Opus BT-C3100 charging voltage

Hey everybody,

I’m making a new topic, because this is about a very specific characteristic of the BT-C3100 and I didn’t want to hijack the other threads.

I got one from GB and started testing it with some cells. I connected a DMM to one slot to monitor the charging voltage and decided to stop charging, when I saw this:

The voltage applied to the battery almost reached 4.5V and the charger still made no sign of reducing the current (1A).
Later I did some tests, which showed that the charger begins to decrease the current slowly after hitting this value (a couple of mins later) and does not overcharge the cells much. At the end, the resting voltage of the cells is about 4.25V, which should be ok.

Then I PM’d Henry Xu (btw.: thanks Henry for the interesting conversation!) about this to ask him, if I got a faulty unit and should contact Gearbest or if I can do something about it.
But to my surprise he told me that this behaviour is normal, because otherwise it wouldn’t be possible at all to charge the battery with 1A.

Now I was confused because I always thought, that the critical value of 4.2V (4.25V should be ok) may never be exceeded and the charger must reduce the current as soon as it reaches 4.2V (CC/CV). In HKJs graphs you can see that, as soon as the voltage curve reaches the 4.2V, the current starts to drop.

Henry suggested that I should post this on the forum to discuss with you guys and get some opinions on this.
So what do you think, is it okay for the battery if the charging voltage is this high (the termination voltage must not be above 4.25V of course) or does this charger have a major fault?
Maybe you can do some testing with your chargers and check how high the voltage rises during charging. (Preferably with old high Ri cells or cells with a bad PCB, as the difference between resting voltage and charging voltage should be high.)

To clear some things up and prevent misunderstandings in the first place:

- DMM Battery is full and DMM is accurate (double checked with other DMMs)

  • this is not about the difference between displayed and measured voltage, only about the actual charging voltage

Thanks everyone!

FF

I have seen something similar with cheap crappy fandyfire 14500, I have 2 protected and 2 unprotected. They tested for 170 mah, and when charged again it stood still on ~4.14 for quite some time, and when I pulled it out to check with my dmm it was overcharged, might have been 4.35 v or something.

After all, this doesn’t look like a dreamcharger, not even close?

don’t stop dreaming :wink:

I just tested mine on a discharged cell, charging it with 1A rate, it was showing 3.35V, slowly climbing up.

Do you get those readings on all bays?

Shouldn’t the voltage be 4,2V through whole charging process?

I have tried this before on my Pila (the voltage shows when DMMed even without a cell) and the charge voltage varies, fluctuating continuously between 4.13 to 4.18.

In the 3100 I just tested it for a very short time to satisfy my curiosity and that was the reading that showed. Perhaps it will climb upwards gradually just like my hobby charger?

In my iCharger, I have tried it several times. The cell being charged is, let’s say showing 3.9V, but the hobby charger terminals are showing 4.1V, because it takes a while for that electrical voltage coming from the charger to be converted and absorbed by the cell into chemical energy. It is not an instantaneous conversion.

I did test one old battery with high Ri and got this curve:

As you can see the voltage goes above 4.3 volt for a short time.

This is not very good charging behaviour and is a combination of the high Ri and to high charge current for the battery and, of course, because the charger measures voltage without the current on.

A real CC/CV charger will never do it, but with simulated CC/CV it can happen.

It is probably time to recycle that battery. Edit: Not because of the charging, but because it has way to high Ri.

Is this an accurate way to measure though? HKJ uses some resistors when testing the charger and battery How do I test a charger

The resistor is for measuring current.

Thanks for always coming to our rescue, Henrik!

Oh ok, I don’t understand what you mean by this then

When I test chargers I measure both current and voltage at the same time, i.e. two DMM's connected.

With that setup I can either measure the battery voltage or the charger voltage, due to the sense resistor they will not be the same. I usual measure the charge voltage.

Maybe what is meant is that a piece of electrically conductive protruding material should be placed at the ‘plus’ of the cell inside the charger’s bay in order for the DMM’s probe to touch any part of the plus side since there is not enough space for the DMM’s probe to make contact without it. No problem with the negative side though.

Is this the V2 version?

I guess this happens because the charger measures voltage only once a minute (or even longer?) and with low capacity cells a short time without checking voltage is enough to overcharge.

Yes it’s the same on all 4 bays.
The voltage is climbing slowly on mine too. The critical point is when the voltage reaches 4.2V (this can be sooner or later, depending on the cells capacity and internal resistance). Then the charger doesn’t decrease current as it should and voltage rises further.

The ideal charge method for Li-Ion is constant current/constant voltage. In the constant current phase the charger should apply a certain current until the voltage reaches 4.2V. From this point on the voltage should stay at 4.2V (constant voltage phase), so the current will drop until the charger terminates the charge process.

Thanks for that!
So I guess my charger is not defective and the high voltage is indeed normal.
But the used cell (Nitecore NL186) is not that old and should be ok. I did not measure Ri though, I will do that. Maybe the PCB has a high resistance…

But would you say this is tolerable and the cells don’t suffer too much or should we select the current so that the voltage stays below, lets say 4.3V all the time?

Yes, V2 from Gearbest groupbuy.

The high voltage will only happen when the cell has a very high Ri and you use to high charge current. It will not happen with a good 18650 battery, except if the charger is faulty.

Definitely select a lower current!

Would a current of 500ma on good 18650 batteries be better than a 1amp charger current? Would that eliminate the high voltage during charging?

HKJ has said in this thread that the charger will not apply such a “high” voltage to good cells. If the internal resistance is so high that the voltage reaches 4.3v / 4.5v / whatever during 4.2v charging, the cell is not a good cell.

If your going to toss that cell, could you remove the protection circuit and see what happens? Just curious.