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I am working on a small 12V boombox project and trying to figure out how I should approach the power supply. 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 using 10 Eneloop batteries. To me this seems to be the lightest and most easily configurable setup for a small boombox, but if you have other ideas, I'm open to that.

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 think it will take to charge the pack? I'm a beginner so ideally every step should be explained.

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    \$\begingroup\$ NiMH are a bit of a pain to charge; if I were doing this I'd use one of the lithium battery chemistries and find corresponding charge control ICs. How much current do you need, and does it really need to be 12V? \$\endgroup\$ – pjc50 Jun 9 '13 at 9:51
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    \$\begingroup\$ 10 of 2500mAh NiMH batteries? Good luck charging them with your iPhone charger which will take about 6 times the time it takes to charge your iPhone. I would consider LiIon since it will be eaiser, safer to charge and may be lighter. \$\endgroup\$ – abdullah kahraman Jun 9 '13 at 10:19
  • \$\begingroup\$ I hear you. I'll be running a TA2020 chip, which uses 12v.. amazon.com/electronics/dp/B003P534SW. The charging time is a big problem indeed. But I'd really like to use an USB charge for this project. Any idea on how they solved it with the Minirig? minirigs.co.uk \$\endgroup\$ – INT Jun 9 '13 at 12:32
  • \$\begingroup\$ Could you even charge 12V batteries with USB? If anything the batteries would drain through the USB because it is only 5V. We need a DC-CD step-upp \$\endgroup\$ – fuzzyhair2 Jun 9 '13 at 13:42
  • \$\begingroup\$ Have you looked into boost regulators - they take a voltage like 5V and up it to values like 12V no problem. \$\endgroup\$ – Andy aka Jun 9 '13 at 15:35
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By "iPhone charger" I assume you mean 5V 500 mA power source. Note that some USB outlets only provide 100 mA until the device actually answers the USB protocol and requests more than that, although wall outlet based ones will have all the juice all the time.

You can use a boost converter to take the 5V input to a 15V output. Either build one on your own using a switching controller, an inductor, some capacitors, and some amount of diodes/transistors, or buy a ready-made one from vendors like CUI or Murata or RECOM. Note that the current at 15V will be about 150 mA with 500 mA in at 5V and 90% efficiency.

Then, use a charge circuit for NiMH batteries. It's OK to parallel the charge circuit putting current in, and the device drawing power out from the batteries, as long as the charge circuit is properly current limited and follows the battery chemistry charging profile.

Actually, given that 1.2V NiHM cells have a max charge voltage of between 1.4V and 1.5V, 15V may not be enough, given the likely drop in the charge circuit. You may need to go to 18V. If you're ambitious, you could control the output voltage of the boost converter based on the current charge level of the batteries. That'd take some careful design tuning, but would make the whole system more efficient.

Stacking NiMH to get to 12V seems bad, though. I'd rather use something safe like a LiFePO4 battery, and charge that from the 5V (using a direct charge circuit, or using a buck converter that is controlled as a charger for higher efficiency.) Then use a boost converter from the battery to generate the 12V output needed. Again, such boost converters can be hand-built, or can be bought from providers. The charger would connect to the battery. The on/off switch for the whole device would connect/disconnect the battery to the boost converter that goes to 12V.

In both cases, the device could run on charger power, assuming the total draw at 12V is less than 150 mA (meaning total power draw of about 2 Watts or less.)

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  • \$\begingroup\$ Thanks for such an elaborate answer! I only have one question before I start researching on what solution might fit me the best. Boosting from 5v to 12v, are there any negative effects? \$\endgroup\$ – INT Jun 10 '13 at 20:10
  • \$\begingroup\$ When "converting" voltages, you loose efficiency (as already stated by JonWatte). Other than that, you (obviously) needs extra parts. Wouldn't it be an option to just use 5V for the whole design? It would make things much easier i guess. \$\endgroup\$ – Rev1.0 Jun 11 '13 at 7:08
  • \$\begingroup\$ The two main problems with a boost converter are efficiency loss (between 80% and 98% efficiency is typical, depending on design) and larger/more expensive components. If you're just building one for yourself, I think that's not likely to be a problem. And if this actually answers your question, you probably should press the "accepted answer" checkbox... \$\endgroup\$ – Jon Watte Jun 12 '13 at 0:04

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