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My point is Nexus 4 needs 2.2 Amp for charging properly however the unrated USB port on FM transmitter will not be providing that much current as it is meant for a pen drive. When I connect it to my phone it shows charging. Will using such a charger hurt my phone. Is it safe to use such a mechanism.

Considering the point that we can charge using the computers USB port, it must work but I read somewhere that if USB port if providing less than 2.1 AMP current, it can hurt your phone? is it correct??

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closed as off-topic by Tog, Mokubai, Dave M, Marcks Thomas, Shakehar Oct 11 '13 at 19:03

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3 Answers 3

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Your car battery would be more than capable of supplying 2.2A to the Nexus, so no problems there.

First of all it's not so cut and dried that the Nexus will draw 2.2A, whatever the power supply is rated for.

Then, most such devices can sense source voltage and adapt, so that if the power supply has a sane power curve, they will never draw so much as to damage anything (much less themselves). This doesn't hold for cheapo widgets, of course, but I'm pretty sure it will for a Nexus 4.

If the FM power supply isn't too well designed, though, there is a definite risk it might overheat. Where it will go from there, I couldn't say: just keep calm and stay hotter than normal, blow something and die on you, or (attempt to) set fire to the car. I'd place chances of everything working fine at around 90%. To be sure, though, I'd advise you to keep an eye on the charging process and finger the power supply from time to time (and/or check charging status/current draw) to ensure the charger won't overheat, especially at the beginning when the current draw will be at its maximum; risks are proportionally lesser during the final trickle charge phase.

I read somewhere that if USB port if providing less than 2.1 AMP current, it can hurt your phone? is it correct??

No, it is not, unless you're working with off-spec USB ports or phones. USB current is topped by standards at 0.9A. You might well damage your USB port or computer, if the charged device doesn't sense nor respect the port limits. By the same token, the battery might be damaged (its useful life shortened, the battery overheat or even explode) if the port doesn't supply a stable and constant current and the device doesn't sense this and stop charging. I don't think either is likely to ever happen with a Nexus (or any other modern and well-engineered device).

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You mean to say that the device connected will draw current as per its requirement, which implies the current remains constant. Does it mean that when I connect it to computer I still charge it with same 2.2?? Why does it charge it slowly then??? –  simar Oct 9 '13 at 6:35
    
No, the device ought to select the most "suitable" constant current depending on power supply characteristics. You might say that it negotiates the amperage. So it might decide that a given USB port can't reliably supply more than 230 mA, require no more, and charge slowly; or it might step up to 460 mA or 690 mA. With some power supplies, the controller might be led to believe suitable (and require) a current which the supplying unit can't really manage. In this case the port might overheat and be damaged, or even just blow. –  lserni Oct 9 '13 at 7:30
    
How should I find about what current my FM transmitter is supplying instead of just guessing by estimation of charging speed.. –  simar Oct 9 '13 at 7:58
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On the first page I linked they say there's a widget to do that, and that Nexus can dial the magic number *#*#4636#*#* . I don't have a Nexus to test it, though. –  lserni Oct 9 '13 at 8:59

It should be okay, but expect it to charge very slowly, if at all. It might even drain the battery, but more slowly than it would drain without being plugged in.

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Apart from charging slowly there shouldn't be any issues.

One observation I've had with a tablet (Nexus 7 2012) is if the current is not high enough you can get funny issues if you run completely out of battery, then try to start it on USB power. The screen would flicker during boot and eventually the device would just power-down.

No lasting harm, just wait until its charged a little more before powering it up.

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