Suitable battery for a supersmall/light boombox? - diyAudio
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Old 9th June 2013, 10:59 AM   #1
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Default Suitable battery for a supersmall/light boombox?

Working on a small 12v boombox project and I'm trying to figure out how I should approach the power part. I have some goals that I want to achieve:
  • Should be able to be charged with an iPhone charger (USB)
  • Should be able to play while being charged.
  • As light as possible
  • Small footprint
At the moment I'm thinking of making a battery pack made out of 10 pcs of Eneloop batteries ( Amazon.com: Eneloop XX AA batteries 2500 mAh NiMH x4: Electronics - Amazon.com: Eneloop XX AA batteries 2500 mAh NiMH x4: Electronics) For me this seems to be the lightest and most easily configurable setup for a small and boombox (if you have any other idea, I'm all open).
But what do I need to set it all up and what do I need for the charger part? The batteries will be built into the box so charging and playing must be able to happen without any interaction with the batteries. How long do you reckon it will take to charge the pack?
I'm a beginner so ideally every step should be explained.
Thanks so much!
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Old 9th June 2013, 11:25 AM   #2
ATAUDIO is offline ATAUDIO  Austria
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Actually, I had your same problem.
That was my answer (thzat is ready and will be tested in a couple of weeks, if my job lets me):

Very simple battery charger for TA2020 & Co

That does everything you ask , but needs a 15 V P/S that you can recover from an old laptop. Some of them worked at 15 V, otherwise there are some second souce universal ones that can be set up for that voltage.
I must tell you that simple solution does not seem to be of interest on the DIY Audio community.
Othewise, if you really want to charge through USB, you have the following limitations:

1) You need a step-up DC/DC conerter to at least 14V to charge a 12 V battery pack.

2) The current limitation of a standard USB port (500 mA) limits linits your max ower in output to about 1W on 4 Ohm when playing with a low battery, draining power only fromn the charger.

3) Even if you limit your solution to a 4 cells pack, the 5V supply of the USB is NOT enough to charge them correctly.
You need at least 1.4 V x cell. A fully charged Ni-Mh cell is about 1.39 V.
With 5V you can charge 3 cells, but then your power is limited to about 6 W on 4 Ohm whyile, playing on battery, the previous point limit still standing when battery is low and you play only from the charger.
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Last edited by ATAUDIO; 9th June 2013 at 11:49 AM.
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Old 9th June 2013, 11:29 AM   #3
FoMoCo is offline FoMoCo  United States
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The best choice for a battery would be Lithium-ion, if you use a dedicated charge controller for safety. Light, small, high energy density, but not easy to use.

Assuming your battery pack is as you planned, there's a way to figure out charge time, assuming that the boombox is not playing. You'll need to charge them to about 14V, and your supply is 5V.

That's a step up of 2.8, which means current is stepped down by that much. Assuming the charger is 90% efficient, that's 32% of the input current.

USB allows 100mA without negotiation, meaning the batteries get 32mA.

2500mAh divided by 32mA is 78 hours. Add in an additional 25% for overcharge and you get 98 hours. We'll call it 100 hours.

With negotiation you can get up to 500mA, but a lot of USB ports can't deliver. So... I wouldn't count on it. This means that it would take about 100 hours to charge those batteries, not using the boombox.

I'd say that using USB to power a charger is an exercise in futility. Get yourself a 15V wall-wart.
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Old 9th June 2013, 11:48 AM   #4
ATAUDIO is offline ATAUDIO  Austria
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Quote:
Originally Posted by FoMoCo View Post
T
I'd say that using USB to power a charger is an exercise in futility. Get yourself a 15V wall-wart.
I concour with most your post, that is thechnically sound (pun intended!) and very interesting .
Only I am not a fan of Lithium stuff.
LI-ion is really dangeros and I want NOT give it to my childrem. Proof enough is that even Boeing could not find out an affordable solution with it and had to keep on the ground for three monts its newest aircraft just for that.
The other Li chemistry, the LI-Fe is much more reliable, (the same used in your top - notch battery drill) bur it is heavier and comes often in automotive standard form factors that add useless container weight to the battery itself.
In practice, a 4 A/h Li-FE, that comes in the the automotive (motorbike) PB SLA 7A/h form factor ) weights the same as a 10 - Cell SUB-C Ni-Mh pack @ 4000mA/h, where you get the pure battery instead..
Only the shelf price of the Li is more than double.
I understand, that in the long term the Li-Fe has advantages (durability, flatness of the discharge curve) but I do not think it is yet the solution for a basically ludic application like a boombox.
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Old 9th June 2013, 11:57 AM   #5
FoMoCo is offline FoMoCo  United States
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Originally Posted by ATAUDIO View Post
LI-ion is really dangeros and I want NOT give it to my childrem.
Would you give a laptop to your children? That's what's in most of them. Li-Fe is definitely an alternative, but Lithium-ion is safe when used properly.
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Old 9th June 2013, 01:28 PM   #6
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I'm in a bit of a hurry right now so I'll read up and reply better in a while, but I saw that you wouldn't advise on using USB, FoMoCo, 100 hours is a very long time indeed. How do you think he solved it with the Minirig? Minirigs | Portable, Rechargeable Speaker
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Old 9th June 2013, 02:03 PM   #7
FoMoCo is offline FoMoCo  United States
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Originally Posted by voyage View Post
I saw that you wouldn't advise on using USB, FoMoCo, 100 hours is a very long time indeed. How do you think he solved it with the Minirig? Minirigs | Portable, Rechargeable Speaker
My assumptions are as follows:
1) He uses 500mA in his spec, which is quite optimistic.
2) The output is rather weak and rated at only a half meter. I'm guessing a 2 amp hour battery at 5V, not 12.
3) Like my calculations, the charge time is with the unit not playing.
4) Based on the specmanship that he's playing, the marketing department solved his problem.
5) Look at the frequency response curve on the technical page. There's no scale on the Y axis. That's more proof that he's not being forthcoming.

Most likely the charge time isn't even for a fully discharged battery. And, most USB ports won't provide his optimistic 500mA. If it did, your charge time would be down to 20 hours, and you most likely have a bigger battery to charge.

In my opinion, he didn't violate the laws of physics. He twisted things to say he did, without actually lieing about it.

That thing reeks of Bose type engineering...
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Old 9th June 2013, 02:15 PM   #8
ATAUDIO is offline ATAUDIO  Austria
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Quote:
Originally Posted by FoMoCo View Post
Would you give a laptop to your children? That's what's in most of them. Li-Fe is definitely an alternative, but Lithium-ion is safe when used properly.
NO, I WOULD NOT f they charge overnight on a chalet (say bungalow).
Li-Ion now are even not transportable in aircraft, as freight or in the luggage.
Their failure mode is not yet well understood, and estinguishing the fire is very difficult (they call a type "D" fire).
Boeing had to make a reinforced, valve vented, external steel container around them in order to get the 787 approved to fly again after a 3 months stop by FAA that costed them multi-millions. Root cause of the battery failure still undetermined.
And we are speaking about Boeing, not the lastcomer. Battery was made by Yuasa, another name thet should be in the know.

I was involved in testing some thousands of units of Li-Ion units for home appliances, and believe me, it is NOT a safe technology. Tolerably safe when kept under watch, at best. I do not let my laptop on the sofa when I go to sleep, call me paranoid if you wish.
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Last edited by ATAUDIO; 9th June 2013 at 02:18 PM.
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Old 9th June 2013, 11:26 PM   #9
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Originally Posted by ATAUDIO View Post
... Li-Ion... and believe me, it is NOT a safe technology. Tolerably safe when kept under watch, at best. I do not let my laptop on the sofa when I go to sleep, call me paranoid if you wish.
...true words...
Also I am surprised how relaxed the entire RC model world is acting with huge packs of these energy wonders.

PS
Still I am not storing my laptop in a sand filled stone box.
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Old 10th June 2013, 12:46 AM   #10
FoMoCo is offline FoMoCo  United States
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Originally Posted by ATAUDIO View Post
I was involved in testing some thousands of units of Li-Ion units for home appliances, and believe me, it is NOT a safe technology.
I must disagree. However, that sort of flame war is off topic, as the OP has already decided on a different battery technology. Being that as the case, I won't argue pros and cons of battery technologies. Things get out of hand pretty fast here.
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