I recently got an XTAR VX4 charger and am doing some initial testing of some LiFePO4 cells (AA and smaller). To set some expectations: I’m new to LiFePO4’s and to be honest I’m not particularly knowledgeable with rechargeables in general. I’m mostly a “charge 'em and hope they work well enough” kind of guy.
When I’ve done some grading of NiMH’s in the VX4 it does a charge/discharge (and count mAmps)/charge cycle then says the cell is FULL and reports the mAh counted in the discharge phase. That matches what I expect to happen.
With the LiFePO’s it seems to run that cycle repeatedly, eventually giving the FULL notice and mAh report after 3 or maybe more repeats of those cycles.
Is that normal and I should just relax, or is it an indication that either the VX4 or the LiFePO cells are defective in some way?
do the results match expectation?
Not familiar with the VX4. Does it have a specific setting for LiEePO4?
They charge to a lower voltage than Li, have very flat discharge curve, and should terminate at a lower voltage. That may mess with a cycling protocol.
Can’t say if it matches expectation - the thing is still going. These are 10440/AAA cells and claim 500mAh if that gives anyone an idea of what should happen.
The VX4 does have a special setting for LiFePO4: a long press of the “C/V” button toggles between the default 3.6/3.7V Li-ion mode and the 3.2V LiFePO4 mode.
Thanks for pointing that out. Do some LiFePO4 cells have an overvoltage protection circuit? I’ve only heard about it in regards to Li-Ion cells, but I’m pretty new to all this. If so, is there a way I can determine if these do (other than the VX4 behavior?). It seems that most info about LiFePO4 relates to things like solar arrays and other things that are on a larger scale than the rinky dink AA and AAA’s I’m messing with.
I’ll ask the vendor I bought the cells from about the OVP possibility and ask XTAR about their thoughts on the VX4’s behavior.
At the moment I have stopped the perpetual “grade” that was occurring and put the cells back in with the VX4 set to charge mode. That has completed and I put one of those cells into a basic ZB2L3 tester with a 30 Ohm load to draw down the cell with 100mA until a 2.6V cutoff. I’ll report what that test says about the mAh capacity for that cell.
Select Grade
Expected result:
Charge, Discharge, Charge cycle to occur. Capacity to be displayed after the Discharge step.
Actual result:
Charge, Discharge, Charge cycle keeps repeating.
Are you using protected LiFePO4 cells? The Discharge step might cut-out if the voltage drops below a certain voltage and break the C>D>C cycle. And the Charge step might cut-out if the voltage exceeds a certain voltage. Ideally the VX4 would work within appropriate voltage levels for LiFePO4 but some protected cells might have different cut-offs.
The steps you listed are exactly what I did with the actual result you describe.
I have asked the vendor. There’s no indication on the cell itself as far as I can tell; there is a single line of text printed on the cell:
“IFR AAA 10440 500mAh 3.2V”
A couple cells that I have run through the ZB2L3 tester with a 100mA draw and 3.6V 2.6V cutoff have resulted in a readings around 220 mAh. About half the claimed capacity. It wouldn’t surprise me if the claim is exaggerated (to put things nicely). But it also wouldn’t surprise me if I have done something wrong.
I plan to put the cells that have been discharged by the ZB2L3 back into the VX4 for another plain charge mode to see if anything changes (for the better hopefully).
Also, I’m pretty sure even if the cells truly are 220mAh capacity, it’ll probably be OK for my planned application: a couple of multimeters. I want to avoid storing the meters with alkalines in them to avoid damage from battery leakage. A LiFePO4 with a dummy cell should make the meters happy with the voltage and my understanding is the LiFePO’s are not prone to leakage.
This looks correct. From a Google search, for small cells, at least, the LiFePO4 cells seem to have half the capacity of NMR (14500 LiFePO4 500-600mAh while NMR 1000mAh to 1250mAh).
Just to add, for devices that have auto-shutoff you could use Energizer Lithiums, which also don’t leak and would provide longer runtime than LiFePO4 cell and dummy.
Typo I think. Should be 2.6v.
If they are ‘generic/no name’ cells, the capacity could be well below claimed. I RARELY see ANY cell other than top end brand names come close to spec.
FWIW I have an IFR14450 (AA size-claimed 600mAh), that tested with discharge on a ZB106 tester to 520mAh.
No way a 10440 cell would have 500mAh…none. Bogus claim. I think a good cell of that size might be 300mAh. Your results are within my expectations.
I seriously doubt they are protected.
You guys have been amazingly helpful. My take-aways are:
the 500 mAh claim is bogus (as I suspected even when purchasing)
220 mAh is OK-ish, but not great for this size LiFePO cell
VX4 can have trouble in some cases with (likely related to protection) grading cells
I’ll send a note to XTAR about the VX4 grading issue. I have no idea if the VX4 firmware is upgradable, but XTAR could maybe fix it for new production.
Don’t rule out a problem with the charger. I bought a Xtar VC4 that worked fine for a little while and then either tried charging Eneloops like Li-ions or indicated the cells were still being charged at 0 MAs way before they were fully charged. According to Xtar, this is how the charger is supposed to work.
I bought two SB2307 18650 3.2V LiFePO4 1600mAh cells and ran Test mode at 500mA in the Vapcell BL4 and Grade mode in the XTAR VX4. 1961mAh and 1996mAh capacities were measured respectively.
They both peformed a Charge, Discharge, Charge cycle and stopped.
I’ll test again when I get time and make some graphs. It’ll be interesting to see what voltage these dropped down to. But first I’ll test an upcoming charger with these LiFePO4 cells.
