My Xtar VP2 will overcharge

EDIT: There appears to be a design flaw in the Xtar VP2 manufactured as of March 2022 that causes it to overcharge LiFePo4 (LFP) cells when then the proper 3.2V charge mode is set. See posts:

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I have an Xtar VP2 with the switch set to 3.2V (for charging LiFePo4 chemistries). It turns green at 3.5 volts as I would expect, with the charging animation stopped. But I recently left a pair of batteries green on the charger for a day and they read 4.2 volts when I took them off!

Now, clearly I shouldn’t leave cells on, but I expected the charger to stop charging once the battery is at selected voltage. Is this by design? Defective? Does the selector switch just control when the light turns green, but the charger goes right on dumping current into the battery?

This excellent review of the charger seems to show that this shouldn’t be happening, stating “Again the charge does a nice CC/CV and turns off when the battery is full.” Test/Review of Charger Xtar VP2

I am certain this wasn’t a charge selection mistake, because I was able to reproduce the overcharge, stopping when the cell reached 3.7.

I’ve used my VP2 since 2004 and I’ve never had an issue with the 3.7v/3.8v cells I’ve charged up, even on ‘maintenance’ charging.

I still have two NIB VP2s, but I haven’t played with them.

I love the charger.


Thanx for the ‘heads up’ post. I have no answer re: what you experienced, but it’s good for me to keep this possibility in mind. I regularly use a pair of VP2s for most of my charging, and also use / charge LFP cells often. Although I don’t often leave cells in the charger after ‘normal’ charge termination, I’ll at least be mindful of this possibility. Like CG said, I love my VP2s, but of course anything can malfunction.

I did overcharge some LFPs one, single time years ago - however in that case, it was definitely due to my failure to check the switch, which I found to be at the wrong setting. BTW - Charging an LFP to 4.2V generally won’t result in a catastrophic failure, but as I recall I did believe that those cells had been degraded by the event, and I retired them.

Edit: BTW, in addition to double-checking the switch, I also always check the charge setting indication on the display during initialization, as a simple bad or flaky switch could also result in an incorrect selection - even if the switch appears to be in the correct position physically.

LFP is pretty safe and charging them to 4.2 shouldn’t hurt anything but the cell itself. If someone has an LFP they’re willing to put at risk to test their VP2, I would appreciate it. I wanted to have a second on hand, but won’t buy another if this is by design.

I got this charger because it is the only one I could find that allows a permanent setting for LFP. All others require changing a setting each time you start a new charge. I’d also happily take suggestions for a different charger.

I’ve never had any issues leaving batts on charger. VP2 should shut down charging when senses full capacity

As long as your unit is not too old, I’d contact Xtra CS. They are good for warranty and fast to deliver from Socal warehouse

Did you not only check the switch physically, but also note the displayed charge voltage during initialization when this occurred? (which is a key question). If not, I highly recommend doing so as standard practice for the reason I cited. I have no doubt that’s (Edit: likely) a cheap slide switch.

If I knew of another charger I would prefer for this application, I’d probably already have one, but I don’t. CG probably doesn’t either or he likely wouldn’t have 2 NIB (which I’ve also considered since they’re discontinued without a replacement I’m crazy about).

I do have a pair of new 16340 LFPs that I never even charged or used, because I didn’t like the lack of specifications available for them after buying them. They’re still in the wrapper. When I have time, I’ll try to run a test with those (leaving them in the charger long after termination) just to see if the issue occurs. I don’t plan to use those cells, so I wouldn’t be risking anything of value to me. They’re just sitting in my retired cell bin. If / when I do that, I’ll post the result.

I use K2 Energy cells, and for the record (at the link below), you can see that the specs indicate under ‘Maximum Operating Conditions’ that they aren’t likely to do the China Syndrome thing on you if you do charge them to 4.2, but I remember having good reason to believe that the experience degraded them (probably a capacity test, but I don’t recall for sure). I would not recommend anyone doing it intentionally.

Thanks Desertcat, I appreciate it. Yes, I did check the display, and it is correctly displaying the position of the switch when no battery is in the unit.

“is correctly displaying”

That’s present-tense, and I actually meant ‘at the occasion of the failure’, but maybe you checked then too. That’s the time we need to verify, though. Any problem it had could have been intermittent and OK now. Not being picky, just precise:-). I’ll try to run that test and let you know.

At charge end, the display always shows the cell voltage, not the set voltage (because a new charging cycle has not begun). I had both bays populated so I don’t know what would have displayed for the switched voltage. It’s definitely possible I didn’t notice it displaying 4.2 before pulling the cells off.

I was able to reproduce an overcharge a second time after verifying the switch and display voltage was correct, but I only let one cell go to 3.7 (and the display did show that for the populated bay).

I’m going to try bringing it up to 3.7 once more with a sacrificial cell just to make sure I’m not going crazy, and hopefully I can get a photo of that along with the empty bay displaying 3.2 to show Xtar customer service.

Status update: I unwrapped 4 unused LFP 16340s (Tenergy) that were in my ‘backup’ stash last night, checked voltages, which were correct / nominal, discharged them to ~2.8 V using my Opus 3100, and let them sleep. Today I set the switches to the ‘3.2V’ nominal voltage setting for this chemistry, loaded the cells into all 4 bays of my 2 VP2s,and powered up both chargers. All indications as displayed during initialization were normal / as expected. After normal indications during the charge cycle, it was displaying the correct ‘3.6V’ CV charge value during the final stages, and the charging cycle terminated around the expected time with green LEDs on. The displayed cell voltages have drifted downwards exactly as expected so far - to the expected current displayed values of 3.35 / 3.36V at this point, which is 2 hours after termination. Since I’m home and awake for the duration of the evening, I’m going to leave them sitting, untouched, to monitor for anything unexpected. So far, all is / has been exactly normal and as expected. I’ll update after letting them sit until ‘down’ time. Just wanted to update you at this point.


Note: I have edited this post to eliminate some text I had erroneously inserted which incorrectly stated the initial voltage displayed upon power-up / initialization.

Thanks again, Desertcat. Here are my most recent findings.

I was mistaken about the display of the end voltage: Mine had been sitting green on the charger for about 24 hours now, and the display read 3.60V and 3.20V (on the empty bay). But as soon as I re-started the charging cycle, it displayed the actual battery voltage of 4.22, confirmed with a multimeter.

So, once it reaches normal charge voltage, it either freezes the display, or stops checking its voltage (or both), but then continues charging it. I’ve got it on video for Xtar support.

Just making sure: your charger’s displayed voltage at end-of-cycle drifts down without any action taken on your part? That alone points to a defect or a manufacturing difference with mine.

No, it doesn’t. Cells will settle down a bit, after charging subsides. With older cells, or shitty cells, the amount might be more profound.

That doesn’t explain your ‘4.2v’ termination with 3.2v LiFePO4 cells, but settling is common.


Hi Chris, I’m referring to the voltage display of the cell at end of charge. I expect the cells to settle, I’m just trying to clarify if the charger displays the actual voltage of the cell as it is settling. Because mine does not… it seems to stick the display at 3.60 regardless of the actual cell voltage, until I restart the charger.

That must be how I missed the overcharging at first, and if any others’ chargers are behaving the same, it’s possible they are missing it too.

Are you measuring things with a quality volt-meter?


And is the DMM battery still good?

The display voltage, multimeter, and Anduril voltage display agree.

Let me get back to you later after I review this further, but I think from the discussion I’m beginning to understand what you’re seeing - but unfortunately, not what’s causing it (root cause). Here’s something to consider for now. If you’re seeing what you indicate, and I understand correctly, your charger is indeed likely overcharging (too high voltage), and I’d recommend you stop using it until you get it sussed out.

It sounds like your charger is indeed charging to an incorrect CV value which is much higher than the correct 3.6V, and is likely exactly what your voltmeter indicates. If it is higher than 3.6V, which it should NOT be by any significant amount, the VP2 will not show the correct value - it will only show 3.6V, although your voltmeter certainly will and apparently does.

The key is this: Go back to your LYGTE link to the test / review, and I think you’ll see why after completion of the charge cycle, what you see on the VP2 display, and what you see if you check the batt voltage w/ your voltmeter do not agree, and it’s apparently because the charger is exceeding the correct CV phase voltage. Here’s the key text:

The voltmeter has a good precision, when charging it will show slightly above the battery voltage, as expected. Sadly it will not really work above max. charge voltage: it will show one value and then stop updating.

So, if it’s overcharging (voltage), you will not see it on the VP2 display, but will only see it with your meter. Believe the meter, because it will be the correct value. Good thing you have one, just for situations exactly like this one.

So that only leaves the question of why your charger is charging to CV value approprate for a 3.6/3.7 V chemistry cell (which is 4.2V), instead of the CV value appropriate for your 3.2 V (LFP) chemistry (which is 3.6V). If so, it’s a serious malfunction of the VP2, which is why I recommend not using it at this point.

Given that text from the review (thank you LYGTE), this is starting to make sense to me. It probably means your charger has a fatal problem, which is not good news, but at least it’s making sense.

Is this making sense to you too, or is it just me:-)

It makes perfect sense, thank you.

I noticed that my VP2 did not have that yellow anti-counterfeit sticker that’s pictured in the review thread (Test/Review of Charger Xtar VP2). Maybe mine is a fake. Illumn might want to know about that. I’m bringing this up with them first, and will check with Xtar if they don’t have an answer for me.

I’m actually watching / noting some numbers on a charge right now, and will have a few further comments at some time, but that stuff is all academic at this point Your observations noted in your posts have essentially proven that your specific VP2 is defective (which is rare, it must be said), and LYGTE’s excellent (and important) observations I quoted above have determined exactly why what you were seeing was (justifiably) confusing (due to a design deficiency that your failure exposed in the VP2), so you’re ready to take action on the charger resolution. You have all the information anyone you excalate to should need - unless they want photos or something). Again, I would not continue using that charger at this point given what you’ve seen.

Yes - Both of my VP2s DO have the yellow stickers on the front which appear to be the same as the one pictured in LYGTE’s test. Its absence on yours is questionable I guess. Just FYI, I purchased both of mine ~5 years ago from Battery Junction.

Anything can fail. That said, anecdotally I don’t recall hearing of any serious failures with VP2s - primarily just dimming of the displays over long periods of heavy use. Perhaps a non-critical failure I’m forgetting, but the falure you’re seeing is what I consider a “serious” and “critical” failure (one that can cause further damage). That said, it may sound crazy, but in your situation I would still consider replacing the failed unit with another properly-functioning VP2 - and that would be underscored further if the authenticity of your unit proves to be suspect.

Again I’ll follow up a bit on this, but you pretty much know what you need to know to deal with the unit you have.

I hate to tell you this, but I just noted that some sort of malfunction happened with one of my VP2s tonight while I wasn’t paying attention. It may be similar to the overcharge problem you observed, and is nothing I’ve seen before. Mine appears to have over-charged a cell that I left in the unit unattended for quite a while following a normal charge cycle (knowingly), and when I started to put things away a few minutes ago I just noticed it. There may be more to this than I thought, and it’s possible that your unit may not be just an isolated failure, and one of mine may have exhibited a similar failure - which means this could be a design issue here rather than a one-off failure as I thought. I’ll be investigating further, but for now, be aware that there could be more to this story than just a single failure with your particular unit. It doesn’t change what you saw or the failure you experienced, or your efforts to possibly get your unit replaced, but the issue may go deeper than that. Obviously more testing is now required on my chargers, and I’ll let you know what I find. If what I think happened actually happened, I may be looking for a replacement for both of my units - which of course isn’t good news:-(