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.