unless otherwise noted)
As long as you dont try this with a AtSAMD21 or D20 you should be fine... the AtMega2560 is a pretty robust cpu for its size, because I managed to kill the pin port drivers with a defect LM1117 3.3 which had voltage spiks up to 3.82V and the specifications go only up to 3.62 so...
Plus the manufacture will give you a value of which he is ablosutly sure that the compunent will survive because if you would by a million of these devices and would kill half of them because the voltage rating in the datasheet wasn't correct you could get your money back...