# Audio Amplifier Battery Life

Discussion in 'The Projects Forum' started by Alicec63, Jun 21, 2013.

1. ### Alicec63 Thread Starter New Member

Jun 20, 2013
12
0
Hi,

Say I have an audio amp(MAX9701) connected to a set of 8 ohm speakers - rated at 1W - powered via 2x AAA Batterys...

Assuming the batteries had a rating of 1000mAh. How long would the battery last under constant operation i.e. playing music through an ipod?

The only other thing in the circuit is a boost convertor (MAX856) which keeps the supply voltage at 5V and obiovusly the amps input is connected to a music 'source' i.e. ipod.

I'm sure this is pretty simple, but I think I am probably complicating things in my head... Friday afternoon!

Any help would be much appreciated.

Thanks

2. ### MrChips Moderator

Oct 2, 2009
12,646
3,458
Use the formula for power P = I^2 x R
I^2 = 1W/8Ω
I = 0.35A

1000mAh/350mA = 3h

You will be lucky to get 2 hours from two AAA batteries.

3. ### Alicec63 Thread Starter New Member

Jun 20, 2013
12
0
Thanks.

Is it really that simple. That will give me the output power. Does it not take into consideration the input voltage i.e. 3V from 2 x AAA batteries? Or the efficiency of the boost convertor and the amplifier?

4. ### #12 Expert

Nov 30, 2010
16,704
7,354
MrChips ignored the efficiency of the converter because they are about 90% efficient and the uncertainty factor of the batteries and how high the loudness will be turned up are a lot more than 10%. It's an estimate. It's close enough.

5. ### MrChips Moderator

Oct 2, 2009
12,646
3,458
Think conservation of energy.
.3A @ 3V is coming from the battery.
The energy has to go somewhere, either as acoustical energy or as heat loss.

6. ### crutschow Expert

Mar 14, 2008
13,508
3,384
1 watt may be a higher average level than normal listening. If so, then the battery life would go up accordingly.

7. ### bountyhunter Well-Known Member

Sep 7, 2009
2,498
507
1W average power would blow your eardums out if listening through ear phones. I suspect the average power would be less than 0.1W. The easy way to determine the battery life is measure the current when the volume is at the typical level.

8. ### GopherT AAC Fanatic!

Nov 23, 2012
6,322
4,049
0.1 watt is the normal max for headphone listening (per ear) for most commercial devices.

Most headphones are 16 ohm each side. Your milage may vary. I think the OP is trying to connect 8ohm loudspeakers though, not headphones.