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219 posts

Master Geek

Topic # 162082 28-Jan-2015 16:15

Hi,

I'm hoping there are some electronics gurus out there who can confirm that I am measuring this right.

I have wired up a very basic solar charger for camping purposes and was trying to work out if my USB charger is actually putting out a 2.1 amp charge as it is labeled as being able to do.

The simple setup is a 12 V 6.5 Ah SLA battery, a car cigarette lighter socket, a USB cigarette lighter charger with a 2.1A and a 1A output, and then a small solar panel to charge the battery.

When I connect my iPhone 5 to the 2.1A output and measure the current on the 12V side I get just under 500 mA.
If I connect to the 1A socket, I get the same 500 mA current on the 12V side.

12 x 0.5 = 6 Watts?  is that right?

Can this calculation then be used to work out the current of the 5V USB side?
6 watts / 5 volts = 1.2 amps?

I'm trying to work out if I am getting a 2.1A charge or not... looks to me like not.
There's a chance the USB charger is just a cheapie that doesn't actually do what it say's it does.

I would expect a 2.1A charger x 5V = 10.2W.  (e.g. a 12W iPad charger)
10.2W / 12V = 0.85A on the 12V side.

Is my math correct?

Cheers,
N

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## Dynamic

2456 posts

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The only way to really check is to get an ammeter which must go in one of the lines so the current flows through the meter.  A multimeter capable of measuring Amps will do the trick.

"4 wheels move the body.  2 wheels move the soul."

“Don't believe anything you read on the net. Except this. Well, including this, I suppose.” Douglas Adams

219 posts

Master Geek

Dynamic: The only way to really check is to get an ammeter which must go in one of the lines so the current flows through the meter.  A multimeter capable of measuring Amps will do the trick.

yep, so i have that, but can only measure current on the 12V side as I have no way (yet) of getting 'inline' on the 5V (USB) side.

219 posts

Master Geek

kiwitrc: Where are you? I can lend you one of these if you like http://www.aliexpress.com/item/BESTOPE-USB-Power-Meter-Voltage-Tester-Current-Monitor-Volt-Amp-Reader-Multimeter-LED-Screen-Portable-Durable/1723031707.html

cool gadget!!  I'm in Palmerston North

## kiwitrc

4123 posts

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kiwitrc: Where are you? I can lend you one of these if you like http://www.aliexpress.com/item/BESTOPE-USB-Power-Meter-Voltage-Tester-Current-Monitor-Volt-Amp-Reader-Multimeter-LED-Screen-Portable-Durable/1723031707.html

cool gadget!!  I'm in Palmerston North

PM me your address and I will fire it up tomorrow, boomerang it when you are done.

## Dynamic

2456 posts

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Dynamic: The only way to really check is to get an ammeter which must go in one of the lines so the current flows through the meter.  A multimeter capable of measuring Amps will do the trick.

yep, so i have that, but can only measure current on the 12V side as I have no way (yet) of getting 'inline' on the 5V (USB) side.

Cut open a USB cable.  I have spares if you need one.  :)

"4 wheels move the body.  2 wheels move the soul."

“Don't believe anything you read on the net. Except this. Well, including this, I suppose.” Douglas Adams

219 posts

Master Geek

Cut open a USB cable.  I have spares if you need one.  :)

true... didn't want to cut into my lightning cable, but i suppose i could always get an extension cable, add that and cut into that.

## Niel

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One person supports this post

Apple does not follow the USB charging standard.  It detects a resistor network on the data lines.  If you don't have the correct divider, then it will not draw more than 1A (or could be 500mA).  Everything else follows the USB charging standard by detecting the data lines are shorted together.  The 2 methods are not compatible, so you will see some adapters will have a socket labelled iPad and another socket labelled Other (or something like that).  With an iPhone you probably don't notice, but with an iPad the lower current limit is less than the power consumption so you end up running the battery flat while "charging".

It is easy to modify the adapter to have the Apple-specific resistor divider.

You can never have enough Volvos!

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Master Geek

Thank Niel... that probably explains it.
I'll have a hunt around for an Apple compatible version.

Cheers

## richms

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Also devices will not pull the full current when they are getting near to full. They will back off at about 70% from what I have seen.

If only apple had added qualcomm quickcharge 2.0 to their products. That kicks arse for charge speed. If only USB powerbanks would take it, since those take _forever_ to charge up.

Richard rich.ms

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