My Xtar VP2 will overcharge

Just send it back in and buy another one, or not.

It’s not functioning correctly.


I get you Chris. Aside from the OP’s unit however, I’m very interested in my two units which I currently use daily for something. If there’s ‘fatal’ problem that can bite me hard - I want to know about it, and what it is, and when it happens (if possible). I could just trash both of mine at this point (and it’s tempting), but I wouldn’t know what to replace them with (and any suggestions in advance would be appreciated:-). I’m testing so I can make a judgment about what to do with my two units. I’m pretty sure I understand it now, but I’ll wait until that’s proven in a while. If it’s what I suspect, and if you do LFPs on yours, I can assure you that you will want to know about this too.

Would be nice if it were that simple. The VP2 is the only one I have found that doesn’t require a separate action to charge LFP cells. Drop in and go. I don’t want it replaced with another defective one.

My tests are still in progress, but if they end as I expect……

Here’s a little preview of what I think I may be ordering soon:-)

Don’t laugh. K2 makes great cells that I use, but just as important, LFP stuff isn’t just a distraction or niche sideline or charger spec for them; it’s a core part of their business (check their site). I’ll bet they’re quite capable of building a purpose-built / dedicated charger that’ll do a fine job of properly charging my LFPs without worry, and 16x34 is the only size LFPs I use, so…….

EDIT: If I’m guessing right, other chemistries may[ Edit insert:] ‘not’ be an issue for the VP2.

achilles’ spiel I have not tested or tried the following. So, take it for what it’s worth.

In my thread:

Gun Light, CR123 or CR2, Need LiFePO4 EXACTLY 3.25 V - 3.30 V

Jeff51 mentioned using the idea of a simple power supply to charge batteries. He mentioned this product from DROK.

Again, I have not tested the theory or the product. I have some DROK products I like and some I don’t. This is an inexpensive unit. It’s not going to have super advanced safety features or warranties. I know from the description that the voltage resolution on that unit is 50 mV. That probably means on the display. But, as jeff51 in the other thread said, you might be able to calibrate it with a voltmeter. So, if you calibrate the current to 1/2 C or whatever relative to your batteries and the voltage to 3.7 V, you might be able to just attach a battery and let it run. I would never leave it unattended and you might need to keep a voltmeter attached for precision measurement. I linked to some battery holders in the other thread that allow attaching to CR2 or CR123 batteries.

I don’t know if this is a better option than what you have, but it might be ONE option.


I have 2 concurrent, but staggered tests running on my two VP2s which were started last night and which I want to continue to let run for a good while yet. I’m charging one LFP 16340/123 cell in the left bay of both chargers. At conclusion of my tests I’ll have most of the info I want, but also want to check some other things when the chargers are available again. That will take me through this evening, but because I currently already have most of the info and answers that most people will likely be most interested in, I’m posting a summary of pertinent facts and data now, and may follow up with more info later if appropriate / desired.

Both of my VP2s are behaving similarly, and both exhibit essentially the same over-charging malfunction noted and described by the OP in his posts. This suggests that the malfunction is not the result of a defective unit, but is rather the result of a design issue.

If left unattended following normal termination of a charge cycle (status LED turning Green) using LFP cells, the chargers will ultimately ‘overcharge’ the cells to a significantly higher voltage than the normal 3.6V CV parameter appropriate for the ‘3.2V’ (LFP) switch setting. I currently show >4.1V on the first of mine, with the second following suit, and the OP measured 4.2V on his. I’ll find out how high it will ultimately go, but it’s already well exceeded ‘unacceptable’. Note that as previously mentioned and verified by the earlier HKJ quote in an earlier post, at this point the VP2 display will only indicate the rising voltage to 3.6V, and after that it must be monitored / checked with a voltmeter.

The initial charge cycle appears normal and proper in every respect up to the ‘green light’ status indication / termination point. The cell voltage then decays / settles in the normal way for LFP cells (and is reflected accurately on the the VP2 display). The malfunction is ‘triggered’ / precipitated at a point which appears to coincide with this ‘feature / bug’ referenced in HKJ’s test report which I’ve copied here:

“Charge will restart if battery voltage drops to 3.9 volt (3.3 in LiFePO4 mode).”

Note first however that this function appears to start at a display-indicated (and accurate) voltage of ~3.36 - 3.37V, not at ‘3.3V’ as stated. The actual point can be determined by observing when the indicated cell voltage hits a ‘floor’ and ceases to drop slowly, and instead begins to slowly increase. If you watch the LSD of the display, you’ll see it - and that’s the only indication you’ll see. Also note that this ‘restart’ of charging in no way resembles a normal ‘restart’ of a charge cycle as one sees when, for example, removing and reinserting the cell. It is not indicated to the user by the status LED, as that remains Green throughout this process. I also does not appear to use any normal charging algorithm (which would start at CC and determine the SOC (state of charge) of the cell. It is seen as simply a very slow ramping of voltage from that trigger point and continuing for many hours until the proper CV value has been far exceeded. The unit appears to be ‘out of control’ and in some invalid (and certainly improper) state.

If a user charges LFP cells and adheres to best practices and removes the cells in a short amount of time following charge termination, no problem should be experienced. Unfortunately that period of time is fairly short with LFP (where the voltage decays more and more rapidly than most other chemistries after charging termination), likely varies with different cells, and is therefore difficult to predict. Within an hour of charge termination, one could be in the ‘danger zone’. One can probably get away with a delay of 1/2 hour, but there’s no guarantee. The only guarantee is that it is unforgiving, and without intervention will malfunction and overcharge the cell(s) at some point. Again, you can watch the VP2 display and determine this yourself using your cells.

I haven’t fully tested these specific conditions using 3.6/7V chemistries, and do not know if the malfunction would be likely to be encountered in normal use with those cells and the 3.6V switch setting. Given the ‘3.9V’ number for other chemistries implied in HKJ’s note above, I think it would take a very long time for any of my such cells charged to 4.2V to ever decay / settle to that value, so it may effectively be a non-issue with such chemistries. In that respect the malfunction may in practice be effectively (if not specifically) LFP-specific.

Let me know of any questions you may have, and if I have answers I’ll share them.


What are you talking about?

My VP2 has a slider switch on the rear of the unit, for 3.2v, 3.6v and 3.8v.

You need to set the slider switch to the appropriate voltage and then insert the appropriate cells and then the charging rate.

All separate steps, in my mind, at least?

It’s certainly not an automatic setting and if you’re under the impression that it works like other ‘automatic’ chargers, than you didn’t read the manual.


I just leave the switch on 3.2 at all times, and the VP2 defaults to the lowest available charge rate (500mA), which is fine with me. I bought the VP2 to dedicate it for LFP charging since all other chargers require an action after inserting the cell to change default of Li-ion charge profile to LFP.

You mention other chargers being “automatic” but I can’t imagine any charger could automatically distinguish an LFP from an Li-ion: the entire voltage range of an LFP is somewhere within the discharge curve of a Li-Ion.

So this is the best I could do…

I grabbed one of my two NIB (white box) VP2s that I’ve never used.

I plugged one in at about 3:00am this morning and let it sit for a good hour, so see if it worked and to let it stabilize a bit.

While it sat, I took a white VapeCell IMR 800mA 16340 and ran it in direct drive for 5 minutes a stretch, in my EagleTac D25C Ti. clicky from 2014.

After an hour, I got the cell down to 2.8x volts, let it rest for 15 minutes, set the Xtar to 3.2v and 250mA and let her fly.

I was right near 3.6v when it was bedtime at 5x am. I checked this morning at 11x am and it displayed red and showed 3.59v and read 3.59v on my 22-805 Rat Shack DMM.

No problems with that charger.


I am satisfied to call this a design issue (hardware bug) with the VP2, and not just a manufacturing defect.

I didn’t think to try it with a Li-ion cell, ChrisGarrett. That you did is probably more informative than you thought it would be.

A Li-ion charged to 3.6V will not sag like a LFP charged to 3.6V: This suggests Desertcat is correct with the theory that the LFP charge cycle works as expected, ending at 3.6V. But, as the cell immediately begins sagging in voltage, it triggers the 3.9V voltage for the charge maintainer circuit that seems to be shared with all charge modes. It then trickle-charges the battery past the initial cut-off voltage and continues toward the normal Li-ion stop voltage, thus overcharging the LFP.

To be sure, I will test again with a Li-ion cell, to see if my unit acts like yours did.

My suggested fix to Xtar is to decrease the charge-maintenance trigger voltage to 3.1 volts when in 3.2V mode (along with adding a 3.6V ceiling), or if not possible, to disable the charge maintainer entirely when 3.2V mode is selected.

Please don’t flame me if this is a naive question. Why can’t the charger just maintain the final voltage (3.7 V or 4.2 V depending on mode) on the cell until it’s removed. I don’t think the batteries would care. Am I missing something?


achilles’ spiel:

[…I just leave the switch on 3.2 at all times, and the VP2 defaults to the lowest available charge rate (500mA), which is fine with me…]

I think you meant to say 250mA(?) (at least that’s the lowest / default setting on my older ones), and is great for these small LFPs BTW.

Not to be picky, but just to keep things accurate; the evil, out of control “maintainer” mode was said by HKJ to kick in at a threshold of 3.3V, but my emipirical testing showed that it actually kicks in at a threshold of 3.36V-3.37V.

HKJ’s quote from his test report:

“Charge will restart if battery voltage drops to 3.9 volt (3.3 in LiFePO4 mode).”

My quote from post 33:

“Note first however that this function appears to start at a display-indicated (and accurate) voltage of ~3.36 – 3.37V, not at ‘3.3V’ as stated.”

By the way, at the completion of my test runs last night, both chargers had overcharged their cells to over 4 V (4.21 & 4.13)…just as you stated in your first post in this thread. Both of those cells will be retired, as I believe they have been degraded by that experience.

They have both since settled to an open-circuit voltage of 3.7V, which is of course far above normal for an LFP cell. I don’t want to use them further. They’re dead to me.

1 Thank

To quote myself from my ‘results’ post (which is always dicey:-):

“In that respect the malfunction may in practice be effectively (if not specifically) LFP-specific.”

I honestly doubt that I could think of a way to even force a VP2 to malfunction using an IMR cell.

BTW: I use those same Vapcells a lot. They seem to perform well. For the record, I also always charge them in my VP2s.

Illumn has replied and found their chargers stop at 3.8, and have contacted Xtar to see if something has changed.

The mystery continues…

Well, I can’t say there’s any real “mystery” to what I observed over 24+ hours. It’s quite clear what my units are doing, and when, and how much. I believe in the original charge cycle mine terminate and go green very close to exactly 3.60V. I haven’t verified that with a meter because that has not been in question. Mine always stop there in the original charge cycle. The cycle never fails to terminate (green status light) at that level. When the charge cycle terminates I immediately see the voltage start to sag to 3.59, etc. (on the display), so the termination voltage has to be very close to 3.60.

It’s only after they sit in the charger undisturbed for quite a while (like yours in your original posting) [let’s say several hours for example] and have dropped in voltage to the threshold of this ‘problem mode’ (~3.36-3.37V) that the voltage begins to increase, from about 3.36-3.37V, to over 4.1 volts during the following 24 hours. Obviously it’s a very slow rate of increase.

Are you sure they clearly understand the problem scenario That statement you’re quoting them on sounds so simple as to be ambiguous to me, and doesn’t really reflect a full understanding of this scenario. I could be wrong, but……I guess if their unit doesn’t behave like our 3 units, the next question would be’ ‘Why?’ I suspect the issue is their test procedure. Anyway, I guess it matters little to me since I do know that our 3 units all have the malfunction, and those are the facts I need to deal with personally. That’s not opinion, but observed fact (certainly the two I observed carefully myself over 24+ hours). I understand exactly what they’re doing, and know that it’s NOT cool, and although I don’t know the internal component / designed root cause, I don’t need to care or know that.

Since you mentioned the distributor: What’s the story with the lack of the sticker on yours? Now that’s something I’d be interested in their opinion on, and an issue I’d expect them to be qualified to address. What have they said on the sticker thing? Do the stickers not mean anything after all?

EDIT: BTW - Illumn does know that the VP2 display will show 3.6V during the entire (phantom charging mode) problem scenario don’t they?

I gave them the details, so I’m giving them the benefit of the doubt.

Let us know if they have anything enlightening to say regarding the presence or absence of those stickers. That seems like a process issue / question that should be answer-able, might be important, and either the response or lack therof may be indicative / valuable info. If the distributor doesn’t have the answer, perhaps XTAR will. BTW - How long have you had your VP2? As I mentioned, mine are likely a good 5 years old. Thanx!

On my two white box VP2s, the sticker isn’t affixed to the charger housing, but rather is printed on a tab of stiff paper, maybe 1.5”x2.5”.

Maybe users weren’t enamored with the aesthetics?


Yesterday I found this old thread for the first time after searching for “Xtar VP2 overcharging”. I just discovered this issue the hard way, luckily without catastrophe.)

I left two 16340 lithium iron phosphate cells on the charger (thankfully in a reasonably fire safe location) and forgot them charging for a week! When I got in next, I was surprised to see the left channel displaying 3.20V even though a cell was in. Now these cells do not like to make contact with that charger and prefer to rotate down out of contact, so without bumping it I examined it closely and sure enough, it was metal on metal both ends. I then reinserted the cell and got no indication I had done so whatsoever.

Wondering if the cell had somehow gone open circuit I measured it with first one, then another meter and the cell was at 4.57V (!:flushed::rage::face_with_symbols_over_mouth:) No flame nor even leakage had occurred. The right hand cell bay was still showing a reasonable looking voltage though I forgot to note what that was. Taking it out it measured 4.49 V :flushed: also with no visible degradation.

I tested several other known good cells of various voltages with those meters, and one of them with Anduril firmware, and they all were within correct range. Two hours later now, those two 16340s are still at 4.43 and 4.38 V respectively. Obviously they’re now retired, I’ll discharge and dispose of them.

What I take from this (other than the yikes of it and that I wish I had found this thread sooner!) is that, at an extremely high voltage between 4.50 and 4.57 the charger stops displaying the 3.6V and reverts to nominal charge voltage, with or without continuing to apply the trickle charge (I don’t know).