Powering a mobile phone with no battery

Posted in Electronics on 2011-11-25 at 20:42:16 by Chris – 43 Comments

A friend wanted me to build a power supply that would enable him to permanently wire a GPS tracker into his van, with power supplied by the van's battery. The GPS tracker he bought uses a regular SIM card to SMS its location to a designated phone. It uses one of the 3.7V BL-5B batteries normally found in Nokia mobile phones. Charging the tracker's battery each week would quickly get boring, and leaving it on charge in the van permanantly would eventually degrade the battery, so I wanted a solution that eliminated the BL-5B battery altogether. I have an old Nokia kicking around so I thought I'd use it for testing.

You might think the task of building a power supply to give 3.7V from a 12V car battery would be fairly trivial but it's not without difficulties, chiefly the fact that even though mobile phones only draw a few milliamps most of the time, when they need to talk to the cell tower to make or receive a call or text message, they draw a significant current (several amps) for a short periods.

To deal with this I have used a heavily-decoupled MIC29152WT variable-output LDO regulator. In the diagram above, the left column pair of capacitors are 0.22uF ceramics, the middle column pair of capacitors are 10uF tantalums, and the right hand capacitor is a big 47000μF electrolytic. The choice of resistors selects the output voltage; devices powered by Nokia batteries appear to work best at around 4.1V; a little bit of experimentation with the resistor values may be necessary.

The components are probably over-spec'd for what they need to do, but I only need to make one, and space is not an issue (within reason!). The whole thing mounts nicely on a piece of stripboard, with no need for cutting tracks:

"Why hello there, Nut. Meet my good friend Sledgehammer."

The new power supply connects directly to the battery terminals. An 82kΩ resistor between the middle terminal and ground is necessary to fool the phone into thinking a real battery is present:

Note: I'm disabling comments on this post because there seems to be a fairly repetitive trend going on. Comments fall into two categories:

  • "Can I use different components?" - answer, probably. Try it and see!
  • "Can I do this with a similar device XYZ?" - answer, possibly. Try it and see!

43 Comments

Does mobile phone works with an ac adapter...here power is from car battery...but what if from ac and then use an adapter...does it work?..

Hi Raj,

No, a phone will not work with just the AC adapter. You may be able to get it to power on by fooling the phone into thinking there's a battery present (on Nokia phones that means soldering an 82kΩ resistor between the middle terminal and ground), but when you try to make or receive a call or a text, the phone will reset.

Why? Because the AC adapter alone is able to supply the small (~5mA) current necessary to run the phone's user interface, but it is unable to deliver sufficient current to allow the phone to communicate with the cell towers. Ordinarily the phone's battery provides this short burst of current, but when the battery is not present the voltage drops and the phone resets itself.

To make it work with an AC adapter you would need a voltage regulator and a big smoothing capacitor as I have shown here. If you want to build one, let me know and I'll publish construction information. The component cost is about £10.

Chris

Interesting work... I would like to build a dc supply to power my automobile bluetooth device thus eliminating the device battery. The battery supply is an Li-ion 3.7v 1650 ahm - small 3 cell pack. Ideally I would like to contain the footprint of the supply within the battery space provided by the device if possible however I'll do whatever is best to make it work. Saw your info concerning the cell phone supply. I am not an electronics pro however I believe I could easily assemble something such as this with instruction. Could you help me by schematic/instructions and possible alternatives?

Michael

Hello Michael,

My gut feeling is that a bluetooth device would be much easier than a cellphone to power directly from a vehicle's 12V supply. I can infer this from two facts:

  • The ubiquitous bluetooth USB dongles are tiny (therefore do not contain large capacitors).
  • They work fine powered from USB, which has a strictly limited current-supplying capability.

What is the make and model of your bluetooth device? How many terminals does its battery have? What are the battery's exact dimensions?

To build something like this you will need a soldering iron, a small pair of wire cutters, possibly a scalpel or other small but very sharp knife, and a voltmeter. If you have all that stuff I would be happy to spec something for you.

Chris

The bluetooth device is built into the rear view mirror and has no brand ID nor any reference to manufactor other than "Made in China". It has many features that are useful, I use it to pair with my phone then manage calls through FM on my radio. It also has an optional earpiece for private listening and embedded display in the mirror that IDs the incoming call, etc. The battery pack only has two terminals (+ & -), 3.7v 1650mAh is the info from the battery pack cover. The dimensions of the battery are 1 15/16" x 1 7/16" x 3/8". Upon closer examination I have found that the device has a USB port on top although I must admit I am unsure as to its purpose. I do have all of the tools you stated plus and would be grateful for your expertise.

Michael

i saw this website they havethe gadget to connect car battery to cell phones directly.

http://boondeeworkshop-dot-com/cellphone

Wow, they're charging $100 for something that cost me less than $10 to make!

Hi Chris,

Im trying to do a similar thing as you with a PDA, I have removed the lipo battery from its case and obtained the small pcb from inside the battery with the contacts that connect to the pda, I soldered some wires to the battery connections of the pcb and then wedged the pcb in place to meet with the contacts of the PDA, I powered it up via a variable bench supply set at 3.7v and hey presto the PDA fired up straight away, it all seemed fine, then today I check and the PDA is OFF, now when I try and turn on the PDA it starts up ok and then drops out randomly, if i put a battery back in the PDA all is well, so Im assuming that my bench supply and long cables etc cant provide the momentary current spikes that the battery could handle, Im assuming that I'd just need to add a capacitor close to the PDA supply wires ? could you provide me with a schematic of your supply and part numbers ?, many thanks ;-)

Hi Dave,

I added a circuit diagram above. It's drawn from memory because the PSU I built is now installed in my friend's van. I'm not 100% sure the resistor values shown in the circuit are the values I actually used, so I suggest you do some experimentation with the resistor values, using a voltmeter to check the output voltage.

Chris

Hi Chris..
U have done a wonderful job. Can u please explain why that 82kΩ resistor between middle and ground terminal is used in order to make the mobile think that a battery is present? I mean why doesn't the phone switch on without the resistor?

On Nokia phones, the middle terminal is called VBAT. The phone uses it somehow to detect the presence of a battery, I don't know exactly how. So in order to fool the phone into thinking there's a battery present, you must connect the 82kΩ resistor between VBAT and ground.

Thank you for the explanation. I understood in a better way now.

Great job Chris. I want to do exactly that for a piece of junk called Motorola Droid X (MB810), which can no more be used as a cellphone. I hope I can use it as a GPS device in my car. The problem is that the phone has four battery contact points and I don't know what each is for. How can I find out the pinouts? any help will be greatly appreciated.

On modern Motorola batteries, the inner pair of contacts are for voltage detection and the outer pair are + and -. I just tried covering the inner pair of contacts on my Milestone A853's battery with a small piece of insulating tape, leaving the outer pair free. The phone started up OK, but was unable to detect the battery level (it shows a ? in the battery level indicator). Strangely enough, it also behaved as if there was no SIM installed, but I guess you're not worried about the SIM anyway.

Please let me know how you get on.

When you have the electronics done and you're ready to mount your Droid X in your car, you might want to make one of these!

Thank you Chris for these tips. It means I probably just need cigarette lighter output directly connected with the battery terminals of the phone. If that works then it will be under $ 10 solution. I will post the results when I am done. Your Phone holder is great but I got one of those ready made plastic devices to hold my phone in place. However, in some situations your proposed solution will be really handy.

Edit: cigarette lighter USB adapter with output reduced to 3.7 volt.

How do you intend to reduce the output of the USB adapter to 3.7V? For that you'll need a 5V->3.7V regulator. You may as well just build a 12V->3.7V regulator and eliminate the USB adapter. The only difference between the circuit you need and the circuit I built is that you won't need the big 47000μF reservoir capacitor because you don't need to install a SIM.

Hello Chris,
you mention this:
"To make it work with an AC adapter you would need a voltage regulator and a big smoothing capacitor as I have shown here. If you want to build one, let me know and I'll publish construction information. The component cost is about £10."

I'm interested in this construction information so please can you publish this?

Thanks.

Since I wrote that I did in fact add construction information to the blog post. There is now a circuit diagram with component information listed above.

Hi Chris, If we are speaking only about Regulator and capacitor, the maximum MIC29152WT current is 1.5 A, charging only (phone is not connected) 47000uF capacitor can be over 1.5A, regulator can be damaged?.

From the datasheet:

The MIC29150/29300/29500/29750 family of regulators are fully protected from damage due to fault conditions. Current limiting is provided. This limiting is linear; output current under overload conditions is constant. Thermal shutdown disables the device when the die temperature exceeds the 125°C maximum safe operating temperature.

Personally, I suspect overvoltage conditions and sustained high power dissipation are the things which are most likely to kill such a device. The short spike of current whilst the capacitor charges is unlikely to hurt it.

I tried this circuit with output capacitor (10000 mF) which is enough for mobile phone, it seems to me. But LM317 requires radiator, very hot.

I'm guessing you mean 10,000uF?

Yes 10000uF

I would like to charge a hand phone by just using an input power of battery such as 3.7V straight. Is it possible?

You're asking if it's possible to power a mobile phone using a 3.7V battery? Isn't that what mobile phones are designed to do in the first place?

A better design would have been to modify a battery housing to contain the components. Then pop in the modified "battery" husk when you want to run off mains, but you don't have to un-solder when you want to go back to a regular battery. An interchangeable, component based solution.

I'd pay $75 for something like that for a Nokia 3310 compatible "battery" shaped power supply.

"Better" is pretty subjective. Design is all about choosing what you're optimising for. I was optimising to minimise the time taken for me to get something working so I could get it out the door and move on to more interesting stuff. Space certainly wasn't a concern for me. That said, although I didn't bother working out what a minimum safe value for the reservoir capacitor would be, I'm guessing it would be pretty hard to cram enough capacitance inside a package that small.

Hi Chris,just say ur grt works,op u don't mind me takin u back to 2011? Pls how do i power a phone directly using solarcells without battery?

You could try a circuit like mine, but with the solar cells instead of the car battery. I can't guarantee that it will work because I have not tried it. I live in the UK where there is not much sun. Oh and remember to tell your friends not to try calling you when it's dark!

thanks alt Chris & sorry for the late reply. havnt been able to try it out cos i havnt gttn a suitable solar cell. will keep u postd though. thanks again

Hi Chris that was a great job there, can a different component other than the MIC29152WT be used to accomplish this project? I'm trying to power a phone using solar cells but I haven't been able to get the MIC29152WT component where I reside, I would like to have this performed with something not as big as the circuit you used in yours, what do you suggest?
thanks.

Have you tried Farnell? Another variable-voltage regulator should work OK; you should be able to get away with any regulator that can be configured to give about 4.1V at 500mA. Shrinking the physical size is more difficult; you may be able to get it to work with a smaller valued (and therefore physically smaller) capacitor, but I can't promise anything. Let me know how you get on.

ok, thanks

Hi Chris, I was only able to get a regular 3pin regulator (LM317) where I reside, is there a way one can reconfigure the circuit with a regular 3pin regulator? also if you don't mind could you explain to me the exact function of each component in the circuit, thanks so very much

Any regulator will do, as long as it can be configured to output the required 4.1V at 500mA. Of the components in my circuit, the resistors and the four small capacitors are what are suggested in the application circuit in the MIC29152WT datasheet. The only thing I added was the big electrolytic capacitor to provide a reservoir to smooth the big spikes when the mobile phone tries to talk to the cell tower. If you use a different regulator, you just need to follow the application circuit in its datasheet. You will still need the big electrolytic capacitor on the output though, otherwise your phone will switch itself off as soon as it tries to talk to the cell tower.

Hello Chris:

I have a mobile phone with TV and other options. The problem is that the duration of a new Nokia battery (890 mAh) is too short running these applications. What I would like to do is to create some kind of external power pack (using rechargeable batteries of 2100 mAh)in order to obtain 5.2 V. I am planning to use the same connection for the battery charger (battery inside the phone). Do you think is that possible?

Thanks in advance,

George

Definitely possible. I'm not sure how easy it would be though. It might be as simple as bringing the battery connections from inside your phone out to the external battery pack. I don't know whether the battery-charge circuitry inside your phone would be able to correctly recharge the external battery though.

Dear Chris:

Thank you very much for your answer.

Dear Chris:

Do you know, please, if exist Nokias batteries (specifically of the type BL-5B) with extended capacity (2000 mAh or more)?
I have tried to get this information several times but I failed.

Thanks,

Jorge

Sorry, I don't know.

hi man. was wondering, i got this router that uses batt. but it runs low so often that i wanna get rid of the batt and power it up using an adapter. Its got a three terminals battery. I tried connecting it directly, the positive and negative terminal alone minus the resistor. the device powers up but it doesnt work. is there a way i can do it without the resistor coz where i am i cant access a resistor. its a chinese model with no name. thanks

I have no idea, sorry.

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