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[on:]

It's a quandary...a 4-cell AA battery is too high voltage for the 5v input, but almost too low for the 78M05 regulator input, adn will drop out fairly quickly despite battery capacity left.  A 9v doesn;t have the endurance of multiple batteries. 
I do see however there is a 5v switching arrangement on the Nano at least, to switch, from the USB 5v to the 5v main bus?  Not sure that is an alternative...

So what are people doing in this regard?

[Reply #1 on:]

It's a quandary...a 4-cell AA battery is too high voltage for the 5v input,

1.2 * 4 = 4.8 volts.  Why do you believe the voltage is too high?  You are talking about NiCd batteries, right?

but almost too low for the 78M05 regulator input, adn will drop out fairly quickly despite battery capacity left.

1.5 * 4 = 6 volts.

The recommended range is 7 to 12 volts.

"Almost too low"?

So what are people doing in this regard?

Two AA alkaline batteries directly to the processor.

[Reply #2 on:]

In general, not a good idea to power a microcontroller from a battery directly.

For most of my battery-powered projects, I use a board with 5V LDO v.reg and power
with 6 NiMH AA-cells in series = a bit over 7.2V fully-charged. These cells have 2200-2500
mA-hr of energy, so I can run motors and servos.

7.2V will barely power a regular 7805 with 2V dropout.

[Reply #3 on:]

In general, not a good idea to power a microcontroller from a battery directly.

Why?

[Reply #4 on:]

Exactly Oric...(4) AA Alkalines start out at like 6.75 volts, and it is enough at first for the 05 regulator, but will drop out somewhere too soon before the batteries are depleted....If I wire the batteries directly to the 5v bus it exceeds the processor's voltage spec of around 5.5v.   I have a simple project, but the lcd display has a large drain led backlight.   Most cases don't come with capacity for (6) AA or even AAA...so I think it is 9v for me at this time...I need to let a supplier know for prototyping samples.  I suspect a 9v won;t last long.

[Reply #5 on:]

Coding, I said almost too low, because I have bench tested 4 batteries through the vreg. It works fine bespite being below the 7v spec.  But will drop out before we want it to....

[Reply #6 on:]

If this were a 3.3v design, all would be fine, just use (2) AAs and be done with it.  But 5v is a bugger.

[Reply #7 on:]

I use cell phone battery chargers that I hook up the to the USB port.  At the moment, I'm using the EZOPower 5000 maH charger that provides two separate USB ports.  In the past, I let my UNO R3 run with the blink  sketch and it lasted for 4 days or so using no special techniques for saving power. I picked mine up from Amazon: http://www.amazon.com/EZOPower-Universal-External-Rechargeable-Generation/dp/B004I0KY68/ref=sr_1_14?ie=UTF8&qid=1355430888&sr=8-14&keywords=ezopower+5000maH+battery.

[Reply #8 on:]

Yes  interesting...I may end up just offering a DC input jack and let people power it how they see  fit...like you suggest or otherwise...thanks

[Reply #9 on:]

Yes  interesting...I may end up just offering a DC input jack and let people power it how they see  fit...like you suggest or otherwise...thanks

To answer two questions, if you let them "power it how they see fit" and don't include a v.reg,
be certain they'll blow the whole thing. Risk goes up by 10X if you have a power jack on the
board. They can connect anything.

Batteries direct to microcontroller may be ok in a very carefully controlled situation and where
you're not using A/D converter, but risky for a general case. I think it's better to use a v.reg
and have a good overhead margin in voltage so it keeps running as the batteries discharge.
With 6 NiMH AA-cells and an LDO v.reg, the cells are depleted when down to about 1V, which
is where the LDO v.reg also drops out. 4 alkaline cells are also a usable choice.

NiMH AA-cells have up to 2500 mA-hr, the measly 9V batteries maybe 1/10 that. I might use
a 9V battery with a microcontroller operating with a 32-Khz xtal.

[Reply #10 on:]

Batteries direct to microcontroller may be ok in a very carefully controlled situation and where you're not using A/D converter,

So using an AVR's A/D converter when powered directly by batteries is a bad idea.  Why?

but risky for a general case.

Why?

[Reply #11 on:]

OP,

If you worry about wasting power with 1.5*5=7.5V arrangement or 1.5*6 (easy to find such housing), you can use a switched-mode dc-dc converter to converter instead of linear dc-dc converter such as 7805. Switch mode converters have higher efficiencies so you are not losing power due to a voltage drop from say 7.5V to 5V. Look it up.

[Reply #12 on:]

Yeah, for that matter, Pololu has some nice "boost" regulators that will run off 3 or 4 rechargeable
cells, and boost the voltage to 5V and higher, and continue to operate as the batteries run down.
[disregard the step-down devices].

http://www.pololu.com/catalog/category/84

[Reply #13 on:]

Oric, maybe lost in translation, my statement meant I would provide a jack to power the device through the built-in 78M05.
I will be using Nano's, they are cheap in compatible version, why reinvent the wheel?  Of course that could change if I find another cheaper option just as good.  It'll be nice to keep the USB port for updates in the field.

There are certainly lots of options and boost and buck converters are great, but when producing a mass produced piece, you have to consider all the costs and complexity as well of course.  Like the power switch discussion...is it worth it to eliminate an on-off switch, if it means adding another button anyhow, along with a couple transistors and a bunch of other glue parts? 

I want to keep this as low cost as possible.

[Reply #14 on:]

It's a quandary...a 4-cell AA battery is too high voltage for the 5v input, but almost too low for the 78M05 regulator input, adn will drop out fairly quickly despite battery capacity left.  A 9v doesn;t have the endurance of multiple batteries. 
I do see however there is a 5v switching arrangement on the Nano at least, to switch, from the USB 5v to the 5v main bus?  Not sure that is an alternative...

So what are people doing in this regard?

So nobody can blame me, I won't suggest that anyone do this, but *I* personally have run an Uno R3 board on 4 AA lithium cells in series. As you may or may not know, a brand new lithium AA cell is around 1.80 volts DC, which is 7.2 volts for 4 of them.

The 4 AA cells in series power the board's "5 volt" supply directly. It does not go through the regulator.

I've made several projects powered like this and they are running fine.