# USB meter help

#### tinpanalley

##### Golden Member
Sorry if this is in the wrong forum. Help me understand this, I know this is very basic stuff but I'm reading different sites and getting myself confused.

I've got a USB power meter and it's plugged into my charging dock which can give up to 12V at 2A
I then charge a rpi2 remote with the following battery specs:
3.7V - 2.96Wh - 800mAh
Charging voltage: 4.4V - 5.2V
Charging current 300mA

and the remote's input specs:
5V -- 300ma max -- operating voltage 3.3V

In reference to what I'm charging, what is the display on my usb meter telling me exactly when it displays V, A, W, and mAh?

The instructions are so confusing I actually don't understand what I'm looking at anymore, but trying to learn.

#### LTC8K6

##### Lifer
V is the charging or discharging voltage
A in the charge/discharge rate in amps
W is the power in watts
mah is the total amount of current that has flowed through the meter.

You can determine if charge/discharge voltages and rates are what they should be, or if battery capacity is what it should be, etc.

#### Leesajohnson

##### Junior Member
Good platform for technical users. They learn a lot from problems posted by different users.

#### Billb2

##### Diamond Member
OP, I think you're assuming the the charger "pushes" the charge into the battery - it doesn't. The battery "draws" whatever power it requires from the charger and the dongle reports what it is drawing.

#### tinpanalley

##### Golden Member
OP, I think you're assuming the the charger "pushes" the charge into the battery - it doesn't. The battery "draws" whatever power it requires from the charger and the dongle reports what it is drawing.
No, no, I get that. I know what current, voltage, etc are, I just wanted to understand what they meant in context to this particular operation. The point of the meter is to show what the wireless remote (in this case) is drawing while it charges. What I've used it for in the past is to make sure that individual adapter/chargers were outputting what they claimed and to test if USB cables were working properly. But I wanted to plug in this remote and for some reason I couldn't understand, between all the specs, what it is I was reading. This is the meter.

#### LTC8K6

##### Lifer
No, no, I get that. I know what current, voltage, etc are, I just wanted to understand what they meant in context to this particular operation. The point of the meter is to show what the wireless remote (in this case) is drawing while it charges. What I've used it for in the past is to make sure that individual adapter/chargers were outputting what they claimed and to test if USB cables were working properly. But I wanted to plug in this remote and for some reason I couldn't understand, between all the specs, what it is I was reading. This is the meter.
I have that same one. It seems to be very good. I use it to make sure my phones are charging properly, my usb ports/adapters are putting out the correct voltage, and to test the capacity of my lithium ion power banks. I use it along with a USB load, which puts a load on your USB charger or power bank.

For example, if you have a USB charger that says it is 2.1A, you can use the meter and the load to see if the charger can actually supply 2 amps at 5 volts.
Many times you will see that cheap USB chargers are not able to actually supply what they claim. When you put the appropriate load on them for their rating, the voltage drops too low, or actually cuts out.

#### tinpanalley

##### Golden Member
I use it along with a USB load, which puts a load on your USB charger or power bank.
Sorry, I've never tested under a load before, how does this work in combination with the device you're charging? Wouldn't it be better to place the load on while you're testing/charging a device?

#### LTC8K6

##### Lifer
Sorry, I've never tested under a load before, how does this work in combination with the device you're charging? Wouldn't it be better to place the load on while you're testing/charging a device?
No, when you are charging or testing a device, the device is the load.

The advantage of the dummy load is that it is fixed and constant, and it does not risk your expensive iPhone.

You can plug the dummy load and the USB meter into that cheap USB charger you got from Ebay, and find out if it works properly, without risking your expensive devices.

You can plug that meter and the load into that fully charged "30,000mah" power bank you bought for \$10.00, and fully discharge it, and learn that it's really only 8,000mah capacity.

One of the cool things the USB meter shows is how your phone or tablet "negotiates" with a USB charger to determine the max charge rate. I can actually see my Samsung phone slowly ramp up the charge rate until it gets a voltage drop, then it backs off the rate until the voltage comes back up, then it settles right on the sweet spot.

You will be surprised at how some USB cables have such high resistance, that only 500ma flows through the cable, even with a proper 2.1A rated charger. Change the cable, and magically you are fast charging again.

I have learned a lot about USB charging, power banks, and devices using the USB meter and the load. I actually have the adjustable load with the fan on it.

My nephew swears by the long 10' USB charging cables for his phone. I showed him that the long cable is limiting him to 500ma, no matter what type of charger he uses, and making his phone take way longer to recharge.

#### LTC8K6

##### Lifer
https://youtu.be/zP3jnzlfgnQ

Here is bigclive showing that a 3.1A usb supply is not actually capable of anywhere near that output, using a USB meter and a dummy load.

#### aigomorla

##### CPU, Cases&Cooling Mod PC Gaming Mod Elite Member
Super Moderator
Sorry, I've never tested under a load before, how does this work in combination with the device you're charging? Wouldn't it be better to place the load on while you're testing/charging a device?

The Term "Load" means there is current going though, and typically when LOAD is implied its the maximum current possible from the Device.

Hence why we call it Load, because like a truck with cargo, its loaded.

So when your charging a device, it will be under "Load" because in DC circuits, a device will take 100% of the Amperage possible at the given specified voltage, unless u limit the amperage.

So when charging the device again, the device will pull the full amperage it can handle to recharge the battery, hence its under load.

If device A was specified to charge at 2.1A, then it will pull 2.1A from the charger specified at 2.1A.

If the charger can provide 2.1A, and plugged into a 1.5A charger, it will only recharge at 1.5A because it was amp limited by the charger.

If the device is specified to charge at 2.1A, and you have a 5A charger, the device will still pull at 2.1A because that is the max current the charger can draw.

Basically your down to what is the limiting factor, and what brick wall you run into before you are limited.

This is why a lot of products claiming they can do X amps may not really do that many amps, because the device you connect to it can now draw the max stated.

A perfect example is when you go into Power Supplies for PSU, and when you look at cheap ones vs quality ones, at high end load.