Measuring Power, 12v RGB LED strips, 7805, atmega328 and 8 AA's

This is a repost from the general electronics section, but this seems like it might be a more appropriate forum.

I've been playing with electronics for a few years now and have build a lot of things that I like, but I'm still a self taught rank amateur and could use some advice.

I'm building a battery powered RGB LED display (for a bike). It consists of 4 strands of 12v RGB LEDs, a 7805 voltage regulator an atmega328 running the UNO bootloader and some transistors.

It currently drains 8 AA's (1.5v*8=12v) in about 2 hours and I want it to last at least 8, or ideally 12 hours.

Each alkaline AA should be 2700mah or 21 Ah for the 8 AA's.

So, now I'm learning about power. I've never tried measuring power and so I think I'm doing something wrong.

I placed my multimeter between the battery and my circuit board and with the lights plugged in I'm getting a .6 reading when the meter is set to 10A and a 2.8 reading when it's set to 20m. Does that mean that I should multiply .6 * 10A to get 6 Amps or is at .6 A or 600 mAh. Most people think that it should be .6 amps which makes some sense to me since the light should use about 115 mA per color per meter and I have about four meters and one color is on at a time (on average).

If that were true then I my lights should last for days instead of hours (21600 Ah / .6 = 36 hours)

To move things further along when I measured without my lights the circuit (the 7805, 16 mhz atmega and a bunch of transistors switching nothing frenetically) is using .25 * 10A). I thought that an arduino should use something like 25 ma while it's running so I'm off by a factor of 10 or 100 depending on how my meter is reading.

That leads me to think that I should remove the 7805 and give the Arduino it's own power source. Is that recommended for minimizing battery drain.

Summary: * 4 meters of 12 RGB LED strip @ 114 mA / meter (http://tinyurl.com/12vrgbled) * 16 mhz 'breadboard arduino' (on a protoboard) * 7805 giving the arduino 5v (and the bypass to get the lights 12v) * 8 AA batteries (21600 mAh)

Power reading @ 10A .6 with lights, .25 without lights @ 20m 2.8 with lights 1.15 w/o lights

Battery life * 1.5 -> 2 hours with 8 alkaline AA's

Questions: 1. How do I read my ammeter (is .6 @ 10A setting .6 amps or 6 amps)? 2. How long would you predict the circuit would last? 3. Should I avoid the 7805 and power the arduino from a separate power source? 4. How much would I gain if I did that?

Thanks again, your struggling friend,

Zeke

Hi,

Your AA cells are connected in series, therefore the capacity of the stack is still 2700mAh, not 21Ah. However, capacity is usually measured at something like the 10 hour rate (i.e. if you draw 270mA you will get 10 hours from a 2700mAh cell). At higher currents, capacity is lower, especially for alkaline cells - see http://www.greenbatteries.com/chart.html. So if the current consumption really is 0.6A then 2 hours might be about right. For longer life at high current, use NiMH rechargeable cells, which retain capacity better at high current than alkaline cells.

PS - 0.25A without LEDs sounds rather high. What else do you have connected to the Arduino, and how are you driving the LEDs?