My Xtar VP2 will overcharge

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.

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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).

Thanks for adding your findings. I never did get a resolution to this, and gave up on the charger (and Xtar in general).

I won’t use it for regular li-ion either, because I need to be able to trust that my charger isn’t cooking up 18650-shaped firebombs.

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This is serious. I would like to see @XTAR (@xtarflashlight?) response…

Reposting the link to the summary post for clarity: My Xtar VP2 will overcharge - #37 by achilles_spiel

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Thanks for the link. I can see you contacted XTAR regarding this; Did they ever respond?

We are so sorry to hear about this issue! This VP2 charger has been discontinued. After checking with our engineers, they told they rarely received such complaints regarding this charger. The engineer suggests that you could toggle the switch to different voltage modes for multiple times. The charger display should correspondingly show 3.2V, 3.6V, and 3.8V. If the voltage displayed on the screen doesn’t change as you toggle the switch, there may be something wrong with the switch part. After toggling the switch position several times, please set it to the 3.2V position and try charging your LFP batteries again to see if it charges them properly.

If the LFP batteries get fully charged with staying in the VP2 charger for long time, the LFP batteries may sag in the voltage, and the charger has the automatic recharging function. In fact, it will recharge batteries according to the voltage setting on the switch, that means it will still charge the LFP batteries normally again, fully charge to around 3.6V.

I contacted Illumn, not Xtar, and they said they could not reproduce the issue.

They either tested with a variant of the VP2 without the issue, didn’t follow the steps to reproduce it properly, or didn’t bother to try reproducing it at all. Illumn stated they contacted Xtar, but I never received a follow up.

@xtarflashlight there are at least 3 people in this thread with a VP2 that will overcharge LFP. I’m not sure what else to say at this point.

I use a different brand of charger and just deal with the inconvenience of manually setting it to LFP mode each time I want to charge LFP cells (and the risk of overcharging if power is lost and restored while one is sitting on the charger).

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