I wanted to verify the output of the DJI USB charger since I thought it was taking a long time to charge the batts.  


Here are 2 pics with my USB voltmenter attached.  One shows the batts & controller being charged using the DJU supplied USB cube and the other with another USB cube I have.  The DJI supplied US cube has 8.8V output with .33A and my other standard cube shows 4.98V at .52A.   I am suspecting the DJU provided USB cube is defective and am nervous to use it.  I have never seen 8.8V output from a USB cube.

Comments?

DJI moderator, can you please check on this.  I have had the mini about 2 weeks, not much flying yet due to very cold conditions here in Boston area.

Bob

8.80

4.98

What you seè there in the second photo is the controller  tapering off the current as it reaches full charge.
The voltage then rises because the load is reducing. The open voltage would be much higher than that as well. In the first photo more current is drawn  as the controller is only at 2 leds. Both are in fact  drawing similar power, ie approx 3 watts for the first(8.8v x .33 A)  and 2.5 watts in the second  (5V x .5 A)
Ideally the current would be constant until the last stage of charge where the voltage must be held constant and the current tapered.
But these usb chargers are  cheap and not very cheerful and work on a questionable algorithm and vary enormously in quality.
Because the are such a compromise , they have to attempt not to overcharge, done by  taking more time. A proper charger does this sort of batt in  1hr, less if required at some expense to batt life,  but is more complicated and so more expensive.

Bright Spark Posted at 12-20 10:44
What you seè there in the second photo is the controller  tapering off the current as it reaches full charge.
The voltage then rises because the load is reducing. The open voltage would be much higher than that as well. In the first photo more current is drawn  as the controller is only at 2 leds. Both are in fact  drawing similar power, ie approx 3 watts for the first(8.8v x .33 A)  and 2.5 watts in the second  (5V x .5 A)
Ideally the current would be constant until the last stage of charge where the voltage must be held constant and the current tapered.

Very well put Bright Spark. Agree 100%

Bright Spark Posted at 12-20 10:44
What you seè there in the second photo is the controller  tapering off the current as it reaches full charge.
The voltage then rises because the load is reducing. The open voltage would be much higher than that as well. In the first photo more current is drawn  as the controller is only at 2 leds. Both are in fact  drawing similar power, ie approx 3 watts for the first(8.8v x .33 A)  and 2.5 watts in the second  (5V x .5 A)
Ideally the current would be constant until the last stage of charge where the voltage must be held constant and the current tapered.

The intelligence to control the rate of charge is invested in the DJI battery itself, not the charger. The 'charger' is not really a charger as such, it is simply a power supply. And Lipo batteries require a constant current charge regime, not constant voltage.

True. The constant voltage however takes over from (ideally) constant current which dominates until max voltage is reached, but more charge at a dimishing rate is still possible.
Yes obviously the power supply has no intrinsic charge control circuitry, but it's behaviour causes it to vary its supplied  current, because of the load's varying requirements.
I gather mini's batts are lion, and they appear light on internal charging regulation and cells may not be balanced at all, looking at some tear down videos where people are already fitting larger capacity batts.
In case I'm rightly called out again, I feel I must add that we have looked only here at the controller batt.  If this were a 1 amp/hr batt, it would take 1 amp for 1 hr to charge from flat.
The mini's batts are I think 2.7 a/hr capacity, and therefore require 2.7 amps to charge in an hour.
At the currents shown that would be over 5 hrs to charge. 18 watts from usb charger is c 2.5 amp at 7  volt  and barely do 1 in 1 hr.