Weird charger behavior

[marauder]

Hi Everyone!

I've just bought myself a USB-multimeter - a really awesome thing to find out what is actually going on between Retrostone and a charger.
And this far I observe quite weird results.

I'm testing 2 different chargers:

1. A charger from one of my old noname Chinese smartphones which claims to be 5V and 1.5A.
First I test with my phone, and it outputs 5.25V and around 1.1A. I don't know if this is the maximum or just my phone doesn't take any more, but this shows that the charger is able to supply at least 1.1A.
Then I plug Retrostone and it outputs 5.25V and 0.7A. I assume that this is maximum that Retrostone takes.

2. A brand new charger that I bought from a RaspberryPi accessories store, with 2 USB ports, claiming to be 5V and 3.5A (maximum 2.4A per port).
With my phone it outputs 5.1V and 1.2A (so almost the same current as the 1st one, maybe a little bit more).
But when I plug Retrostone, the result is surprising: 4.9V and 0.3A!
So the current is significantly lower, and it looks like the reason might be in the voltage that dropped lower than 5V.
But why does it happen?
If this is a charger's fault, than why it's not happening with my phone plugged, though it takes even more ampers?

Therefore if anyone has issues charging a Retrostone with even seemingly good charger, keep in mind that you might have the same issue as I'm having.

Besides, it looks like even a good 1A charger should be enough for Retrostone since it never takes more than 0.7A. BTW I checked this also with my native 2.5A RaspberryPi charger, and it was also only 0.7A.

@Pierre, what do you think about the issue with my 2-ports charger?
I'm afraid my knowledge of electronics is not enough to explain that.

[Admin]

Hi Marauder, thanks for sharing those results.

Your tests were with the unit ON or OFF?

The charge current of the battery is around 0.6-0.7A
If you add the system load (if system is turned on) it should go up to around 1.2A (depending on load)

So if you have a 5V 1A charger (real 1A) and that you'r not using the RetroStone it should charge correctly.

What is strange is your results with the raspberry pi charger, it's strange that you had only 0.3A. Have you done these test at the same time? (ie battery was approximately the same level?)
Do you have same results on each of the charging port of this charger? You were not using other ports at the same time I guess?

[marauder]

Your tests were with the unit ON or OFF?

Unit was OFF all the time. Yes, makes sense, with the system load the current should be higher.

What is strange is your results with the raspberry pi charger, it's strange that you had only 0.3A. Have you done these test at the same time? (ie battery was approximately the same level?)
Do you have same results on each of the charging port of this charger? You were not using other ports at the same time I guess?

Yes, it was at the same time, same cable, I tried both ports, with nothing plugged to the other one, and even switched it 3-4 times between 2 chargers to confirm the result.
No idea how can this be, maybe I should return that charger to the retailer..