Cell Phone "AA" BATTERY Replacement...

[NewToElectronics]

My cell phone battery uses 3.7V. I want to use 2 "AA" batteries (3V) to replace the 3.7V cell phone battery. Does anyone know where I would start to try to design a dc-to-dc converter for this? How important is it to use 3.7V as opposed to a 3V power supply? I'm sorry for the complete newbie question, but I don't have much experience with electronics.
Thanks, Mike

 

[audioguru2]

Two AA battery cells are 3V only when they are new. Their voltage quickly drops to about 2.4V then slowly drops to 2.0V.
I don't think your cell phone will work with a battery that is between 2.0V and 3.0V.

Where are you going to put a dc-dc converter circuit?

 

[NewToElectronics]

I thought that a dc-to-dc converter would be required to meet the 3.7V battery requirement. If I had 3V, then that would have to be converted to 3.7V to make the device work properly... at least I assume that to be the case.
I never thought about a voltage drop-off being a problem... good point. So what about the devices that uses "AA" batteries in order to operate? They are designed to operate between these voltages?

 

[audioguru2]

So what about the devices that uses "AA" batteries in order to operate? They are designed to operate between these voltages?

Two AA battery cells supply a voltage of from 2.0V when old, to 3.1V when new. Devices that use this as a battery must be designed for the range of voltages or they won't work, will they?

A guy posted a circuit on the web that uses an ordinary Cmos gate IC that has a minimum operating voltage of 3.0V, and he powers it with two AA battery cells. I tried it and so did other people and we found that it doesn't work with a battery that is slightly used.
I built it with a 74HCxx high-speed Cmos gate IC that has a minimum operating voltage of 2.0V and it works fine until the two AA cells are at the end of their life.

 

[NewToElectronics]

Great Insight. I would be interested to see what operating voltage range my cell phone would work. I don't want to hook this up to a variable DC supply: do I run the risk of destroying my cell-phone if I put 4.5V into it? My plan was to completely charge a cell phone battery and measure the voltage accross the leads to see the operating voltage range of my cell phone.

 

[NewToElectronics]

Luckily, on queue, my cell phone started beeping me indicating a low-battery. I measured the leads of the cell phone battery at 3.45V. If I can assume that this is my minimum voltage, then the range of voltage I want to get from "AA" batteries is 3.45V - 3.7V. The question is, can I get this range of voltage with 3-4 "AA" batteries. The additional constraint would be not to damage cell phone with excessive voltage.

 

[NewToElectronics]

Actually, I have a LG VX3200. A very basic $20 phone from Verizon. Horrid battery life. This wasn't even really intended to fit inside a cell phone. I don't see how it could either. I imagine I could tape/velcro the batteries to the back cover somehow. It wouldn't be for looks. I just really like the idea of using "AA" batteries for a cell phone.

 

[ante1]

A circuit diagram would help, some phones have a step-up regulator right at the input and some have a step-down. It is a good start to know which you phone has.

 

[NewToElectronics]

A circuit diagram would help, some phones have a step-up regulator right at the input and some have a step-down. It is a good start to know which you phone has.

Speaking of a step-up voltage regulator, are there any voltage regulators capable of converting 3V to 3.7V?

 

[audioguru2]

Are you planning to run your cell phone from throw-away alkaline battery cells, instead of using a rechargable battery? Are you foolish and rich?

AAA cells have less than half the capacity of AA cells and AAAA cells or a 9V battery (it has 6 AAAA cells inside) have 1/5th the capacity of AA cells.

 

[NewToElectronics]

I guess I don't really care about the practicality about it! I figure there are worse things I could be doing with my mind and my money... and if I happen to learn a couple things on the way, how bad could that be for me? What non-linear voltage regulators can you recommend?

 

[ante1]

Speaking of a step-up voltage regulator, are there any voltage regulators capable of converting 3V to 3.7V?

Put in your requirements here:

http://para.maxim-ic.com/search.mvp?q=dcdc_all&an_1=Family&av_1=Switch-Mode%20DC-DC%20Power%20Supplies

and you will be recommended some alternatives to chose from.

 

[NewToElectronics]

Thanks for the link. Looks like I'll have to do a little bit of research to figure out what all the parameters mean... in the meantime, what are some of the favorite and more popular switching voltage regulators out there?

 

[NewToElectronics]

I have decided on:

TPS61122: Synchronous Boost Converter with 1.1A Switch and Integrated LDO

Is this a good choice?

The specs:
"3.6-V 95% Efficient Boost Converter with 3.3-V 200-mA LDO for 1-Cell LiIon or Dual-Cell Applications"

1. Topology: Boost
2. Min Vin:    1.8
3. Max Vin:  5.5
4. Min Vout:  3.6
5. Max Vout: 3.6
6. Preset Vout:  3.6
7. Iou Maxt: 0.5
8. Iout(2): 0.2
9. Operating Frequency: 600 kHz
10. Iq(typ) 0.04 mA

Looks like the Max, Min, and the Preset Vout is almost exactly where I want it to be: near 3.7V.

 

[NewToElectronics]

Can anyone recommend a 2 "AA" battery holder that doesn't take too much space? My current designs for a prototype will be to use a 2 "AA" battery holder, solder some wire to the leads to my little switching voltage regulator, and then connect the output of the voltage regulator to the cell phone battery terminals with very small aligator clips.

 

[audioguru2]

I have decided on:

TPS61122: Synchronous Boost Converter with 1.1A Switch and Integrated LDO

Is this a good choice?

It is in a tiny surface-mount package. Can you solder it?
Its regulated output voltage is only 200mA which is only 0.74W at 3.7V. The RF output of my cell phone is 2W so its battery power is about 3W at 3.7V. The IC isn't powerful enough for my cell phone. Will it work with yours?

 

[NewToElectronics]

It is in a tiny surface-mount package. Can you solder it?

I have no idea! I hope so... I won't know until I see it. I'm getting a free sample off of the TI site.

Its regulated output voltage is only 200mA which is only 0.74W at 3.7V. The RF output of my cell phone is 2W so its battery power is about 3W at 3.7V. The IC isn't powerful enough for my cell phone. Will it work with yours?

I had no idea that a little cell phone would be drawing that much power?! So maybe it's back to the drawing board.

I found some battery holder's online through batteryspace.com. It is a little expensive for shipping, but the holder's themselves are cheap. Also, I didn't realize what you guys were saying about the rechargeables being only 1.2V each... so maybe I could use 3 "AA" rechargeable batteries for 3.6V and not worry about the regulator. However, I also like the idea of using 2 "AAA" batteries to cut down on size, so I am still interested in switching regulator.

 

[indulis]

Be careful here...

Yes, it's a boost converter, but by definition, a boost converters output will always be bigger than it's input. Just because it says the input range is to 5V, doesn't mean you can put 5V in and get 3.7V out... won't happen. Your input voltage would have to be less than 3.7V. Also, you don't "have to use" the "on-board" LDO. The internal "switch" is good for 1A, so you can get more than 200mA out of it.

 

[audioguru2]

[i didn't realize what you guys were saying about the rechargeables being only 1.2V each... so maybe I could use 3 "AA" rechargeable batteries for 3.6V and not worry about the regulator. However, I also like the idea of using 2 "AAA" batteries to cut down on size, so I am still interested in switching regulator.

The Lithium-Ion rechargable battery that is made to fit in your cell phone is 3.7V. Most people use it and charge it with the charger that is made for it. Why don't you?