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Author Topic: Unregulated battery power ?  (Read 683 times)
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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
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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 *)
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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.
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@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...
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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.
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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.

http://www.mouser.com/ProductDetail/Power-One/DFC6U5S5/?qs=sGAEpiMZZMtwaiKVUtQsNRDnN%252bEM4QEQE%252b97sN4FQrA%3d
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What about elevensies? Luncheon? Afternoon tea? Dinner? Supper?

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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...
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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 smiley
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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
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