Powering LEDs & Strips With Battery Packs

Round two of Cooper confusion concerning things related to making a cardboard box robot head for Halloween: Trying to figure out if my battery source will be adequate to power my in-head LEDs for the duration of the Halloween party .

I'm not completely settled on which and how many LED sources I'll be using, but it looks like it'll most likely be a mix of 5mm pre-wired LEDs (20mA/ea), and old 4.7" 3mm LED strips (80mA/ea) .

First, the 20ma figure comes from the LED spec page for just the raw LED, not the pre-wired version . Can I assume that this only applies accurately if no resistor is used ? If so, any idea of what kind of power consumption I should count on from the same 5mm red LED in the pre-wired version that's bundled with a resistor ?

Let's say for simplicity's sake that I'm running five 4.7" strips, and four 5mm LEDs, all red, for an approximate power consumption of 500mA .
... And my battery source is 8 AA's, which don't give a capacity rating on the battery (thanks a lot, Energizer ) but my e2 rechargeables list a 2500mAh capacity, so again for simplicity let's say the alkaline versions carry at least 2000mAh apiece ? (I hope that guess is somewhat close) . So with eight AA's total I'm drawing from a pool of 16000 mAh .

So a 500mA demand pulling from 16000 mAh seems pretty trivial, right ? I realize that the batteries aren't regulated in any way that will keep the LEDs at full brightness, and that they'll dim on some kind of curve as the batteries drain, but how pressing of a concern is this given the numbers above ?

I only need maybe six hours of juice of the batteries, but I'd like to keep close to full brightness for that length of time . Are my expectations realistic, or do I need to seriously reconsider my power source ? This is my first time experimenting with battery-powering any LEDs . I can't step up to C or D cells without making my cardboard box head heavier and less balanced than I'd like, but I do plan on keeping a back up set of 8 AA's in my pockets to 'reload' the head mid-party if necessary .

Any thoughts / comments / pointing out of important factors that I'm overlooking are much appreciated .

actually when u put them inseries like that to get 12 volts u stick with the original mAh...

so if all have 2700mAh and in series for 12V source then it stays at 2700mAh...

now if u were to take 8 AA and put them into a set of 4 in series and then make both sets of 4 paralell then u would have 6V with 5400mAh (1 set of 4 batteries would be 6V at 2700mAh)

http://www.zbattery.com/Connecting-Batteries-in-Series-or-Parallel


also rememver, resistors are just for voltage drops not amps... so if you find a led that runs off 6V w/o a resistor then u would be better off with the 8 batteries in the 6V paralell/series for the 5400mAh (if u need i can draw a diagram for you.)

Crap.. talk about important factors that I'm overlooking .

Thanks for pointing that out, zans .. So with a 500mA draw and a 2000mA capacity obviously I'm not going to make it six hours on one set of AA's . What about with swapping them out mid-party ? That would put maybe three hours on each set of batteries.. but three hours into powering a source that's pulling a quarter of its available capacity each hour is there any way to predict how much those LEDs will have already faded ?
If after three hours they're at 75% brightness, for instance, that would be acceptable . But if after 3 hours they're at 20% brightness, that would suck Any way to calculate this ?
Also, is there some point at which the current draw becomes unsafe, or will make the cells too warm ?

I Used my Drill battery pack , and a set of alligator clips .... 1.8Amp hours FTW

I would suggest that you don't use prewired LEDs. You're just wasting power w/ the resistor. Buy non-prewired LEDs. Hook 2 AAs in series for 3V. Then run the LEDs straight off the batteries w/ no resistors.

Done that way, 8 AAs will give you 8Ah (8000mAh). With a draw of 500mA, it would last for 16 hours. It'll easily last for 6 hours w/out fading. The down side is that you won't be able to run LED strips.