How to charge a mobile phone (5v500mAh) from 6v 1300mAh Ni-Mh battery

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Discussion Overview

The discussion centers around the feasibility and methods for charging a mobile phone (rated at 5V 500mAh) using a 6V 1300mAh NiMH battery. Participants explore various circuit designs, voltage regulation techniques, and the implications of USB charging specifications.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • Some participants suggest using an LM7805 voltage regulator to achieve the necessary 5V output, while others caution that it may not be the best choice due to power dissipation issues.
  • One participant notes that a 6V NiMH battery can peak at around 7.5V, which necessitates careful consideration of the voltage regulation to avoid overheating.
  • There is a discussion about the need for a stable voltage rather than a constant current, with some proposing alternatives like low dropout regulators (LDOs) such as the LM2940.
  • Some participants express uncertainty about the relevance of USB specifications for charging, with differing views on whether a simple 5V source is sufficient for most phones.
  • Concerns are raised about the power dissipation in regulators and the necessity of using heatsinks to manage thermal performance.
  • Participants debate the complexity of USB charging protocols and whether they impact the charging process when using a simple voltage source.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the best method to charge the phone, with multiple competing views on the choice of voltage regulator and the importance of USB specifications. The discussion remains unresolved regarding the optimal approach.

Contextual Notes

Participants highlight limitations related to the assumptions about voltage regulation, the need for heatsinking, and the varying requirements of different phone models regarding charging inputs.

Meera
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i need to use as a portable charger(POWER BANK) for my mobile
can i use LM7805 to deliver my rated output !
please guide me with circuits !
 
Last edited by a moderator:
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Looks possible - one approach is just to get the battery to supply what the phone's AC adaptor socket expects.
There are other approaches - so you want to read around and pick one.
If you are unfamiliar with electric circuits then the advise is: don't do it.

Also see:
https://www.physicsforums.com/showthread.php?t=575310

Don't know how good these are - only skimmed them.
http://www.Newton.dep.anl.gov/askasci/gen06/gen06628.htm
http://www.instructables.com/id/Easy-How-to-charge-a-cellphone-battery-from-other-/
 
A 6V NiMH battery is a 5 cell battery pack. At peak charge it can be close to 7.5V.

Most cell phones have a 5V charger, and you need to be able to supply the current supplied by the charger.

You need to calculate the maximum power dissipation in the regulator and use a proper heatsink.

http://www.jameco.com/Jameco/Product...DS/1731278.pdf is the wrong voltage, but has a nice section (page 10 and 11) on thermal considerations.

Looking at a 7805 spec, if your phone draws 500ma, then the regulator will dissipate 1.25 watts. (2.5V X 0.5A). A TO-220 package with no heatsink would heat to 65 x 1.25 = 81.25 degrees C above ambient. (which means you should probably use a small heatsink)

You should read some application notes.
 
Last edited by a moderator:
IC-7805-as-current-source.jpg

Will this help me to produce constant current from LM7805 !
also tell me what is input should be provided for this circuit !
 
Why do you want a constant current?

You edited your original post after people responded.

I no longer have any idea what you are trying to do.
 
I edited his original post to get rid of fancy formatting - wording has been not changed.
 
ahh --- the information I thought disappeared was in the title.

Sorry, When I say "supply the current" I don't mean you need to build a current source.

When I said "supply the current", I meant you need to create a 5V Voltage source that is capable of supplying the required current without overheating. Carefully analyze my 7805 example and read the Thermal section on the other data sheet I linked to.
 
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Meera said:
Will this help me to produce constant current from LM7805 !
No you don't want constant current, you just need a somewhat stable voltage around 5 volts. That is, you need to go from the somewhat stable voltage of around 6 to 7 volts that your pack provides to a somewhat stable voltage around 5 volts that the phone's internal charging circuit requires.

A simple linear regulator will probably give the best results, but if you want something really quick and nasty then two series silicon diodes would probably get the job done (eg 1 or 2 amp general purpose diodes).

If you do go for a linear regulator then don't use the 7805, there may not be enough input to output voltage differential for it to work properly. Get a low dropout type like the LM2940 for example.
 
Regardless of what regulator you use (chemnoob is right, 7805 is marginal to bad, LDO is better) make sure you understand the power dissapation as I described.
 
  • #10
Why not just look at the spec for what's available from a USB power out? Phones can cope with that.
 
  • #11
Knowing the USB spec is going to help him, how? Read the title to the OP.
BTW, Do *you* understand USB charging? Not a simple topic.
 
  • #12
My phone does all the necessary clever stuff. Any USB will serve it.
 
  • #13
Provide your phone with 5V and a few 100mA and it will be more than happy to make the best of it.

@MeBigGuy The OP asks how to charge a mobile phone. I read that much to know he doesn't wasn't to get inside the phone battery - hence he just wants the simplest of voltage sources. The phone's charger is smarter than you think.
 
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  • #14
Phones can definitely deal with chargers that don't meet the USB specification; in USB there's actual communication that goes on where the slave device requests an amount of power from the master end. Many chargers don't care and are happy to cram 2A into the phone(way more than USB allows), with the phone handling it just fine. IPhones seem to come with a 2A charger standard, but most Android phone chargers seem to be the 500mA type, but the phones themselves take 2A no problem(might degrade the battery slightly, but I haven't had any issues charging that way). TL;DR: The USB specification isn't very relevant here, and supplying more than 500mA shouldn't be an issue at all.
 
  • #15
I see, somehow knowing the USB spec will help him charge a phone from a 5 cell NiMH battery (read the title).

Actually, all he needs to know is in the title (again, read the title). Build a 5V 500ma capable regulator.

BTW, the USB will only provide 100ma (at as low as 4.4V) unless you enumerate and ask for 500ma. Phones are not "smarter than I think" in that I build (among other things) LI-ion battery chargers, USB interfaces and chargers, and USB Audio devices for a living.
 
  • #16
meBigGuy said:
BTW, the USB will only provide 100ma (at as low as 4.4V) unless you enumerate and ask for 500ma. Phones are not "smarter than I think" in that I build (among other things) LI-ion battery chargers, USB interfaces and chargers, and USB Audio devices for a living.

Sounds like you know the specification off the top of your head. I know that the device requesting the power has to request it from the source through the data connection, but is there anything special that the receiving device needs to know about how much power it's receiving? I've seen chargers that don't appear to even implement USB signalling and the phone couldn't care less.

Or put more simply, is there any reason that he would need to worry about the data component, or will any 5V source capable of providing the requisite current do?
 
  • #17
From the phone's viewpoint (most phones), it just wants 5V 500ma (or whatever it actually uses). "Legal" USB compatible wall chargers short the USB D+ D- pins to indicate they are a charger. Apple iphone/ipad/etc chargers use a proprietary signalling method. Some phones might care (I know some USB products care and will not charge if The USB is not active or shorted), most do not. I don't know what *all* the phones actually do. Some vendors might only work with their charger. For this thread I'm assuming that simply supplying 5V with 500ma capability will work. If it doesn't, then I'd short DP and DM. If that doesn't work then I'd research the specific phone.

As far as the USB port is concerned, all will supply at least 4.4V at 100ma by default for a while. If you try to draw more, it might shut down (some hubs care, some don't). If you enumerate and request 500ma, you can draw 500ma. If you do not enumerate at all, it might also shut down (probably will, eventually).
 
  • #18
meBigGuy said:
Knowing the USB spec is going to help him, how? Read the title to the OP.
BTW, Do *you* understand USB charging? Not a simple topic.

You seem to be suggesting that the cheap and cheerful car, solar and mains chargers are more complicated than 5V voltage regulators. Are they? I cannot imagine phone manufacturers risking a charger input that couldn't deal with the very crudest of inputs.
What do I need to know about USB charging, in this context (and what, exactly does that phrase mean, aamof?)?

You clearly know about charging Lion cells but is that necessarily relevant if you are not planning to go inside the phone to modify its internal charging circuit? You keep referring us to the title and the title is about supplying a phone not battery charging.
 
  • #19
sophiecentaur said:
Why not just look at the spec for what's available from a USB power out? Phones can cope with that.

That comment has no relevancy given that the OP wanted to know how to charge a cell phone from a 6V 1300mAh battery. The answer was: use a regulator to create 5V. Be carefull about voltage drop and power dissipation. Then you chimed in with the above. How does knowing anything about USB going to help him with that?

I'm done. You are much too difficult to communicate with.
 
  • #20
meBigGuy said:
That comment has no relevancy given that the OP wanted to know how to charge a cell phone from a 6V 1300mAh battery. The answer was: use a regulator to create 5V. Be carefull about voltage drop and power dissipation. Then you chimed in with the above. How does knowing anything about USB going to help him with that?

I'm done. You are much too difficult to communicate with.

It's a shame you started to interpret my comments as some sort of criticism of you. You get abrasive rather quickly.
My comments about USB were intended to put the difficulty of the problem in some context. A good reason for the OP to believe the opinions he read on PF is to connect them with the fact that USB works fine as a power source. Is that being contentious?
You should try viewing what you read here a bit more generously. It will make you a happier person.
 
  • #21
I don't feel in any way criticized (at least not technically and ignoring your last very blatent criticism). You are just wrong, but will never give up. You do that a lot.

USB does not work fine as a power source unless you carefully obey its rules. You are confusing USB itself and chargers that happen to have a USB connector and may or may not be USB compliant. Two very different things.

Want to learn about battery operated devices and USB? http://www.usb.org/developers/docs/devclass_docs/BCv1.2_070312.zip will keep you busy for a day or two. Check out the state diagram on page 29.

Before you respond, please re-read your sig a few times.
 
  • #22
I don't need to 'understand' USB to know that millions of people use very basic (pretty dumb) chargers for their phones, that have a USB connecting leads for them. They seem to work. That is all I'm getting at and I don't remember writing anything in detail about the functions of a phone's charging circuit.
How can I be wrong about that? That was really the only point I've been making.

You are making this a battle about being 'wrong or right'. I haven't claimed to be right about anything other than that this is a simple problem to solve and referring to the generic 'USB' charger. I think you may have mis interpreted me this time. And, yes, I do like a good argument but I seldom get ratty about it.
 
  • #23
sophiecentaur said:
Why not just look at the spec for what's available from a USB power out? Phones can cope with that.

Again --- There is no relationship between what is available from a USB power out and what is available from chargers with USB connectors. Period. End of discussion.
 
  • #24
It sounds to me like you two agreed on the topic of the thread and are now off on a tangent. If I'm following the conversation, then: "Generic USB Charger" == 5V between power and ground pins, maybe the two data pins are shorted, and it doesn't catch fire or otherwise fail when you use it. If we were trying to draw power from a computer's USB port that would be a *very* different story, but that isn't relevant here.

@sophie: There is a difference between having a USB plug and using USB signaling for data. There are numerous examples of the data part of USB being done over different connectors(controller ports for a couple game consoles come to mind), and of the power and connector being used without the data component(standard micro-USB phone charger). Basically, the issue isn't understanding or not understanding USB, it's understanding that this isn't really USB beyond them sharing a connector.
 
  • #25
FireStorm000 said:
@sophie: There is a difference between having a USB plug and using USB signaling for data. There are numerous examples of the data part of USB being done over different connectors(controller ports for a couple game consoles come to mind), and of the power and connector being used without the data component(standard micro-USB phone charger). Basically, the issue isn't understanding or not understanding USB, it's understanding that this isn't really USB beyond them sharing a connector.

I made a simple remark, way back, about the commonality between a 'USB Phone Charger' and the OP's requirement. I can't think why it could have started all this stuff about the signalling details of the USB. It's obvious that all that's needed here is a 5V regulator. I only made my initial post because it looked like people were starting to talk about battery charging circuits and all phones have one of their own. Why introduce the USB, idea? Because everyone has one and it brings the problem down to being very straightforward and not a scary project.
 
  • #26
sophiecentaur said:
I made a simple remark, way back, about the commonality between a 'USB Phone Charger' and the OP's requirement. I can't think why it could have started all this stuff about the signalling details of the USB.
I'd have to re-read everything, but I think it's because you initially defended that statement and never clearly conceded the point. I think this comic nails it. IMO the USB talk was a bit of a derail, but regardless of that, I don't think there was every anyone saying a 5V regulator wouldn't work, only that a heatsink might be necessary depending on the details. Of course, there's no substitute for actually building it and trying it on an old phone... curious to hear how this goes.
 
  • #27
FireStorm000 said:
I'd have to re-read everything, but I think it's because you initially defended that statement and never clearly conceded the point. I think this comic nails it. IMO the USB talk was a bit of a derail, but regardless of that, I don't think there was every anyone saying a 5V regulator wouldn't work, only that a heatsink might be necessary depending on the details. Of course, there's no substitute for actually building it and trying it on an old phone... curious to hear how this goes.

Absolutely. :smile:
 

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