MiBoxer C4 v3 charging issue

Hi folks, I’ve had a MiBoxer C4 v3 (2019 model) for a few months and it sees frequent use on an “every other day” basis where I’ll use it to charge between one and four cells at a time. I was using it this morning and came across what appears to be an intermittent fault.

I was charging a pair of cells - an ncr18650b and a vtc6 (both unprotected), whilst doing another task nearby. I was periodically monitoring their progress and knew that the vtc6 was due to finish. When I checked, I found the charger was showing 4.20V / 97%, but somewhat alarmingly, the charger was still pushing 1.5A (max current per slot on that charger). The charger was not set for HV li-ion/4.35V.

I pulled the cell as soon as I saw this as the charger seems to have failed to switch from CC to CV. I can only assume this would have resulted in an overcharged cell had I not been monitoring.

Unfortunately, I blew a fuse in my DMM just yesterday and had no accurate way to check the cell voltage, therefore I popped it back in briefly to see 4.20V displayed and a much more sensible charge current of 0.25A before pulling it again. I then put it in my old xtar vp1, which also said 4.20V. I didn’t charge the vtc6 any further, although I have marked it for future reference. The “B” was finished off in the xtar.

This is the first time that I’ve had a problem with the C4 charger, but I have since seen a few threads on here regarding issues with predominantly slot three of the C4-12. Interestingly, the vtc6 was in slot three. Unless this is just a coincidence, one can probably assume the issue lies with some piece of shared design and/or firmware.

The suggestion in the other threads (again, regarding the C4-12) is to just stop using slot three, however this begs the question: why can this happen only in slot three, or is it just a case of that slot “fails” first? I’m not really looking for answers to this as I’ve already decided that I’m going to retire and replace the charger as I believe that even occasional overcharges are wholly unacceptable. I did think this may be useful information for the community in general though.

Miboxer seems to be plagued with problems.

After this issue I would stop using the charger altogether. It could have resulted in something serious like a fire if you were not around. Think about that.

I have noticed that here this charger with some kind of problem seems to pop up quite often.
Knowing this charger doesn’t seem to have a good track record, lets give it the bennefit of a doubt.
When most chargers switch from the constant current stage to the constant voltage stage that happens at a cell voltage of 4.2 volts. Then another 10 to 20 minutes of constant voltage until the charge reaches usually 10% of the set max current where it terminates. It is possible I guess that you checked the cell at approximately the switch between the two stages. I’m not saying that’s what happened but it is possible.
To test this, deplete the same 2 cells again let them rest a few minutes and then try charging in the same way again. Once it reaches 4.2 volts continue to let it charge for another 10 minutes watching the charging current and touching the cell to check for heat ever 30 seconds or so. If the current doesn’t drop during this time or the voltage climbs above 4.2 volts or the cell gets hotter than warm pull it immediately and discard the charger.
If it acts normal it should be okay but continue to monitor it while charging in the future, seems to be a problematic charger.
Just my two cents :wink: .

Need to get your DMM back in operation to really know what is going on. As 007 points at 4.20v is the change over point from CC to CV. Actual battery voltage will be lower at that point if the charging process is terminated right then. In order to push current into the battery, the charger has to put a voltage higher than the actual battery voltage for current to move. Typically on chargers, I see the max voltage of 4.20 not exceeded during the charging process which means the actual battery voltage at the point of switching to CV is with a battery around 4.10v or lower. (depends on how high the charge current is).

Assuming the Miboxer charger was at fault; then if it truley was putting in 1.5A and the resting voltage of the battery after you pulled it out was 4.20v……….then the charger had to have been putting something like 4.30 volts or higher on that battery. Only with a DMM would you be able to verify. It makes me wonder if it operated improperly or measured improperly.

I’d repeat with another charge while keeping an eye on things and see if you can measure what’s going on. What you’ll probably find is it won’t repeat and you’ll lose confidence in the charger. I haven’t had problems with mine but then I don’t use it a lot and mainly got it for 21700 size cells.

Without proper measurements bashing a charger is not a good idea, especially for a new member, do proper investigation then post your results
Posting I could not measure because I had blown my DMM fuse recently does not really speak for your skill with electrical gear

It could be a simple display error and no harm to the cell or sort of user fail up accidentally getting 4.35V enabled, even if you doubt it

if it had overcharged to pull out the battery at exactly 4.2V is like a lottery on 1.5A charge rate, a minute later and it would be higher, a minute earlier and it would be less

Also as you say the charger displays 4.2V and 1.5A can’t be right, because if you pull a 4.2V battery with 1.5A charging current the voltage in the charger would be at like 4.4V already
If you can’t measure current just measure the voltage difference between charged and uncharged cell, if you can’t proof at least twice >4.25V inside the charger with different batteries stop making assumtions

Even good chargers can get fooled by Ultrafire or very old or bad batteries if you look at HKJ´s tests

Don’t look at 4.20V, it’s not a full load
Look to the left of the display, 90, … 99, 100% = Full and you can see that it is 4.18/4.19V.

Cleaning all contact points on the charger and batteries might also help with a charger misreading cell voltage.

I probably should not ask here but all the knowledgeable charger people seem to be here. I have an on board charger that takes hours and hours to terminate. Cell comes out at 4.15 volts. Watching the usb meter, the current starts at 2 amps and decreases to .1 amps and then stays there for 4-5 hours. Should I be concerned? The battery is a 26650 5500 mah Shockli. The flashlight is a Astrolux FT 03.

You’re absolutely right and I appreciate the input, but I would normally expect this charger to switch from CC to CV at about 90, not 97. Pulling the cell and reinserting it, the charger recalculated a 0.25A charge current, which is much, much more in line with what I would expect at this stage of the charge.

Again, absolutely true and I agree with everything you say. I admit that without a functional DMM, I am very much making assumptions based upon the information available to me.

I’ve used this charger a lot and it has never done this before. If this is indeed an intermittent problem with the charger, rather than a total failure of the slot, I won’t know until I see it happen again. When will that be? Will I be as lucky to catch it if/when it does happen again? Intermittent issues are by far the hardest to diagnose.

Thanks for your input. Based upon your condescension, I’ll wager that you’ve never made the mistake of absent mindedly switching from voltage to current without switching leads. Good for you. Looks like I’m not the only one making assumptions, though.

You are correct. 4.20V is not a full load, the CV phase (saturation charge) would also need to complete. Once a full charge is complete, one would then expect the cell to settle to a lower level.

Li-Ion does not tolerate charging past 4.20V, which is why the CV phase of charging exists. Unfortunately without a DMM, I can’t know what actually happened and as I have freely admitted, I have made assumptions based upon the available information.

Not something that I had considered, however the contact on both the cells and the charger are clean as a whistle.

pennzy: I’m not familiar with that torch and I don’t use 26650s, but so long as it terminates before 4.20V, it should be fine. 5500mAh is a big cell, which will take a while to charge.

Thanks, I thought so but wasn’t sure.

Pretty much no, mostly using clamp meters as they do not add additional resistance to the measured circuits
Pretty much anything down to 30mA I can measure spot on with my expensive clamp meter in the 300mA range with 0.1mA resolution and 1% accuracy

When I hook things to my 1200€ bench DMM I rather think twice before blowing a 5€ fuse or wreck my meter in worst case, using my lab power supply I always limit the current below the fuse value on uA/mA measurements in the 400mA range

By the way do you know most DMM still can mesure voltage with a blown fuse! The fuse is there for over current protection in current measurements.

I wouldn’t trust the 90, 97 fuel guage for cells. Different cells have different characteristics, its hard to predict percentage of a partial charge unless using the same cell that you have all ready collected voltage to % charged data. How is the charger predicting this charge percentage and what cell was it based on or is it a average comprimise.
Its predicting a close estimate. There are alot of variables that can effect the prediction.
I still think you might have pulled the cell during the start of the CV stage. Testing it in the same environment would be the only way to know.
4.2 volts is a recommended termination voltage, its what you’ll will find in most datasheets. Slight overcharging shouldn’t cause anything bad to happen but it does shorten the life of the cell drastically. On the opposite side, terminating at a lower voltage extends the life of the cell. Many here have overcharged a cell or cells just to get the absolute max lumens out of a light even if it just last for seconds. :person_facepalming:
I would be interested in what you find out if you do try the charger again with the same cells.

Umm… good for you, I suppose? I’m not really sure what you’re adding to the conversation here, except perhaps an attempt to “flex”?

Personally I can’t remember what mine cost, but I know it was a lot less than that. I did buy it about thirty years ago though, and in that time I have blown a single fuse. But you know: if you feel the need to judge people and attempt to belittle them when you know nothing about them, then that probably says more about you than me.

Thanks, but the pertinent part of that statement is “most DMM”s. Not mine, as it did in fact occur to me to test the obvious. The DMM was not expensive when I bought it, so this is not entirely unexpected.

Whilst I understand what you’re saying about the fuel gauge, I have a selection of 30Qs, VTC6s, NCR18650Bs, LG MH1s and a variety of laptop pulls (plus a good number of NiMH, but this is not relevant here). The panny Bs are from 2013 to 2016 and are the oldest (apart from maybe some of the laptop pulls, but they are still sufficient for their use) and the vtc6s are the newest (purchased in November of 2019). I have found that this particular charger has always gone from CC to CV around about “90“. Not following this pattern is an unusual situation that I have not seen before. I’m 98 sure that if I put the exact cells back in to charge in the exact slots, they would charge normally the next time. It’s the 2% that is the issue, and as it sees regular use it’s not an expensive device to replace for my peace of mind.

As a test (one that I can do without a DMM), I charged the same vtc6 (which, since OP, had been lightly used) up to the start of the CV phase in slot three of the C4. It reached CV at “94%” (I waited until the charger registered a drop in current, rather than just hitting 4.20V) - which I still consider quite high from previous track records, but accept that the fuel gauge is a “best guess”. I pulled the cell, reinserted it, and the charge current was recalculated as 1.5A (not the 0.25A from the original post). This is obviously far from conclusive, and in fact should be considered “anecdotal evidence” at best. With this being one of the few things which I can currently measure (no pun intended) - assuming the displayed current is and was accurate both times that it was measured - it is interesting, at least. For reference, I didn’t let the charge complete.

You have taken a very moderate (again, no pun intended) approach to this discussion, which I appreciate. The point of this thread was never (as Lexel put it) to “bash a charger”, it is simply for the purpose of data - which the internet is very good at collecting. Other than the implicit assumption that the charger’s display is accurate, I do not believe that I have presented anything as fact which I did not verify, and I have clearly stated when making other assumptions.