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USB plug electrical question


Zero Knievel

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https://www.amazon.com/gp/product/B01MSK7ZAV/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&psc=1

I got this to replace a similar product that worked fine until it got so hot it actually melted a nylon bag with which it made contact.  I’m not sure why my prior unit did that.  I had installed a power distribution block and set that circuit for 10A.  I don’t know if there is a max amperage one should send to this type of unit.  Up to then, I used a pigtail connection to the battery fused at 7.5 amps.  In the rewire, I used an SAE extension cable made in China, so I wonder if the red and black wires were reversed...causing me to reverse the polarity in the wiring by attaching the wrong wire to positive.  The old unit didn’t work after the overheat it didn’t work and the wires were shot as well.

 

I got this one from Amazon, and as a safety, I have the power supply fused at 7.5 amps.  It does get warm, and that has me concerned as during a trip it would be in a closed compartment charging a device.  I don’t recall the last unit being warm when plugged in, and if this is normal, how do I ensure it’s not too hot?  My best guess would be to fuse the device at the minimum input amperage needed to operate.  The specs say what the output is, but it doesn’t say what the input amperage is for a 12 volt source.
 

Can you offer any insight?

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I was looking at something like that this morning. Then I remembered I had this: expensive but worth it. It should plug right in to your bmw. 54D65EA3-8BFD-4D0E-8D3D-AEABAD258295.thumb.png.967cd4428c1a1814606b80a0f5b6e1f3.png

 

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4 hours ago, Zero Knievel said:

The specs say what the output is, but it doesn’t say what the input amperage is for a 12 volt source.

 

Output amps * 5 volts = watts out

Watts out / 12v = watts in

 

A QC 3.0 output means 3a*5v=15 watts.

 

15 watts/12v = 1.25a in

 

Obviously there are some losses to overhead (the heat that's being created), but a 2a or 3a should be perfect.

 

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30 minutes ago, SwampNut said:

Output amps * 5 volts = watts out

Watts out / 12v = watts in

 

A QC 3.0 output means 3a*5v=15 watts.

 

15 watts/12v = 1.25a in

 

Obviously there are some losses to overhead (the heat that's being created), but a 2a or 3a should be perfect.


Thanks.  I think a fuse limiting the power supply would be the first safeguard against it overheating.

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Well, a fuse prevents an over-current situation, and electrical items respond to over-current by over-heating.  If the thing is just sitting there idling hot, you can't really solve that.  But absolutely a properly-rated fuse is important.

 

The VHF/UHF in the Jeep blew the main interior harness fuse when a tree bent and grounded the antenna and I tried to talk.  It's a 10a fuse.  The radio itself had a 7.5a Chinesium fuse, which didn't blow.

 

Buy good fuses.

 

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3 hours ago, SwampNut said:

 

 

Buy good fuses.

 

And make sure that they are DC, not AC fuses. There is a difference.

 

Is there any way to reposition the charger so that it gets airflow? And is not in a flammable bag?

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I never had an issue in the past.  I’m sure converting 12v to 5v generates some heat, but the “fire” incident was out of the blue.  Whether it just went bad and started overheating (it was hot to the touch) or it was damaged based on flawed wiring, I don’t know.  It’s a sealed unit, so it’s no cooler based on placement.  I’m just paranoid now that it happened.  I don’t know how warm these things normally get.

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I think I’ll rig something off bike where I leave it plugged in for a day and see what happens.  At least then if it gets hot, it won’t damage anything.  If it goes a day without anything beyond getting a little warm, I’ll know I’m good.

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5 hours ago, SwampNut said:

 

No, since you won't be duplicating the varying voltage and loads of riding.

 


True, but if the unit is defective, odds are it will manifest just having power.

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Thought I’d update.

 

I did a test off a car battery and used it to charge an iPad while it was in use (playing a video with brightness set to max).  The USB plug got hot to the touch but not as to burn.  Using a candy thermometer (all I could find in the house), I estimate the surface temperature was 125 F.  It never got that warm when charging the phone, but I suspected higher loads would generate more heat.  I’ve not reproduced this by just having it hooked to power.

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27 minutes ago, SwampNut said:

That's a good test, I think you're very safe.  Increasing voltage REDUCES current, so meaning that a battery with a running engine should result in lower temps.

 

Not that I don't believe you but :blink:

 

So the last one caught fire possibly because of low voltage?

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Sorry I did not mean to imply that, but it's possible too.  What I meant is that higher voltage normally only makes it better.  So your no-charging, battery-only test *should* be the harder case.  Does it make more sense now?

 

Watts are volts times amps.

 

Amps create heat.

 

Electronics normally increase amps in order to keep watts and output the same.  So do motors, so they overheat in under-volt conditions too.

 

For example, I have seen a server battery catch fire because it was set for 240v residential/small business input and connected to a 208v commercial power input.

 

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13 hours ago, Zero Knievel said:

 

Not that I don't believe you but :blink:

 

So the last one caught fire possibly because of low voltage?

He's not lying:

 

Probably a combination of low voltage and chinesesium. Some of the better chargers will cut out below a certain voltage. 

It's why a utility brown-out is far worse than a black-out. The same thing can happen with any electronics, or even worse anything with a motor: A/C units, or your refrigerator. 

 

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