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Advice please, iPhone 5 power
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DennisN
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PostPosted: Wed Nov 07, 2012 6:23 pm    Post subject: Advice please, iPhone 5 power Reply with quote

I need power to my iPhone 5 in the van because it won't run all day on battery. I cannot yet find a powered mount (like the TomTom Carkit and Magellan one).

So I bought a spare Lightning to USB cable and this crafty little thingy in B&Q, a ciggy lighter socket USB thing with two plugoles. One is marked 2.1A, the other 1A, which I assume are amps. I know my iPad needs the 2.1A hole, but what about the iPhone?

I've actually plugged iPhone into them both (ignorance is bliss!!). In the 1A hole, it charges so slowly it's pretty much like paint drying - it gets there eventually, but takes best part of the day to get back up from 25% to 100%.

In the 2.1A hole, it charges so fast you can almost see the percentages moving - maybe 1% a minute. And I think it also gets a bit warm.

So far, I've been using the 1A hole, but very occasionally the 2.1A hole. I somehow seem to remember Mike saying a USB device only takes as much power as it needs, but my memory could be sooooooo wrong.

The iPhone didn't blow up (yet). What can I do - will it be OK in the 2.1A plugole?


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M8TJT
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PostPosted: Wed Nov 07, 2012 7:11 pm    Post subject: Reply with quote

Provided that the voltage output from the thingy is right, then the device being charged will take as much as it has been designed to take and no more. You can increase charge rate by increasing the applied voltage, but as the USB standard is standard, then overvoltage is unlikey but possible.
If you plug something that normally charges at say 1.5A into a 2A supply, the device will only take 1.5A but if you plug it into a 1.0A supply, it will charge at the lower rate and thus take longer. (My 1A and 2A supplies assume correct voltage regulation)

Cor, you have got wrinkly hands Dennis
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DennisN
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PostPosted: Wed Nov 07, 2012 7:14 pm    Post subject: Reply with quote

'Cos I'm a bluddy wrinkly, innit?
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Darren
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PostPosted: Wed Nov 07, 2012 7:41 pm    Post subject: Reply with quote

Use the 2A USB port, it will do no home whatsoever. I'm using the TomTom dual USB charger and have mine connected to the 2A iPad port, all good, works fine.
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DennisN
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PostPosted: Wed Nov 07, 2012 8:19 pm    Post subject: Reply with quote

Ah, cheers, Darren. I needed the simple answer.
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M8TJT
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PostPosted: Wed Nov 07, 2012 8:37 pm    Post subject: Reply with quote

I must get a simple 2A charger for my phone. Very Happy
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Guivre46
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PostPosted: Wed Nov 07, 2012 11:26 pm    Post subject: Reply with quote

I looked at these power adaptors on Amazon and a lot of the feedback said they had experienced problems with overheating, so take care with it.
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aj2052
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PostPosted: Thu Nov 08, 2012 2:02 am    Post subject: Reply with quote

The problem with using the 1A uotput particularly if the unit is drawing more than the output is that the voltage will drop and the amps will increase, consequently the charger will overheat thus shortening the the life and probably never reaching full charge due to the voltage never balancing the item being charged, The basic electrical rule is that current is inversely proportional to the voltage.
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M8TJT
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PostPosted: Thu Nov 08, 2012 9:50 am    Post subject: Reply with quote

aj2052 wrote:
The basic electrical rule is that current is inversely proportional to the voltage.
No it's not. Current is directly proportional to voltage providing the resistance remains constant. The formula is I=V/R In the case of a device being charged, R is the apparent resistance of the device.
I=Current (in Amps). V=Voltage (in Volts) and R = resistance in Ohms.
However you are partly rigt, In a well designed power supply (charger), if you try to take too much current (excede its rated O/P) from it, the output voltage will fall (by design) so either the current will will remain at the maximum rated output or, on a better designed device, the output will 'fold back' and reduce the current also.
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aj2052
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PostPosted: Thu Nov 08, 2012 1:20 pm    Post subject: Reply with quote

M8TJT wrote:
Quote:
No it's not.


Oh Yes it is in the context i was using to the show the current draw of the unit being charged i.e using nominal watts which remains constant whatever the voltage:
Amp = Watts/Volts therefore 200w/100v = 2A
200w/5v = 40.4A
Therefore Current is Inversely Proprtional
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M8TJT
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PostPosted: Thu Nov 08, 2012 3:45 pm    Post subject: Reply with quote

Oh no its not, in any context that you like to demonstrate. Your argument is flawed because in your example, the resistance of the load must change. Nominal watts do not remain constant though. If you decrease the voltage applied, the current will also decrease and the power will reduce on a square law. Power is the product of current and voltage by definition. You cannot use your analogy because to generate 200W at 5V you must reduce the resistance of the load that you were using in the first example.

Anyway it does not matter how you try to explain it, Ohms law states that current is directly proportional to voltage if the resistance of the load remains constant. This is a proven fact that cannot be disputed (perhaps someone well versed in quantum physics or the like may be able to though) The only known time that this is not true is in a superconductor at about absolute zero that's resistance is almost zero Ohms and the current is infinite regardless of the voltage applied.
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aj2052
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PostPosted: Thu Nov 08, 2012 4:10 pm    Post subject: Reply with quote

At the end of the day, what should work in theory doesnt always work in practice, so probably the best advice is to stick within the ratings of the charger and the unit being charged.

from yet another tired old man
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M8TJT
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PostPosted: Thu Nov 08, 2012 4:22 pm    Post subject: Reply with quote

aj2052 wrote:
At the end of the day, what should work in theory doesnt always work in practice, so probably the best advice is to stick within the ratings of the charger and the unit being charged.
I 100% agree with that, for a change.
I just tried doing some stuff with my Ace2. Found that it charged from 37% to 100% in about 2 hours using my mains 700mA charger. Ran Sygic with GPS (and WiFi) and full brilliance for an hour off charge. Charge dropped to 76%. Put it back on charge, still running Sygic etc. and it recharged to 100% in less than 1.5 hours (I forgot to check it periodically). So as a 700mA charger can more than hold its own against stuff being on, I'm binning the idea of a 2A charger for the car (I've got a 1A one) unless some tests in the car indicate that the 1A charger is not providing enough to keep up.
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DennisN
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PostPosted: Thu Nov 08, 2012 5:17 pm    Post subject: Reply with quote

Well, I haven't the faintest idea what either of you is talking about - I'll have to review my degree in quantum physics and see if there's an explanation in there.

Ohms law? What worries me is Sod's law. All I know is we managed to install a 6 volt thing in a 12 volt car by putting a 6 volt light bulb in front of it.
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aj2052
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PostPosted: Thu Nov 08, 2012 5:27 pm    Post subject: Reply with quote

Im not sure that you can even rely on the manaufactures rating nowadays, it would seem that most major manufactures chargers are made in China now, and it would take weeks to check all my chargers hanging around the house and probably require a data logger to get true performance, I already have a Pico data logger lying in a drawer for over twenty years and still havent found time to set it up but probably technology has surpassed its serial and dos requirements by now, ah well.
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