Question for battery experts.

Guysakar · Feb 7, 2014

Hi guys,

Long time reader first time caller. I know this is a bit off topic, but you guys here seem to be the most on the ball concerning my topic.

In short, I want to build an emergency cell phone charger out of one AAA battery. Energizer has done this with one AA battery, so I assume that it is very possible. Link

How are they doing this? If I just hook wires up to a AAA battery (let's say lithium) and hard wire that to a micro USB and plug it into my phone, would that work, being that the AAA battery is only putting out only 1.225 volts and cell phone batteries usually put out around 3.8 volts?

My goal is not to actually charge the phone battery, but to power the phone for, say, 10-20 minutes or so.

I know I could just purchase the Energizer contraption with the AA, but I really want to make my own and make it with a AAA.

Any ideas, thoughts or advice will be greatly appreciated.

Thank you in advance.

AnAppleSnail · Feb 7, 2014

I can talk in general about this, but I can't tell you exactly what parts you will need. You will need some stuff to raise that voltage up.

The phone wants about 5V power supply. The way Energizer (And others) do this from any battery with less than 5V is a part that boosts voltage. Well, technically it's several parts, but they come in a chip. Flashlights use them to raise 1.5v (1 AAA) to about 3.5v (LED working voltage). They are common and pretty cheap. The AAA will give you about half an hour's charge at a low-for-smartphones charge rate. Most cell phones only work when the battery has power for the phone. None of my past three smartphones or two feature-phones ran without the battery being charged (Their internal power circuits only ran when the battery was installed and charged).

I should tell you that the AAA charger will be a few millimeters smaller in each dimension than the AA charger, but have about one-third the available power at a likely charge rate. The extra capacity in the AA is good for phone battery managers - My phones will usually complain of "Precharging" and refuse to turn on for about the first ten minutes of charging.

For a reasonable device, you will want a voltage-boosting component with 100-500mA output. Let's pick 350mA as a nice compromise - Some low-end smartphone chargers run at this charge rate. That is 350mA x 5v, or 1.75W power output. These boost circuits are around 85% efficient, so we need about 2W from the battery. This is sustainable for a bit on an Energizer Lithium AAA, but a bit much for an alkaleak. To get 2W from a 1.5v battery, we need 1.4 amps. This will flatten your AAA in around half an hour - So there is definitely a good chance to get a decent charge, even from the AAA. If my current Android phone were off, this would be vaguely like a 20% charge.

The AA is 14mm diameter x 50mm length. The Energizer charger is about $20?
The AAA is 10mm diameter x 44mm length. Cutting down the Energizer charger will be the easiest way to build this.

mojo-chan · Feb 7, 2014

What phone do you want to power? An old dumb Nokia would be best because anything more will draw too much current. The battery will handle the current but your switching boost regulator might not.

Have a look at Adafruit's MintyBoost. It does what you want but I can't remember off hand if it will work with a single cell. If not it shouldn't be hard to find a different switching regulator that will code with very low voltages. IIRC Microchip do some that go down to 0.5V.

Guysakar · Feb 7, 2014

AnAppleSnail said:
I can talk in general about this, but I can't tell you exactly what parts you will need. You will need some stuff to raise that voltage up.

The phone wants about 5V power supply. The way Energizer (And others) do this from any battery with less than 5V is a part that boosts voltage. Well, technically it's several parts, but they come in a chip. Flashlights use them to raise 1.5v (1 AAA) to about 3.5v (LED working voltage). They are common and pretty cheap. The AAA will give you about half an hour's charge at a low-for-smartphones charge rate. Most cell phones only work when the battery has power for the phone. None of my past three smartphones or two feature-phones ran without the battery being charged (Their internal power circuits only ran when the battery was installed and charged).

I should tell you that the AAA charger will be a few millimeters smaller in each dimension than the AA charger, but have about one-third the available power at a likely charge rate. The extra capacity in the AA is good for phone battery managers - My phones will usually complain of "Precharging" and refuse to turn on for about the first ten minutes of charging.

For a reasonable device, you will want a voltage-boosting component with 100-500mA output. Let's pick 350mA as a nice compromise - Some low-end smartphone chargers run at this charge rate. That is 350mA x 5v, or 1.75W power output. These boost circuits are around 85% efficient, so we need about 2W from the battery. This is sustainable for a bit on an Energizer Lithium AAA, but a bit much for an alkaleak. To get 2W from a 1.5v battery, we need 1.4 amps. This will flatten your AAA in around half an hour - So there is definitely a good chance to get a decent charge, even from the AAA. If my current Android phone were off, this would be vaguely like a 20% charge.

The AA is 14mm diameter x 50mm length. The Energizer charger is about $20?
The AAA is 10mm diameter x 44mm length. Cutting down the Energizer charger will be the easiest way to build this.

Thank you very much for that post. That was exactly the information I was looking for. I agree with the AA being the better source but I am going for a specific size and the AA is just too fat to fit in the housing I have in mind. And, if the AAA lithium would provide about 20-30 minutes of power, that would be perfect.

I'm Googling variations of voltage, booster, chip, flashlight, etc... The best I am finding is stuff like this, voltage regulators switching regulators

What do you think the best term to search for to find a source would be?

If I am understanding you correctly, I will need a battery, the voltage chip (one piece) and then of course the micro USB.

I'm going to keep searching and see what I come up with, but your post is invaluable and I kindly thank you.

Guysakar · Feb 7, 2014

Thanks for the reply. So much good information. I'm looking to charge something along the lines of a Galaxy S3, which the battery is 3.8 volts and 2100 mAh. I'm only looking for an emergency 20-30 minutes of power at the most.

I can't go into detail on exactly what I am doing on an open forum, but when PM privileges are granted I will send you two a PM and tell you what I have in mind.

But basically I want to find the absolute simplest and cheapest way to take a AAA battery (Has to be AAA. You will understand why in my PM) and put power to a smart phone.

If it is as simply as running wires to a small chip, and then connecting those wires to a micro USB and a battery, then I am in business (after I take a crash course on how exactly to wire the chip of course. There seems to be many prongs sticking out of them).

Again, thank you both for the help.

Tmack · Feb 7, 2014

Why does it need to be built? I have 2 5000mah battery boosters from TARGUS. They work great. I can run my phone all day.
Just curious as to why it must be AAA. size problem, or do you just want the satisfaction of making it yourself?

RetroTechie · Feb 7, 2014

More importantly: why would one want to power this with an AAA battery? :thinking:

Not that I'd want to interfere with "because we can!",

but for virtually any phone on the market right now, its battery capacity >> capacity of an AAA battery. Read: in an emergency, it might put some small % of charge in the phone's battery. But never charge it fully. In all practical cases, using an AAA battery for this purpose is practically useless imho, as it's simply inadequate as an energy source.

Tmack · Feb 7, 2014

All the battery boosters, or external battery packs output 5v . Mine outputs 5.1v 2.1a. I know your only looking for a short period of run for emergency, but without more power than a AAA could give, I don't know if the phone would even recognize it was being charged. If a different charger (that still fits, but us the wrong output) is plugged into my charge port, my phone gives me a message saying that the charger being used isn't compatible with my phone, even though it fits perfectly. It may not be the case with every phone, but I'd think you would at least need 5v

mojo-chan · Feb 7, 2014

You need 5V-ish and 100mA minimum. Most phones will charge from less than 5V, and may even be more efficient at lower voltages. There is a cut-off though, usually around either 4V or 3V.

100mA is the minimum current that a USB port will provide without any negotiation IIRC. Ideally you want 500mA but in theory 100mA will do. In practice a lot of devices choke with less than 500mA..

AnAppleSnail · Feb 7, 2014

The last time I was out 'in the field' with a phone recharger (Depleted alkaleaks) and a near-dead phone, I turned off the phone, plugged things in, and took a long wait. The battery went from 5% (Emergency power, can't send calls (WTF?)) to about 15%. That was enough to get AAA, although the driver was annoyed that he could not call me back.

While we're on the subject of emergency recharges, what about a 9v battery? The lithium ones have almost 350mAh of capacity. Even if you just throw away 4v (It will take fairly good regulation to charge picky phones. Un-picky phones will take anything), that's almost double the output that the AAA would manage.

Here is a particular component that would work. It is on a "breakout board" that increases the size and ease of use. Sparkfun.com NCP1402-5V Stepdown Widget (Mods please keep the part title if you kill my link). It gives you practice reading datasheets. At Vout of 5v, and Vin of 1.5v, what is the output current? Pretty low, isn't it? The performance won't be great. The stated size gives you an idea of why these often run off 2xAA - The size of the chip can be large, if it isn't a single-package device.

This (Texas Instruments tps612222) is more size-friendly. But harder to work with. The key words I use are Boost 5V.

Question for battery experts.

Guysakar

Newly Enlightened

AnAppleSnail

Flashlight Enthusiast

mojo-chan

Newly Enlightened

Guysakar

Newly Enlightened

Guysakar

Newly Enlightened

Tmack

Flashlight Enthusiast

RetroTechie

Flashlight Enthusiast

Tmack

Flashlight Enthusiast

mojo-chan

Newly Enlightened

AnAppleSnail

Flashlight Enthusiast

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