Is there such a thing? It would be a bonus if a USB tester could also display the voltage of the battery being charged inside a flashlight. Not sure how that would work, but since I thought of it, maybe someone already built it? I'm always lagging behind the latest tech anyways.
Literally impossible to do outside of the flashlight.
I am assuming you mean a flashlight with a USB port? You can’t because you are only reading USB out which is always 5V
Was worried about that! Oh well, it was a nice hoping for it.
Yes as I have a few.
I was looking at my USB tester just now that is between the charger and the flashlight and maybe I could get an idea where the voltage would be based on the drop in amps on the tester display. Was charging at 1.75A and now is down to .74A. I will pull the battery as it slows down more and see if I can make a connection between charigng amps and battery voltage. Might be worth a try, even if it means nothing, at least I will know.
I'm assuming it would be different for each flashlight based on what they have inside for charging, but I could take note of each one and see if it remains consistent for the same flashlight and then remember it. The reason I asked is because I like charging my batteries to 4.1V or around there.
It worked! By disconnecting charging when the UM25C USB tester was displaying around 0.470A when initially charging at 1.75A to 2.13A I got my 4.1V charge that I wanted from both batteries in different USB chargeable flashlights. So at least with two different batteries and two different flashlights, the results were almost identical.
You can;t do it because the USB part [where you are asking for a meter] does not contact the battery - only the charger knows that.
You disconnected the charger in the middle of the saturation charge phase, when the cell was already roughly 85% to 95% charged and the voltage is pretty much going to be between 4.0V to 4.2V. But this was not what you originally asked. If you were to disconnect the charger during the constant current phase, when the current was still over 1A, you would have no idea if the cell was at 2.8V or 3.9V. Basically, during most of the charging cycle, there is no correlation between the input current to the charger and the cell voltage. It is only during the second phase of charging where you might be able to infer the cell voltage from the charger input current, but different chargers, batteries and temperatures can yield different results, unless you’re already near the tail end of charging, which appears to be where you ended your test. This is why others have answered that it is not possible to determine the cell voltage solely by using a USB current meter connected to the charger.
If you only want to know when the cell is at least 85% charged, then yes, a USB meter might be able to tell you that. If you want to accurately determine cell voltage between 2.8V and 3.9V during charging, then no a USB meter would not be able to do that.
KuoH
Yes it wasn't my original question but by pulling the charger when the draw was down to .47A gave me the 4.1V I wanted. I I should have asked a different question in my first post but too late for that! Still, I found it interesting that .47A gave the same results with two different flashlights and batteries, so there may be some consistency with that after all.
All that shows is that charging slows down when the cell is almost full.
It was only consistent because those chargers happen to be capable of high charging current and you pulled the plug near the end of charging. If it were a charger only capable of or set to .5A max charging current, then pulling the plug at .47A would tell you nothing useful about the cell voltage.
KuoH
That's a good point, I should have thought of that. Had a look and all three of my USB chargeable flashlights are fast charge 2A. Going to try it on my UT02 tonight and if the results are still the same, then it looks like that is the norm for fast charging at least. Well, at least with a 26650 maybe and that gives me what I wanted anyway and that was an easy way to get to 4.1V for all the usb chargeable flashlights that I own.
Around how much capacity charged is 4.10v? (eg. if it’s a 2600mAh battery, and it’s charge to 4.10v, how much of the full 4.20v capacity would it generally have? 85-90%)
According to battery university here 4.1V is 85-90% and generally doubles the life of the battery if charged to to that.