$37.99 for Opus BT-C3100 V2.1 with OP3100 ( clearance) & $38.99 for Opus2.2 without using the coupon @GearBest

Yes - don't make me laugh - been thru 8-10 cells on this, few DMM's, few chargers.... It's about as consistent as clouds on a rainy day Smile. Whether it says FULL or not doesn't matter - it displays the voltage to a precision of 0.01v, so would display 4.20v, not 4.2v - I differentiate this because of the engineer/designer quoting accuracy to +/- 0.05v but displays precision on 0.01v, and others have 2.1's that are truly accurate to 0.01v.

Some will win, some will lose. Some were born to sing the blues... Cry

I just might spend some time researching this. If I come up with a fix, I'll certainly share it here. Might just be a resistor swap or something - a friend here at work has designed charging circuits so I'll see if he can help or not.

Display precision is not the same as accuracy. If you read a specification for a DMM it says x%+y count, both x and y specify how the value on the display is different from the correct value. Some of my DMM's has more than 10 count, i.e. the last digit is basically useless (It is not, but that is another story) and it is not because it is a cheap DMM.

On the charger the +/-0.05 is probably because it matches the allowable tolerance on many LiIon batteries, but it is not a very impressive precision.

Yes, totally agree and understood it that way. I work on firmware in systems where we have to clearly differentiate the precision of the hardware and resolution of reporting. Some things are just too obvious to me - didn't mean to deceive anyone.

Updated post #258. Seems my OPUS is reading from 0.02v to 0.04v too high. Not too bad.


I already have buyer’s remorse and mine hasn’t even left Europe yet.

I do hope someone can identify the problem and solution.
It could well be that whatever fixes the meter calibration will also fix the charge termination.

Anyone have an idea of what the last .05V undercharge means in percent of capacity?

I still love my XTAR chargers! :beer:

It means very little. I had some that came off at 4.158 on my Opus and I threw them on a hobby charger to see what I was losing. Both were around 80 mAh to get to 4.2. Even that is misleading since the current going in is always more than what is discharged.

I assume you are talking about a 18650 cell? That would roughly calculate to around 2-4%

If you’re referring to my post…yes it was 2 18650 laptop pulls and it is about 3-4% since both had discharged to around 2200mAh when I initially tested them (both came off around 4.20v on another charger).

Thanks for clarifying. Yes, 3-4% I have to throw into my runtime equations. My XTAR WPII terminates right at 4.20V dead on every time as does my new little XTAR MC0. With the glowing reviews of the Opus, I wasn’t expecting such poor correlation in such a basic function.

+1 - me too

From post #204 here: https://budgetlightforum.com/t/-/24621, the engineer/designer, Henry Xu replied to my questions and said this:

Tom: The charger will charge each cell to 4.20v before terminate the charging processing. Our charger doesn’t supply trickle charge current, so when it stopped charging, the cell voltage will re-settle by itself, and after 12hours, cell voltage can be 4.16v display. So concerning volt meter accuracy issue, please do this: make a charge cycle, and after is shows full, you take out the battery and measure the battery voltage with a DMM. We have +/-0.05v tolerance ( internal control is +/-0.03v at 4.2v condition). There will be volt display variant due to internal voltage resister network components variance reason. With your case, displayed value of 4.18 for 4.2v is possible with this charger.

Sounds to me like you are due a replacement according to this rational and your post #250 above.

And my cells read within .02 volts after charging and a rest time of 24 hours. Again, using my XTAR chargers.
As a matter of fact, I have a 14500 holding within .03V for several days when installed in a device!

Does Xu needs a few quality XTAR cells to test with instead of cheap ChineseLiIon cells.

Mine should arrive tomorrow. I will be upset if it ends at 4.15V.

Both Nitecore and VP2 terminate at 4.17~4.18V. Even my old $4 charger is able to terminate at 4.2V.

Isn’t it better to terminate charge a little early to preserve the life of the cells and get more cycles out of them? Just asking, I’ve read in the battery university that you should keep a cell at peak 4.2v cell voltage as little time as possible to minimise stress and prolong life.
4.15 does indeed sound a little on the low side.

I wouldn’t mind a resting voltage of a cell even at 4.14v (overnight, on each test). In these tests I made on protected Soshine 2900mah cells, it still had discharge capacities ranging from 2871mah to 2915mah, using 1A, 3A and 5A discharge rates.

See for yourself in the review link below:

Capacity is one thing, but internal resistance is another important attribute for me, resulting in lower performance for a partially depleted cell. If that 4.14v was 4.20v, what would the capacity be then? It's kind of hard to compare when there's only one level with data. I've had KK 26650 4000's test at over 4400 mAh, so higher results than labeled is common.

Hi Tom E,

I was only pointing out that having a cell that rests at a “low” 4.14v does not mean a bad cell, nor will a cell that rests at 4.19v be necessarily better and will show way higher than its rating, at least in my various tests with different chargers and cells.

As for KK cells, they have historically from 5 to 10% higher capacities than their rated capacities, as I have owned some before, but seldom with name brand 18650s, mostly lower by an average of 2 to 4%.

Your charger might just be defective as you are saying, unfortunately.

If it's defective, Henry hasn't answered my pm, hasn't been in the forum in 11 weeks, and I have no clue who manufacturers these things ?? Not sure if Henry ever mentioned it.

GearBest just this morn offered two options:

  • $15 refund, I assume in store credit, which will be no problem anyway for me
  • I can buy another Opus BT-3100 v2.1 for $25

I suppose no guarantee the new one will be any better than the one I have... My only choice is the $15 because I'm not taking this gamble again - that's for sure. I was seriously thinking of configuring it for 4.35v so it charges to about 4.28v, then leave them in for an extra 12 hrs to trickle drain... Might work . Should drain to about 4.24v which is within their tolerance. Of course the CC/CV cycle may shorten the cell life a bit... Actually it's bout perfect for 4.30v cells that way and I may have a couple of them...

I'm trying to find an up-side .

I would buy the second one for $25, in a second!

Not for anything else, but I have 2Xv2.0’s, 2Xv2.1’s and all of them have been performing very well, don’t have the same problems as yours.

And Gearbest has almost sold a thousand units so far of both versions, and if complaints similar to yours here in this forum is an accurate gauge, problems such as yours can be considered an ‘isolated incident’ so to speak, (if I am not mistaken, not more than 4 individuals) as they are a very small percentage compared to the total number of sold chargers.

My point is, buying another Opus AND getting another DUD unit is quiet remote.

If you buy and you do get a dud, play the lottery man.