I have a 12v accessory plug socket that has a light on it and I was curious how much keeping it plugged in would affect my battery. (everything I read said it wasn't an issue, but curiosity sent me checking)When I used my harbor freight multi-meter I saw that it was pulling only 0.02 amps with no load. I then tested it with a load, so I plugged in a usb charging plug which also happens to tell me the amp draw through it. ~Here is the problem~My multi meter reads 0.48 amp for the "12v accessory socket" PLUS the "USB charger", when the USB charger alone shows 0.9 amp draw by itself.Am I doing something wrong?I would expect the total draw to be closer to 0.92 amps not the 0.48 that my multi-meter reads. (0.9 amps for the USB charger + 0.02 for the accessory socket that the charger is plugged into)

The USB gizmo is telling you the 5 volt draw (USB is 5 volts). Your meter is telling you the 12 volt draw. There's a voltage converter in the USB gizmo. 5V x .9A = 4.5 W = 12V x .375A. The power conversion isn't 100% efficient, so what you're measuring seems reasonable.

Both look correct. 0.48 at 12 volts is close enough to 0.9 at 5 volts.5.76 watts and 4.5 watts

Could be your Harbor Fright cheapo multimeter too. The Chinese have an odd way of measuring things....

So you are saying that I was DOING it right... I just didn't know what the hell I was doing. Sounds about right My life story :)Thanks so much. That helps out A LOT!

Really simple the multi meter is giving you the total power used 5.76wWhile the USB is displaying 5v power 4.5 watts480ma x 12v , 5.76w input900ma x 5v , 4.5w output/load 1.2w aka 100ma conversion loss from 12v down to 5v20ma at 12v for the ledSo conversion loss is only 80ma at 12v or approx 0.96 WattsApprox 17% conversion loss, typical and normal

Understanding unexpected amp draw. | Good Sam Community

Wampthing
Explorer
Sep 01, 2017
Matt_Colie wrote:
OK Wampthing,

You got answers. You got good answers on the first try.

Now, My Question for You.
Did you understand the answers?? (You are allowed to say "no".)

If you did that is great.
These little power converters are not a simple device. If you think of it as a transformer for DC (with bad efficiency) you will have a closer idea what it is. If you were to crack that USB charger with read out open, you would be amazed as all the miniature stuff that they crammed in there.

Matt

Yes, The answer makes total sense. Knowing USB is 5v, the math makes sense (in hind sight).

That being said, I have a lot more goofing around with the system before I am anywhere close to confident. Its like when a math teacher shows you what you did wrong with one problem, and you understand it enough to move on and get the next question wrong. :)

Always learning.
brulaz
Explorer
Sep 01, 2017
4x4van wrote:
I have several of those 2-port USB outlets mounted in my rig; Anyone aware of how much current they draw without anything plugged in? Was that the 0.02 amps @ 12vdc?

Just measured the current draw of my trailer's TV screen when turned off and only the little red LED showing.

It too was ~20 ma.

Must be a typical draw for those little LEDs.
MrWizard
Moderator
Sep 01, 2017
4x4van

yes.. that is the 'idle' draw / parasitic draw of for that device

yours could be same, a couple MA more or less
wa8yxm
Explorer III
Aug 31, 2017
The Draw of the USB charger depends on the device being charged. For example I have one device that can easily suck 2 amps or more when charging... And operating.. A 1.5 amp charger will not hold it.

Other devices draw 1/2 amp or less

Full battery on "Target" = less draw as well.
4x4van
Explorer III
Aug 31, 2017
I have several of those 2-port USB outlets mounted in my rig; Anyone aware of how much current they draw without anything plugged in? Was that the 0.02 amps @ 12vdc?
RedRocket204
Explorer
Aug 31, 2017
Alternatively, if the LED light bothers you, you could always add in a simple switch on the + side of the wiring for that USB device. I know it is a very small draw but that is what I did when I added two dual port USB sockets in my trailer's bunk areas.
Matt_Colie
Explorer II
Aug 31, 2017
OK Wampthing,

You got answers. You got good answers on the first try.

Now, My Question for You.
Did you understand the answers?? (You are allowed to say "no".)

If you did that is great.
These little power converters are not a simple device. If you think of it as a transformer for DC (with bad efficiency) you will have a closer idea what it is. If you were to crack that USB charger with read out open, you would be amazed as all the miniature stuff that they crammed in there.

Matt
MrWizard
Moderator
Aug 31, 2017
Yep
It's working correctly
You took two measurements at different voltages
And got confused because the amps reading went up, but forgot the voltage went down

Just the reverse of an inverter, but same idea

12v * 50 amps = 600w input
120v * 5 amps = 600w output/ load , not including the conversion loss

It's important to remember that with electricity, it's about power in and power out, always compare the Watts ,(do the math)
There will always be a difference because of conversion loss
But the difference should not be too great
If there is a very large difference, then there is a problem with the conversion device
naturist
Nomad
Aug 31, 2017
Wampthing wrote:
So you are saying that I was DOING it right... I just didn't know what the hell I was doing. Sounds about right
My life story :)

Thanks so much. That helps out A LOT!

Most of us most of the time. Welcome to the club.
MrWizard
Moderator
Aug 31, 2017
Really simple the multi meter is giving you the total power used 5.76w
While the USB is displaying 5v power 4.5 watts

480ma x 12v , 5.76w input
900ma x 5v , 4.5w output/load

1.2w aka 100ma conversion loss from 12v down to 5v

20ma at 12v for the led
So conversion loss is only 80ma at 12v or approx 0.96 Watts
Approx 17% conversion loss, typical and normal

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