I'm building a project that needs to run off battery power - pretty basic, just an Atmega328 and a whole bunch of LEDs with suitible drivers.
Ideally I'd just like to use 4 C or D cell rechargeables and run the whole thing off the 4.8v directly - will the 328 be happy with that?
Is there anything else I need to take into account? I'm not really worried about what happens when the batteries go flat - as long as there's no damage!
Cheers
As the input voltage drops the maximum operating frequency of the '328 is also reduced. At 2.7V the maximum frequency is 10MHz. I am not sure when the brown-out detector shuts off the chip. Are you using a crystal or the internal
oscillator?
Is the ATmega drawing a lot of current in your design (relative to the LEDs)? If not
I would use a regulator (3.3V or 3V) and a supervisor IC (MAX6861 or TPS3836).
(* jcl *)
4 rechargeable may be too much without a regulator. I've powered an Atmega328 off of 3 rechargeables and I get 4.3V with fully charged batteries. 4 batteries will give you almost 6V initially and that is too much.
It would be safer to use 4 batteries with an LDO regulator to get a regulated 5V.
@jcl - no, I'll be driving 96 leds (4 groups of 24 at 10ma a piece) via PWM through various sequencing & fading patterns. Although they could potentially be on all at the same time the average will be much less, but still considerably more than the Mega.
@dogsop - interesting - I was banking on the 1.2v / cell and assumed that would not be high enough to run through a 5v reg...
I'm not sure if you are talking about NiMH or NiCad batteries but near the bottom of this page is a graph showing the voltage for NiMH battieries. I'm sure the graph for NiCad is similar.
http://www.4gdo.com/batfaq.htm
They start out at almost 1.5V per battery and drop quickly to something closer to 1.2V.
This example is a bit expensive, but it takes in 3.5-16 volts DC and outputs 5VDC. Devices like this can be found in the couple-to-couple dozen dollars range.
Was going to be using NiMh - maybe it would be safer to stick to 3 cells then if there's a chance of 4 being over voltage after a full charge. I didn't really want to add the overheads of regulator circuitry if possible...
Unless the LEDs are going to be mostly off most of the time or you enjoy changing batteries every few hours, it sounds a bit heavy to be running from NiMH cells. Just my opinion 
It's for some display lighting in a model - it needs to be portable since there will be no mains at the venue, and it only needs to last a few hours on a charge so I'm not too worried about long term efficiency.
Cheers