# How long will AA batteries power my project

Discussion in 'Power Electronics' started by lpsarsam, Apr 10, 2012.

1. ### lpsarsam

7
0
Dec 23, 2011
Hi
I am trying to build an LED clock, that uses four 7 segment displays for hours and minutes, and a ring of LEDs around the clock for seconds.
The LEDs are going to be 20000mcd blue LEDs (these ones
http://www.ebay.co.uk/itm/50x-Ultra...Supplies_ET&hash=item2ebbdbc1e0#ht_500wt_1413)

As on average there will be 30 LEDs on at a time (not counting the 7 segs) does this mean that the AA batteries will only be able to power the device for about a few hours before running out of power, meaning that it will need to be powered by a DC adapter?

Thanks

2. ### timothy48342

218
1
Nov 28, 2011
A very very quick calculation:
I clicked your link, but didn't right away the see current they need and decided not to research it. I'm going to guess 20mA, but you need to check the datasheet for a better number.

30 LED's at 20mA is 600mA. (I'm going to ignore the 7 segment displays like you did and go straight to the math)

The very first search I did for AA batteries said they hold about 1000mAH. (milli-amp-hours) (That's for 1 AA. If you have more than 1 AA in series, then that DOES NOT CHANGE. If you have some parallel setup, then you would need to describe that more.)

So 1000 mAH / 600mA = 1.6 hours.

You won't get much more than that and maybe way less. We are ignoring some other current draw. (7 segment. The controlling ciruit.)

Unless I'm way off on the my guess of 20mA current for the LED, then it won't last long at all.

--tim

3. ### lpsarsam

7
0
Dec 23, 2011
Yep, that's what I thought, thanks for confirming my equally quick calculations.
I'll get a dc adapter and use that instead