Comments on: How to Build a 9v Battery Powered USB Charger

By: Jesper

Jesper — Fri, 06 Nov 2009 18:35:41 +0000

Good try, Andreas, but not so wise.You have yourself pointed out some of the problems, I’ll just try to clarify and add some more info.For an absolute EMERGENCY charger, it can perhaps work, but there are some serious flaws.1. As already pointed out, you have the battery discharge curves a bit wrong. Batteries are not perfect, especially not 9V cells which are not meant for high-current draw.2. The 7805 will be insanely hot, and probably self-destroy if you hook up something with a max draw of 500 mA.As it is being used in a charger situation, many devices may draw this max current (to pump up the internal cell quickly), even though the device is itself a low-current- draw device.The thermal resistance junction-to-case of a 7805 is 65deg/watt, meaning the temperature of the center silicon will raise 65 degrees (Celcius) for each watt the regulator dissipates. The dissipated effect at 500mA is (9-5V)*500mA = 2W, which gives a temperature rise of 130 degrees. Add to that, the room temperature of, say, 25 deg. That means the silicon will be at about 155 degrees. Anything above 125 degrees is outside the chip specs and seriously bad.3. The 7805 series of regulators are prone to oscillation, so small capacitors are normally mounted close the the device. As you have none, there is a risk the regulator will go into self-oscillation and regulation will be affected, potentially delivering the full 9V to your USB device. Not good, obviously.4. As you already know from calculations in this and earlier posts, linear regulator methods have huge losses.In this case, efficience is 5/9 -> 55%. The rest is lost as heat.A step-up regulator (from 3V or even 1.5V), typically have an efficiency of 90%.There’s a number of erroneous posts here too:cp suggest to use a 78L05. Serious mistake, it’s a low-power regulator for max 100mA and will die shortly.The regulator itself does not “use” any significant power.SansaRulez83 suggest using a car battery. I hope my above calculations of temperature issues show that this would not be a smart move.

By: Dromedary

Dromedary — Wed, 28 Oct 2009 19:05:03 +0000

For those who couldn't be bothered to search:Links to a MP3 player/phone charger with mini USB output. Uses two AA batteries:http://www.amazon.com/Turbo-Charge-TB550-Portable-Charger/dp/B000R1C42E/ref=sr_1_1?ie=UTF8&s=wireless&qid=1256770705&sr=8-1The Ipod compatible version:http://www.amazon.com/Turbo-Charge-Portable-Charger-iPhone/dp/B000R1C42O/ref=pd_bxgy_e_img_cThe two-battery version is said to work better with larger devices. I noticed that rechargeable NiMh batteries provide higher current and do not overheat as the plain alkalines.

By: Dromedary

Dromedary — Wed, 28 Oct 2009 18:53:06 +0000

Nice to see, but there are much more practical chargers based on AA batteries.Amazon and Walmart sell models using one and two AA batteries for about $20.I myself use the old "Turbo Charge" model. Works like a charm with my Iaudio U3 and rechargeable NiMh AA.Just search for:"Turbo Charge" Portable Chargeron Amazon or even Google.http://www.walmart.com/catalog/product.do?product_id=10746818

By: graham

graham — Wed, 28 Oct 2009 18:26:41 +0000

Great idea. Any thoughts on how many charges one might get from a new 9 volt battery? Lets say the player was a Sansa Clip.

By: Andreas Ødegård

Andreas Ødegård — Wed, 28 Oct 2009 17:50:05 +0000

@fred You're right, I've written about that before here: http://www.anythingbutipod.com/archives/2009/08/usb-charging-guide.phpForgot to add it to this article and have now, thanks for the reminder!

By: Allan White

Allan White — Wed, 28 Oct 2009 15:27:03 +0000

Great concept! Simple & compact.For your multi-D-batt design, I wonder if you could adapt a Maglite case or something, so it could be a flashlight and charger. THAT would be supa-cool.

By: fred

fred — Wed, 28 Oct 2009 14:58:48 +0000

some media players like SonyNWZ need to have the middle pins at set voltages to initiate device charging. This design will not charge with them

By: Chris aka sumx4182

Chris aka sumx4182 — Wed, 28 Oct 2009 14:18:41 +0000

I wouldn’t use a resistor because as the battery drains, the voltage will drop and vice versa, a fully charged battery will output a different voltage than a half charged battery. And also, two different batteries may have slightly different voltages. A regulator will convert any of these to the necessary voltage while a resistor or resistor network will only match to a specific voltage.

By: remus

remus — Wed, 28 Oct 2009 13:21:12 +0000

Is it possible to use an resistor instead of your voltage regulator?Thanks for an answer and for this gread article!

By: humorsimpson

humorsimpson — Wed, 28 Oct 2009 12:23:09 +0000

Cool, this might work for me. I was wondering how to power my speakers that I have been using in my shed via my 19.2V rechargeable tool batteries instad of going through AAA’s like I have been doing. I will give this a try! Thanks!