Can I run an Arduino off 8 x AA batteries? The documentation says it can run from 5-12v which is in the limits of the the specs. I wanted to ensure I had more than enough power for anything else I wanted to drive through it. Would it be better to run the Arduino off a 5v regulator from the 8 battery supply then draw anything else off the power supply directly?
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The first thing the power jack goes to inside the Arduino is a 5V regulator.
This can (for the UNO at any rate) supply a maximum of 800mA.
Any more than that and you will have to use the external power.
How much current the AA batteries can supply depends on the chemistry.
But yes, you can run (and many people do) off 8 AA batteries.
What regulator IC does your Arduino use?
Yes, you can PROBABLY run you Arduino from 8 x AA batteries.
If you care about battery life you can get a useful gain in lifetime using a buck regulator to step the voltage down. If your Arduino regulator will really accept 5V then you may get 50%+ xtra battery life by using a buck converter to convert battery voltage to 5V
IF the Vin min spec is really 5V then you would be better off using 6 x AA.
If you only use a linear regulator then using 6 cells instead of 8 has very little downside and you need 6/8 as much battery capacity.
If the spec is 5 - 12 V in then you will exceed that voltage using new AA Alkaline batteries !!!
An Alkaline battery will typically provide 1.55V when new.
Worst case an Alkaline cell can produce 1.6V or slightly more.
A Nickel Zinc cell produces 1.6% V or more when new.
An Alkaline cell is dead dead dead flat at 0.8V, and dead flat at 0.9V
NimH or NiCd should not be discharged under 1.0V and have a max voltage just after charging of about 1.3V + - so inside Alkaline range at both ends.
It has been suggested by @Majenko that the regulator is a MC33269D-5.0
If so, the regulator outputs 5V and needs a MINIMUM of 6.35V to be guaranteed to operate at 800 mA under all temerature conditions. Slightly ove 6V in may be OK in most cases.
If this is the regulator used it is 5 volts out an needs 1.25V headroom worst case at 500 mA and 1.35V at 800 ma.
So Vin min = 5V + 1.35V = 6.35V.
So, 6 cells are getting marginal BUT you get reasonable but not complete utilisation of cell energy.
8 cells will be OK.
If this IS the regulator IC then the spec should be about 6.6V - 20V.
For higher Vin's energy losses get severe. A buck regulator or similar would make much sense.
Energy loss as heat = (Vsupply - Vout) x Iout.