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'[EE] State of Charge Indicator for NiMH and Li-Ion'
2010\11\11@195434 by

Hi all,

I'm looking to build a simple battery-powered charger for my mobile
phone. I'm out a lot where it's not convienient to plug my phone into
a wall outlet.

The charger will simply consist of a power supply that provides 5
volts via a USB socket. I want to power it with 4 AA NiMH batteries
(Sanyo Eneloops). 1.2 * 4 = 4.8, so I will need to use a boost
converter of some sort to output 5 volts.

1. I know that NiMH batteries typically supply 1.2 volts, but the open
circuit voltage is much higher. If I have 4 of them in series, would a
boost converter suffice? Or would I need to use a boost/buck
converter? I am worried about the open circuit voltage surpassing 5
volts. How will that affect the boost converter?

2. I would like to have a simpe state of charge monitor for the
battery pack. It doesn't have to tell me the exact state of charge,
but something approximate. For example, 10%, 20%, 30%... I know there
are several ways of monitoring state of charge, but what method would
you guys recommend for this purpose? The voltage vs SOC for NiMH curve
is quite shallow except for the extreme ends.

Update: Added ta
I tend to go for simple. Use 6 AA's and a linear 5v reg. Check the
dropout and use a low dropout version if necessary. IIRC, 1v is a good
point to stop with NimH (I'm sure someone will correct me ;) . Holders
are easy to find for 4 and 6 cells. The cost of a couple extra cells vs
the parts count and time to do a converter... To evaluate, see if the AA
capacity minus losses are still greater than the charge needs of the
phone. Chances are, with either design, you'll get one full charge. If
you calc the losses are so great that one design gives 2 full charges,
and the other only one, and that's important (not able to charge the
pack between uses), don't go simple... Just remember that using a boost
converter will draw more current from the AA's than the output
(Pin=Pout) plus consider its efficiency % in your calcs.

Maybe consider as well that NimH like full discharge cycles, at least
once in a while, and consider, if it's close, how much room you have for
capacity degradation over time. Eneloops are supposed to be good though.
Those are considerations in "system design".

For the SOC piece, it's been a long time for me, but I might play with a
series resistor, zener, and led with a few different combinations to
light up at different levels. Pushbutton to turn it on and see (like
some laptop packs). There's a chip that does a bar graph too (number
escapes me). I think National makes it.

On Thu, 2010-11-11 at 19:54 -0500, V G wrote:
{Quote hidden}

> Update: Added tag
V G wrote:
> I'm looking to build a simple battery-powered charger for my mobile
> phone. I'm out a lot where it's not convienient to plug my phone into
> a wall outlet.
>
> The charger will simply consist of a power supply that provides 5
> volts via a USB socket. I want to power it with 4 AA NiMH batteries
> (Sanyo Eneloops). 1.2 * 4 = 4.8, so I will need to use a boost
> converter of some sort to output 5 volts.

Not necessarily.  USB voltage at the device can be from around 4.3 to 5.5
volts worst case if I remember right.  Probably the phone contains a
switcher to charge its battery and you don't need to be right on 5 volts in..
Give it a try with a variable power supply to test it.

********************************************************************
Embed Inc, Littleton Massachusetts, http://www.embedinc.com/products
(978) 742-9014.  Gold level PIC consultants since 2000
On Fri, Nov 12, 2010 at 7:53 AM, Olin Lathrop <olin_piclistembedinc.com> wrote:
> Not necessarily.  USB voltage at the device can be from around 4.3 to 5..5
> volts worst case if I remember right.  Probably the phone contains a
> switcher to charge its battery and you don't need to be right on 5 volts in.
> Give it a try with a variable power supply to test it.

Yes, I'm going to experiment with voltage on my \$700 phone.
V G wrote:
>> Not necessarily. USB voltage at the device can be from around 4.3 to
>> 5.5 volts worst case if I remember right. Probably the phone
>> contains a
>> switcher to charge its battery and you don't need to be right on 5
>> volts in. Give it a try with a variable power supply to test it.
>
> Yes, I'm going to experiment with voltage on my \$700 phone.

1: \$700!?  Is it gold plated with diamond studded keys?

2: I was trying to help and could provide more suggestions and explanation
and clear up a obvious misconception you have, but I've got better things to
do with my free time than to help people who just get snippy in return.

********************************************************************
Embed Inc, Littleton Massachusetts, http://www.embedinc.com/products
(978) 742-9014.  Gold level PIC consultants since 2000
> > Yes, I'm going to experiment with voltage on my \$700 phone.
>
> 1: \$700!?  Is it gold plated with diamond studded keys?

That was sort of my reaction as well, especially as my understanding is
that this person asking the original question (and replied with that
quote that Olin answered) is our friend solarwind, who is at university
in Canada. Have I got my connections wrong ???
-- Scanned by iCritical.
I believe that the phones have a high street value as stand alone units but they are obtained for much less if purchased as part of a "plan."

Sort of like Satellite TV......the money is made on the plan, not the phone..

Google for an "unlocked" phone and you will see what I mean.

Although I do not believe I have seen one for \$700.

Mark Skeels
Engineer
Competition Electronics, Inc.
TEL: 815-874-8001
FAX: 815-874-8181
http://www.competitionelectronics.com

On 11/12/2010 9:04 AM, alan.b.pearcestfc.ac.uk wrote:
>>> Yes, I'm going to experiment with voltage on my \$700 phone.
>> 1: \$700!?  Is it gold plated with diamond studded keys?
> That was sort of my reaction as well, especially as my understanding is
> that this person asking the original question (and replied with that
> quote that Olin answered) is our friend solarwind, who is at university
> in Canada. Have I got my connections wrong ??
On 12/11/2010 13:20, V G wrote:
> On Fri, Nov 12, 2010 at 7:53 AM, Olin Lathrop<olin_piclistembedinc.com>  wrote:
>> Not necessarily.  USB voltage at the device can be from around 4.3 to 5.5
>> volts worst case if I remember right.  Probably the phone contains a
>> switcher to charge its battery and you don't need to be right on 5 volts in.
>> Give it a try with a variable power supply to test it.
> Yes, I'm going to experiment with voltage on my \$700 phone.
>

I understand the concern, but USB voltage can vary a lot - at least from around 4.4 to 5.25 (and probably to the figures Olin gives above in extreme cases) and a \$700 phone almost certainly has a dedicated chip to handle the charging, which may handle voltages of quite a margin from nominal. Any USB chargable phone has to handle these variations. If this is only a one off then it would be worth finding out before proceeding (phone manual?), so you know what specs you are designing towards.  It may work fine with 4 AAs (with some basic protection), personally I would be temped at least to see how low the voltage can drop before charging cuts out...
In my experience mobiles are quite robust - I have messed about quite a bit over the years with "non standard" charging solutions (when chargers break etc) for different phones and never managed to break one yet.
One small thing you may or may not be aware of - some phones (my Omnia for instance) need to actually talk (Activesync or whatever) to the PC before commencing charging over USB, for instance if I use a standard USB cable into a portable charger it won't work. With mine I have to use a "special" (I think 5 pin on the phone end or something to tell the phone it's in charging mode) cable to charge without doing this, so it's worth checking how yours works in this respect.

On Fri, Nov 12, 2010 at 8:20 AM, V G <x.solarwind.xgmail.com> wrote:
> On Fri, Nov 12, 2010 at 7:53 AM, Olin Lathrop <olin_piclistembedinc.com> wrote:
>> Not necessarily.  USB voltage at the device can be from around 4.3 to 5.5
>> volts worst case if I remember right.  Probably the phone contains a
>> switcher to charge its battery and you don't need to be right on 5 volts in.
>> Give it a try with a variable power supply to test it.
>
> Yes, I'm going to experiment with voltage on my \$700 phone.

If Olin is correct, which he usually is on these things, then every
time you plug your phone into a USB port to charge, you are
"experimenting" with it.

As an aside, I bought a cheap cigarette lighter inverter rated for 90W
120V, but it also had a USB port on it. Turns out the 5V on the USB
plug was regulated by a 7805 with a tiny heatsink. After 5 minutes of
charging my phone I smelled burning. Nothing was damaged, but the poor
regulator was probably dropping 7V at 500mA-1A, while it was probably
only rated for 2W. Stupid designs involving USB are everywhere, and
I'm sure the phone manufacturers know this and build in some
protections so they aren't getting phones returned under warranty.
On 12/11/2010 15:10, Mark E. Skeels wrote:
> I believe that the phones have a high street value as stand alone units
> but they are obtained for much less if purchased as part of a "plan."
>
> Sort of like Satellite TV......the money is made on the plan, not the phone.
>
> Google for an "unlocked" phone and you will see what I mean.
>
> Although I do not believe I have seen one for \$700.

Yes, I was going to say this - as part of a contract you can get very expensive phones depending on what you spend. I have spent a lot on contract in the past so have had a few phones in this price range - it's usually only very briefly when the best ones first come out, within a couple of months they will be half that.
You wouldn't buy them at this price (it would make no sense if you can just wait a few weeks) but on a contract the "high street value" doesn't really matter to the phone company, as long they lock you in to a new contract for however long they are happy.
On 12/11/2010 15:59, Adam Field wrote:
> On Fri, Nov 12, 2010 at 8:20 AM, V G<x.solarwind.xgmail.com>  wrote:
>> On Fri, Nov 12, 2010 at 7:53 AM, Olin Lathrop<olin_piclistembedinc.com>  wrote:
>>> Not necessarily.  USB voltage at the device can be from around 4.3 to 5..5
>>> volts worst case if I remember right.  Probably the phone contains a
>>> switcher to charge its battery and you don't need to be right on 5 volts in.
>>> Give it a try with a variable power supply to test it.
>> Yes, I'm going to experiment with voltage on my \$700 phone.
> If Olin is correct, which he usually is on these things, then every
> time you plug your phone into a USB port to charge, you are
> "experimenting" with it.
>
> As an aside, I bought a cheap cigarette lighter inverter rated for 90W
> 120V, but it also had a USB port on it. Turns out the 5V on the USB
> plug was regulated by a 7805 with a tiny heatsink. After 5 minutes of
> charging my phone I smelled burning. Nothing was damaged, but the poor
> regulator was probably dropping 7V at 500mA-1A, while it was probably
> only rated for 2W. Stupid designs involving USB are everywhere, and
> I'm sure the phone manufacturers know this and build in some
> protections so they aren't getting phones returned under warranty.
>
Partly the "real" USB spec, not Apple's made up one. says a Max of 500mA. not 1000mA. There are many Apple devices that take more than a standard USB port is supposed to supply. The earliest  USB spec is 100mA

but 14.2 - 5 = 9.2V Thus 4.6W when engine running and full 500mA. You need a serious heatsink for 4.6W on even a  78M05

On Fri, Nov 12, 2010 at 9:31 AM, Olin Lathrop <olin_piclistembedinc.com> wrote:
> 1: \$700!?  Is it gold plated with diamond studded keys?
>
> 2: I was trying to help and could provide more suggestions and explanation
> and clear up a obvious misconception you have, but I've got better things to
> do with my free time than to help people who just get snippy in return.

Seriously? It's an iPhone 4. The street price (off contract) is around \$700..
V G <x.solarwind.xgmail.com> wrote:
> Olin Lathrop <olin_piclistembedinc.com> wrote:
> > 1: \$700!? Is it gold plated with diamond studded keys?
> >
> > 2: I was trying to help and could provide more suggestions and explanation
> > and clear up a obvious misconception you have, but I've got better things
> > to do with my free time than to help people who just get snippy in return.
>
> Seriously? It's an iPhone 4. The street price (off contract) is around \$700.

Hey, *you're* the one who proposed plugging the phone into a *homemade*
power supply ... and you think that Olin's suggestion to plug it into a
bench supply first is riskier??? Get real already.

-- Dave Twee
At 05:54 PM 11/11/2010, V G wrote:
>Hi all,
>
>I'm looking to build a simple battery-powered charger for my mobile
>phone. I'm out a lot where it's not convienient to plug my phone into
>a wall outlet.
>
>The charger will simply consist of a power supply that provides 5
>volts via a USB socket. I want to power it with 4 AA NiMH batteries
>(Sanyo Eneloops). 1.2 * 4 = 4.8, so I will need to use a boost
>converter of some sort to output 5 volts.

My experience has been that you don't need any electronics at all.  Inexpensive battery-powered USB chargers that I have purchased confirm this.

Both NiCd and NiMh batteries have a very flat discharge slope.  The terminal voltage ranges from about 1.25V (fully charged) to about 1.1V (less than 10% charge remaining).  That means that a 4-cell pack will range between 4.4V through 5V.

A standard USB port will supply somewhere between 4V to 5V under load.

Take note that you should NOT ever use alkaline batteries in your box.  One of the battery-powered USB chargers that I purchased took care of that possibility in a typical Far East fashion: they had 3 large (5W?) 5.1V zener diodes connected in parallel right across the battery pack.  The obvious intent was to force alkaline batteries to discharge quickly so that the terminal voltage was less than the zener voltage.

Bottom line: a small battery pack with 4- high-capacity NiMh batteries should be all that you need.

dwayne

-- Dwayne Reid   <dwaynerplanet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
(780) 489-3199 voice          (780) 487-6397 fax
http://www.trinity-electronics.com
Custom Electronics Design and Manufacturing
On Fri, Nov 12, 2010 at 12:06 PM, Dave Tweed <picdtweed.com> wrote:
> Hey, *you're* the one who proposed plugging the phone into a *homemade*
> power supply ... and you think that Olin's suggestion to plug it into a
> bench supply first is riskier??? Get real already.

Real? The reality is, adjusting knobs recklessly on a power supply can
lead to damage. However, a home-made "power supply" using industry
standard components providing a very stable and acceptable 5 volt
output shouldn't pose many risks. The external component count is low,
and obviously one would verify the output with a voltmeter or
something
On Fri, Nov 12, 2010 at 12:40 PM, Dwayne Reid <dwaynerplanet.eon.net> wrote:
{Quote hidden}

That's a VERY sensible solution. Thank you!
On Thu, Nov 11, 2010 at 4:54 PM, V G <x.solarwind.xgmail.com> wrote:
>
> I'm looking to build a simple battery-powered charger for my mobile
> phone. I'm out a lot where it's not convienient to plug my phone into
> a wall outlet.
>
> The charger will simply consist of a power supply that provides 5
> volts via a USB socket.

The Mintyboost supports the iPhone 4:
http://www.ladyada.net/make/mintyboost/

Your homebrew device may not enable charging your phone:
http://www.ladyada.net/make/mintyboost/icharge.htm
On Fri, Nov 12, 2010 at 2:42 PM, Alex Harford <harfordgmail.com> wrote:
> The Mintyboost supports the iPhone 4:
> http://www.ladyada.net/make/mintyboost/
>
> Your homebrew device may not enable charging your phone:
> http://www.ladyada.net/make/mintyboost/icharge.html

Thanks! I want to be able to charge it at 1 Amp. I'll still make a
converter for it, but now I'm aware of the required resistors
V G wrote:
> On Fri, Nov 12, 2010 at 12:40 PM, Dwayne Reid wrote:
>> My experience has been that you don't need any electronics at
>> all.
>>
>> ...
>>
>> Bottom line: a small battery pack with 4- high-capacity NiMh
>> batteries should be all that you need.
>
> That's a VERY sensible solution. Thank you!

Huh.  I wonder what changed from this morning when I made the same
suggestion but it was met by a snotty response instead.

********************************************************************
Embed Inc, Littleton Massachusetts, http://www.embedinc.com/products
(978) 742-9014.  Gold level PIC consultants since 2000
On 12/11/2010 20:23, Olin Lathrop wrote:
> V G wrote:
>> On Fri, Nov 12, 2010 at 12:40 PM, Dwayne Reid wrote:
>>> My experience has been that you don't need any electronics at
>>> all.
>>>
>>> ...
>>>
>>> Bottom line: a small battery pack with 4- high-capacity NiMh
>>> batteries should be all that you need.
>> That's a VERY sensible solution. Thank you!
> Huh.  I wonder what changed from this morning when I made the same
> suggestion but it was met by a snotty response instead.
>

Sometimes it takes a while for the penny to drop.

Sometimes it's the way you say it ;(

Personally I thing the charging solution is to have a Nokia that needs plugged in less than 1hr a week
> Yes, I'm going to experiment with voltage on my \$700 phone.

Indeed you are, every time you plug it into a computer.

--
Wouter van Ooijen

-- -------------------------------------------
Van Ooijen Technische Informatica: http://www.voti.nl
consultancy, development, PICmicro products
docent Hogeschool van Utrecht: http://www.voti.nl/hvu
> Partly the "real" USB spec, not Apple's made up one. says a Max of
> 500mA. not 1000mA.

IIRC it states that 500mA is the maximum *a device can count on* to get. It is **not** a maximum that the host is guaranteed not to exceed! IIRC the text is something like "the host shall limit the current to  safe maximum".

--
Wouter van Ooijen

-- -------------------------------------------
Van Ooijen Technische Informatica: http://www.voti.nl
consultancy, development, PICmicro products
docent Hogeschool van Utrecht: http://www.voti.nl/hvu
On Fri, 2010-11-12 at 21:52 +0100, Wouter van Ooijen wrote:
> > Partly the "real" USB spec, not Apple's made up one. says a Max of
> > 500mA. not 1000mA.
>
> IIRC it states that 500mA is the maximum *a device can count on* to get.
> It is **not** a maximum that the host is guaranteed not to exceed! IIRC
> the text is something like "the host shall limit the current to  safe
> maximum".

Very true, it's a minimum spec, a port is "guaranteed" to be able to
supply up to 500mA, given everything else is right.

In real life, many ports ARE current limited, cheapers ones usually
around 1A (I've seen polyfuses used most frequently). Some will only
supply 500mA, some have NO limit (the cheapest cards I've seen have
polyfuse footprints, but a solder bead across the contacts), and will
supply whatever the power supply in the host is capable of supplying.

TTYL
> adjusting knobs recklessly on a power supply can lead to damage

Well then, reck

Google for buck-boost 5V. Just everybody does switchers these days

*
*
**********
Quality PIC programmers
http://www.embedinc.com/products/index.ht
On Fri, Nov 12, 2010 at 3:48 PM, Wouter van Ooijen <woutervoti.nl> wrote:
> Indeed you are, every time you plug it into a computer.

The difference is. Multi-million dollar companies thoroughly test
their motherboards before selling them. Especially huge ones like
ASUS, like the one I have. It's not really a gamble to plug it into a
USB port on such a motherboard...

On the other hand, sticking it into a bench supply and turning knobs?
I'm not saying the bench supply is unsafe. But see how the message is
worded:

On Fri, Nov 12, 2010 at 7:53 AM, Olin Lathrop <olin_piclistembedinc.com> wrote:
> NOT NECESSARILY.  USB voltage at the device CAN BE from around 4.3 to 5.5
> volts worst case if I remember right.  PROBABLY the phone contains a
> switcher to charge its battery and you don't need to be right on 5 volts in.
> GIVE IT A TRY with a variable power supply to TEST it.

There is nothing in the suggestion that is certain. I don't want to
PROBABLY GIVE anything A TRY to TEST it on my \$700 phone. That's all
I'm saying. There's got to be a better way
V G wrote:
>> NOT NECESSARILY.  USB voltage at the device CAN BE from around 4.3
>> to 5.5 volts worst case if I remember right.  PROBABLY the phone
>> contains a
>> switcher to charge its battery and you don't need to be right on 5
>> volts in. GIVE IT A TRY with a variable power supply to TEST it.
>
> There is nothing in the suggestion that is certain.

I don't have your phone, nor is it my job to do the research.  The point is
that valid USB power voltage already varies over a range.  The "can be"
expresses the range, not the uncertainty that it might vary.  It does vary.
The details are in the USB spec.  I mentioned the numbers I recall off the
top of my head.  If you want to know with certainty what the legal range of
USB power voltage is, look in the spec yourself.  Again, I'm not going to do
your work for you.  I did tell you that 1: Valid USB power voltage can vary,
2: there is a specified valid range, and 3: a rough idea what that range
might be.  If you want to know more, look it up.  It's your project.

Back to the main point.  Since the USB power voltage can vary over a
considerable range even when everything is operating to spec, your phone
must have been designed with this in mind.  At worst it might not function
correctly over part of that range.  It's not going to get hurt by a voltage
in that range because the manufacturer isn't going to design in regular
field failures for this kind of item and price.  One field failure costs
them the profit on many units.  It's also quite unlikely to get hurt by
lower voltages.  The manufacturer would be leaving themselves open to all
kinds of failures if they did that.  At the very least, the phone has to
tolerate low voltages for a little while, since the power voltage can't
instantly go from 0 to in spec.  *Think* about it.

NiMH batteries like you said you were planning to use will put out about
1.2V when fully charged.  1.2 volts times 4 cells = 5.0V.  That's the
*nominal* USB power voltage, so will clearly not damage your phone.  Of
course the batteries will sometimes put out less, which will also not damage
your phone.  NiMH cells are pretty much empty at 900mV, which is 3.6V for
four.  The point of the bench supply test is to see at what voltage your
phone stops charging.  None of this is going to hurt your phone, but it will
tell you whether the phone will take power from the batteries at a low
enough voltage so that you get enough effective capacity from them for the
whole exercise to be worthwhile.  The NiMH voltage discharge curve is pretty
flat until it gets near the end, so it will probably work well enough.
Again, look it up if you want details.

> I don't want to
> PROBABLY GIVE anything A TRY to TEST it on my \$700 phone.

As apposed to what, not testing first and just plugging in your homebrew
power supply?  The point of the test was to characterise your phone so you
know better whether or how well the 4 NiMH batteries in series will do the
job, and what current the batteries will have to supply.  If you don't test
first and the result is "phone doesn't charge" or "phone only charges a bit"
then you can only guess why.

If you want certainty up front before doing anything, forget about
engineering and go find a religion.

> There's got to be a better way.

OK, just plug it in then.  Or buy off the shelf products designed to do what
you want.

********************************************************************
Embed Inc, Littleton Massachusetts, http://www.embedinc.com/products
(978) 742-9014.  Gold level PIC consultants since 2000

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