I decided to test the 7xF cell NiMH batteries in a realistic way. I charged them fully, attached the Arduino and the LEDs, and I ran a test program adapted from dbu’s LED Globe idea and code. Here’s what the setup looks like:
I expected it to run for about 2 hours, and I hoped for three. Anyway, I put a voltage meter on the power bus, and measured every 15 mins or so. Results in the graph below.
Holy cats. The battery kept 200 LEDs blinking for 477 minutes, nearly 8 hours! So nominal 8.4v batteries supplying 13 Ah are more than enough. The power curve is nice and flat until it fails, which is excellent.
A couple of observations here. First, the Polulu voltage regulators totally rock, hella efficient. At no point were they even slightly warm to the touch, so nothing wasted as heat. Second, in real use, the LEDs draw way, way less than their rated max current. Third, the giant, ridiculously heavy batteries are more than enough. No Li-Ion polymer for me, at least in this round.
Wylbur's Inner Geek 
Dude! I’m so happy you posted this stuff and are working on an LED based coat. This was really helpful to see.
I’m working on a similar project for burning man right now and trying to figure out the power supply, with the same challenges: pushing at least 1 – 3 amps from a battery pack without being TOO heavy, expensive, or likely to catch on fire. My power draw will be 4 meters of either the addressable or non-addressable LED strips inside a big furry white coat, like these: http://adafruit.com/products/306
The toughest calls for me right now are:
• Hit the right voltage straight out of the power supply OR overshoot and use a voltage regulator?
• Alkaline / niMH / liPO?
• How big (mAh) of a battery pack to get? Trying to balance cost, weight, and longevity.
Would love to chat more about what works and what doesn’t. I also happen to live in the mission / SF and stop by Noisebridge occasionally. Drop me a line? loren@acleandesign.com
great to hear from you! Our coats are brown, but seems like a similar idea. Ours are also “for” BM, but for me, BM is a giant excuse to build stuff I want to do anyway.
Here would be my reactions:
• Hit the right voltage straight out of the power supply OR overshoot and use a voltage regulator?
— at 3A, definitely a voltage reg. I *love* the $11 one from Pololu, I’ve now used it in two projects. I think the WS2801s and LEDs may be a little fussy about voltage.
• Alkaline / niMH / liPO?
— Depends on your budget, but at 3A, you can probably go to LiPo without killing the budget. I studied this:
http://www.batteryspace.com/howtochoosebattery.aspx
• How big (mAh) of a battery pack to get? Trying to balance cost, weight, and longevity.
— My choice was 1 Ah more than the max draw of the LEDs (ignoring the Arduino). In the test, I found I’d overdone it. YMMV.