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### Topic: How long will a 9v last with 4 LED's? (Read 2226 times)previous topic - next topic

#### bobmcinnis

##### Dec 19, 2012, 03:23 am
Me and my 11-year-old son have a built a school project with four LED's connected to an Arduino as well as a button to turn them on and off. Even when the LED's are off, the on-board LED seems to stay lit to indicate it's connected to power. We have them connected to a 9V battery (the little square ones). He's taking it to school in the morning as part of a class project. The four LED's will remain off (but with the on-board LED glowing) until he needs to show the class, no longer than a 7 hour wait.

I'm wondering how long an Arduino can be powered in this "standby" mode and if, when it's his turn to show his project, if it'll still have enough juice to flash the four LED's for 15 minutes or so.

Any help would be appreciated.

Thanks!

#### Jack Christensen

#1
##### Dec 19, 2012, 03:51 am
Everyone here hates the rectangular 9V batteries because they don't have much capacity, usually 550-650 mAh. The Arduino alone draws about 40mA.

I'd say that's cutting it mighty close, in fact I wouldn't bet it'd make 7 hours. Is there some reason the battery can't just be unplugged until show time?

#### bobmcinnis

#2
##### Dec 19, 2012, 03:56 am
Thanks for the response. Given your comments, I'm going to definitely have him plug it in only at Showtime. How long do you think four LED's will flicker from a 9V. They're more on than off.

Thanks again. This has been a huge help!

#### ibiwan

#3
##### Dec 19, 2012, 04:41 am
A quick google tells me "standard" LEDs draw about 20mA, so with four of those and a 40mA Arduino, your 600mAh battery should last about 600/(4*20+40)=5 hours.

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