# trying to calculate current requirement

I have a Pro Mini clone controlling four MAX7219 which are controlling four 8x8 LED matrices.

calculation:
4 * 64 LEDs = 256 LED * 20 mA/LED = 5.12 A
4 * 330 mA/MAX7219 = 1.32 A
? Arduino Pro Mini

Total current < 7 A

Am I right?

What should I do if I want to run this off AA batteries?

ieee488:
What should I do if I want to run this off AA batteries?

Buy a lot of AA batteries !

You cannot really expect small AA batteries to supply 7A .

Its either a mains powered PSU or a much larger battery like a car battery if you want to run the leds for hours .. ?

ieee488:
I have a Pro Mini clone controlling four MAX7219 which are controlling four 8x8 LED matrices.

calculation:
4 * 64 LEDs = 256 LED * 20 mA/LED = 5.12 A
4 * 330 mA/MAX7219 = 1.32 A
? Arduino Pro Mini

Total current < 7 A

Am I right?

What should I do if I want to run this off AA batteries?

From a quick look at the MAX7219 datasheet, it looks like your current estimate is correct (if you really do need to have all LEDs on full brightness at the same time).

I assume you want NiMH AA cells, like these? → http://data.energizer.com/PDFs/nh15-2300.pdf

The highest discharge rate they show is 2C, and the voltage sags quite a bit at that load, so 4.6A is the max at which they should be discharged. Therefore, you’ll need 2 in parallel to satisfy your max current required.

To keep your MAX7219 at its required V+, you’ll want 4 cells in series. From the 2C discharge curve in the datasheet, you’ll get about 80% of the energy out of the 4 cells when they stack between 4V-5.5V.

So, in the end a 4s2p NiMH battery would be needed to supply your load. It will only last for ~30minutes, however, at max load (although how often will you really have all of the LEDs on at the same time?). If you need more runtime than that you need to add more cells in parallel, use a larger cell than AA, or use a different chemistry than NiMH.

You won't get 5.1Amps from an AA battery.
You may want to use your >=four AA battery group to provide Vin and a few mA to the arduino, and then independently create your 5.1A supply for the LED set ( how many Volts does that really need ? ) and if the required voltage is different then another 2A rated supply for that other bit. It makes your thing better at self-test diagnostics if the arduino is still ok after the supply for the LED group starts to get a bit low.

One low cost way to get 5 or 7 Amps is to find a scrap 12V Pb car battery and (after some care regarding fuses and what-happens-if catastrophe prevention) you wire it with suitably rated cable to a 12A dc-dc buck converter, which can be screwed to set your desired LED voltage anywhere in the 2V to 12v range. It might also be possible to downconvert off a group of say, four LiMH cells, if you are looking for a more slender supply than the Pb battery, which is possibly a bit larger than you really need. I use those to directly power my 15V 0.5A speaker and at times buck down to 5V 1A for a raspberry pi.