Portable Stereo battery life help

#12

Joined Nov 30, 2010
18,224
What sensitivity would you recommend for this project remembering that I'm on a budget? Most of the speakers I am looking at are around 90-95
Do the math. 16 watts is 4 doublings of the power above 1 watt. Each doubling gives 3 db sound increase. A 90 db speaker will deliver 102 db SPL at 1 meter and a 95 db speaker will deliver 107 db SPL at 1 meter.

(That's assuming your enclosure is not strangling the air flow. Imagine a 10 inch speaker in a 1/2 cubic foot box with no openings. No place to move the air on the back side!)

What does 100 db sound like?
The, "book" standard is 70 db for a human voice in a long conversation and each 10 db is a doubling in perception. 100 db would sound 8 times as loud as a person at 1 meter. 110 db would sound 16 times as loud as a normal conversation, so you're working in the range of 10 to 15 times as loud as a conversation.
 

Thread Starter

shlo

Joined May 11, 2014
21
As someone who would be really annoyed to be on the beach or camping near you, my suggestion is to use headphones. Bob
As i said Bob i don't like people who blast music either but most places we go we are very far away from others and if there are others near us we are considerate. headphones... really i might as well sit at home.

Because i was unsure of what to expect volume wise i have brought just the battery's and amp for now. The battery's i got cheap i ended up getting 2 7AH battery's i may only need one but i have two if i need them. The amp is a Lepai LP-2020A+ and i got it today, its much louder than i expected. I tested them with some old speakers which are 6 ohms, 40W and 85 db SPL at 1 meter and they were very loud. I found that i have distortion at half volume but still is very loud. If i end up using speakers like these then i will just use the boxes they come in. Now that i know i have the volume i want i want to focus on finding speakers with good sound quality and good base response.

So far this is what i have spent:
- 2 Battery's 12v 7AH $24 Aud
- Amp $20 Aud
- 12v Battery Charger $6 Aud
Total $50 Aud

My budget is $100 so i have $50 for speakers and the box/esky and other small parts. A Few of my friends have some old car speakers that they said i can have so I'm going to have a look at them and see what they are like.

Have a look at the photos and let me know what you guys think
 

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Thread Starter

shlo

Joined May 11, 2014
21
Silly question, when I'm wiring the battery to the amp do i need a fuse or will the amp only draw the current it needs?
 

MrChips

Joined Oct 2, 2009
30,720
The fuse is to prevent a fire or other catastrophe.
I would expect the amp to have a fuse internally. If not, it doesn't hurt to install a 5A fuse inline with the +ve battery cable.
 

#12

Joined Nov 30, 2010
18,224
And, yes. The amplifier will only use the current it needs. Fuses aren't current regulators and they almost never have any effect on the circuit, they just wait until it all goes wrong, then they blow (hopefully) before something catches on fire.
 

Thread Starter

shlo

Joined May 11, 2014
21
okay thanks for your help guys. According to the manual the amp has over voltage protection if the voltage exceeds 14.4 volts but it doesn't say anything about amps. So should i still put a 5 amp fuse in? Sorry as you can both probably tell my knowledge on this stuff is minimal
 

#12

Joined Nov 30, 2010
18,224
P=I^2 x R
20 watts = I squared x 8 ohms
sqrt 2.5 = I
I = 1.58 amps
Try a 2 amp, slo-blo fuse.
or find that something changed and use the math I provided.
 

R!f@@

Joined Apr 2, 2009
9,918
It says 3amps. Meaning it can supply 3 Amps maximum.

So if 3 Amps is being drawn from the USB device it will be 5V X 3A = 15W.
Assuming 100% efficiency ( which is not ). The input power is 15W too.
So at 12V current is 15W/12V = 1.25A.
A 7AH battery can last for around 5 to 6 hours I believe.

I believe you will be charging phones, and if this the case it will last definitely much longer
 

Thread Starter

shlo

Joined May 11, 2014
21
Yes you are correct it will be used for charging phones. Mainly iPhones. When you say 5-6 hours I that for charging one or two phones?
Also I have two 7ah battery's but at the moment I'm unsure if I will use both. I figure it's better to be able to charge my phone if I need to
 

Thread Starter

shlo

Joined May 11, 2014
21
okay cool thanks. Hopefully I'm getting speakers this weekend and start the build. I may have a problem because my friend has a PA speaker that i can have but it would mean i can only run off one channel because i don't believe i can bridge this amp please correct me if i am wrong. Also if i run off one channel is there anyway to run mono signal so that I'm not just getting left or right audio signal?
 

Thread Starter

shlo

Joined May 11, 2014
21
I found a speaker that has separate speakers one for bass and one for treble wasn't ideal but works great. I have built the system and took it camping on the weekend, the battery's were not fully charged but lasted around 15 hours which is awesome. I am still waiting for the charger i brought to arrive but can anyone tell me if its safe to charge the battery's while hooked up (see pictures)? or do i need to remove them and charge them separately?
 

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MrChips

Joined Oct 2, 2009
30,720
You can charge the batteries while hooked up.
Also there is no need to have both batteries hooked up at the same time.
You can play the amplifier with one battery while the other is being charged.
 
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