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