Batteries for PIC / LED projects, instead of PSUs; how long could they last?

[Hippogriff]

Hi all,

You'll know - if you've seen any of my previous postings - that I'm messing around with PIC 12F683s, 3W RGB star LEDs and 12v RGB LED strips (just getting into it all, really, but enjoying it)... so far I have been using wall wart power supplies and I have recently completed my first project that has a jack soldered firmly onto a piece of stripboard - obviously that's all cool and suits my needs there.

However, I just started wondering about mounting one of my projects on a wall and I thought that would obviously be best served by a battery to avoid wires trailing. Yet, I don't want to go to the hassle of even trying this out if the end result is going to a project that lives for a couple of days... before the battery needs replacing.

If I'm powering a PIC and a 3W RGB star LED does anyone have any gut feel as to how long, say, one of those cuboid 9v batteries would last for? Is there a ballpark kind of duration, or is it really a "suck it and see" kind of question that I'm asking here? I know other devices use these batteries and live for years on the same battery, but I expect my LED is quite a bit more hungry.

 

[Speakerguy]

A 9V battery would die a short death unfortunately They can only do 100-200ma max discharge, and that shortens their life significantly. You would need a switching converter with any battery source to get the most efficiency and even then it wouldn't last very long.

Can you measure the current draw when you have your project hooked up to the wall power supply? I suspect you are right that your high power LED will eliminate the possibility of battery power lasting any meaningful time, but maybe you are driving it with less than max current.

 

[Hippogriff]

Not when it's connected to the wall wart, no, but when it's connected to my bench PSU, yes... rises to a max. of about 780mA when the LED is white, i.e. all at max. output.

So, no go, then?

 

[Speakerguy]

Not when it's connected to the wall wart, no, but when it's connected to my bench PSU, yes... rises to a max. of about 780mA when the LED is white, i.e. all at max. output.

So, no go, then?

Definite no-go, a 9V battey is only good for 3-4 hours at 100ma draw. At 780ma that current I think the battery would fry

Different battery chemistries will help out, but not even a LiSOCl2 cell (the highest energy density chemistry out there) will give you days of operation. The LED is too power hungry.

 

[Hippogriff]

Ah, well... glad I didn't put too much time and effort into trying that out, then... thanks!

 

[crutschow]

Even if you used 6 D alkalines in series to get 9V, which have a typical capacity of 12,000 mAH, your 780 mA draw would drain the batteries in 15 hours or less.

 

[MrDEB]

What about PWM to turn on the LEDs.
supposed to use less current?
the AA batteries will last longer than the 9v block.

 

[Hippogriff]

I am using software PWM, but - even so - the bench PSU tells me around 780mA is being drawn when the fade goes to white and all colours are on most of the time. Looking at the responses, it seems like my only feasible solution for powering a PIC and a LED like this is mains. It's not a major disappointment and - after all - it's what I asked the question to find out.

 

[Speakerguy]

Even with 3.6V lithium thionyl chloride cells (19,000mah) it wouldn't last a day. And those dudes are expensive and hard to find (Saft, Tadiran, and a couple other companies make them).

But like you said, good to ask and find out beforehand.

 

[vne147]

Hippo,

This isn't an answer to your question but I though I'd mention it anyway. I recently did a project with a 16F627A that I powered off of 2 unregulated AAAs. The data sheet says the PIC will only operate down to 3V but I disabled the brown-out reset and tested it down to 2.1V and it worked fine. Just thought that if you are doing projects with PICs and batteries you might find that interesting.

 

[DirtyLude]

You can cut the power use of the microcontroller a decent amount by being agressive with the different sleep modes, but that LED is the real hog. Not much you can do about that.

 

[crutschow]

You could use super bright LEDs, run at a low current with a PWM switching regulator to minimize power, but it probably still wouldn't give great battery life.

 

[Hippogriff]

Thanks gents, I'm rapidly going off the idea. I want the brightness but also the longevity... I also want the moon on a stick. Sometimes, you can't have it all...

 

[Nigel Goodwin]

Thanks gents, I'm rapidly going off the idea. I want the brightness but also the longevity... I also want the moon on a stick. Sometimes, you can't have it all...

No you can't

The PIC takes hardly any power, and isn't a problem - it's bright LED's that sap the power.