CCCV Charging of NiMH Cells

[AA Cycler]

The conventional wisdom says charge LiIon with CCCV and NiMH with -dV. But how bad CCCV charging is for NiMH cells?

Hard to say so I started a CCCV Charging of NiMH Cells test. The results are not conclusive yet, I am getting too wildly different data on my 2 test cells. I will add a 3rd "confirmation" cell to get more conclusive results. Anyway, I can already conclude what I expected - the cells charge to lower capacity, but last longer (more cycles). I will provide more data once I finish the test.

There is a slight problem with this test though. There are no analyzing NiMH chargers with CCCV. The closes I got is the SkyRC MC3000 charger configured for RAM chemistry. It uses CCCV on Von (I wish it was Voff, see my "improved" CCCV algorithm below) therefor it is slow. And it uses pulsed charge: 7sec on, 3sec off (basically 70% duty cycle) therefor it is slow...

My "improved" CCCV algorithm would monitor 2 voltage levels

• Voff voltage with charge current off (open circuit) for the CV phase
• Von voltage with charge current on for overheating protection

The algorithm would look like this:

• start the CC phase with 0.5C charge current (1000mA for AA, 400mA for AAA)
• keep lowering the current to keep Von at max 1.70V
• keep lowering the current to keep Voff at max 1.45V
• stop when the current drops below 10% (100mA for AA, 40mA for AAA)

Could anybody reprogram the firmware of an existing NiMH analyzer and put this algorithm in? I would buy 2 such analyzers

Anybody?

Cheers,
AA Cycler

 

[WalkIntoTheLight]

This should be another interesting test of yours. I'm still watching your AA trickle-charge test, which is finally showing differences after what... a year??? You have more patience than Job.

One note on this CCCV test. You're choosing to terminate at only 1.45v. Why so low? I notice that -dV usually terminates at well over 1.50v. If you terminated at 1.5v, then perhaps there would be less of a capacity drop.

BTW, it looks similar to your trickle-charge test. Perhaps it results in the same thing happening to the cell.

 

[parametrek]

I was under the impression that there is a lot of CC-CV NiMH charging done out there. I've heard it referred to as "bulk" charging and can get up to around 70% of the capacity. Going higher requires dV/dt termination. This is why Eneloops are "precharged to 70%" because they can safely do it quickly with simpler chargers.

 

[sbj]

...There is a slight problem with this test though. There are no analyzing NiMH chargers with CCCV. ...

Take a look at the Junsi X6 (or X8) hobby charger.
https://www.progressiverc.com/icharger-x6.html
https://www.youtube.com/watch?v=gDhGxHfmaZw

It has a NiZn mode, where you can set the charging voltage of 1.2-2.0V and the discharge voltage of 0.9-1.6V. The termination current can also be selected freely (as a percentage of the charging current).
Besides that, it still has a user program 1.0-4.8V; 0,5-4,5V.

The disadvantage:
As a hobby charger, it has no independent charging slots.

However:
With the CC-CV charging method you can charge any number of cells in parallel.
Additionally it can also charge NiZn serially with balancing.

sbj

 

[Bazsy]

You could check HKJ-s charger tests as I recall him noting that some of the cheaper Chinese chargers do use CCCV för NiMh.

 

[InvisibleFrodo]

I'm confused why the OP would want a NiMH charger that uses any algorithm of CC/CV. I don't see any advantage to charging NiMH that way.
Am I missing something?

 

[sbj]

With the -dV charging method, there is always a significant warming of the cell at the end of the charge, which then causes the voltage drop of the cell. The chargers evaluate this voltage drop as end-of-charge detection. But heat means stress and aging for the cells.:huh:

I think, the approach of AA_Cycler is now:
Find the right parameters for the CC-CV charging method of NiMh batteries, to get the batteries full, without getting too warm.

I suspect that the appropriate charge voltage and current values for aged cells would have different amounts, than for new cells. The problem will be to fit that right.

 

[WalkIntoTheLight]

I suspect that the appropriate charge voltage and current values for aged cells would have different amounts, than for new cells. The problem will be to fit that right.

Yup. It will also likely vary by brand. It might even be different depending on the "generation" of the cell. For example, I notice that later generations of Eneloops terminate the -dV charge at a lower voltage than earlier generations. I don't think this is due to the age of the cells, because IIRC new early-generation Eneloops still terminated high. Okay, I know this is after the -dV drop, but it's probably similar for CC/CV.

 

[AA Cycler]

Take a look at the Junsi X6 (or X8) hobby charger.

Thanks for the tip, but the price is too high. And I don't need a hobby charger.

You could check HKJ-s charger tests as I recall him noting that some of the cheaper Chinese chargers do use CCCV för NiMh.

I went through all the chargers HKJ tested and found only a few that resemble CCCV. None were analyzing chargers. After ruling out the dodgy mains powered ones (I don't want to be electrocuted) I ended up with E-SYB E4 (too low charge current), Opus BT-C3100 (kind of), and Yonli Q1 (no real CC phase, CV is 1.43Voff). HG-1206W looked OK, but is mains powered and I don't trust those.

I think, the approach of AA_Cycler is now:
Find the right parameters for the CC-CV charging method of NiMh batteries, to get the batteries full, without getting too warm.

Yeah, pretty much. But I am not looking for the holy grail parameters, I already picked ones I like

AA: 1000mA for CC, 1.45Voff for CV, 1.7Von as overheat protection for aged cells, 100mA termination current
AAA: 400mA for CC, 1.45Voff for CV, 1.7Von as overheat protection for aged cells, 40mA termination current

I just want to test the viability of CCCV for NiMH and how it affects the cycle life.

Yup. It will also likely vary by brand. It might even be different depending on the "generation" of the cell.

and temperature...

What ISDT does for LiIon is very close to what I want to do to NiMH, but not at 4.2Voff level, but rather at 1.45Voff

with an additional Von cap at 1.7V to downregulate charge current for aged cells.

Cheers,
AA Cycler

 

[InvisibleFrodo]

As some have pointed out, if you wanted a full charge, the final voltage in a CC/CV system would vary by brand, by generation, by cell age, and by number of cycles the cells have been subject to. More age and/or more cycles would mean higher internal resistance, which will alter voltage under charging load. I suspect that would lead to premature termination of charge cycle or extremely slow charging in the CV phase while charging older cells.
I don't think there would be any way to really know exactly what voltage to terminate at while still getting a full charge as cells change with age.
It's an interesting idea.

 

[WalkIntoTheLight]

AA: 1000mA for CC, 1.45Voff for CV, 1.7Von as overheat protection for aged cells, 100mA termination current
AAA: 400mA for CC, 1.45Voff for CV, 1.7Von as overheat protection for aged cells, 40mA termination current

1.45v sounds kind of low. My Eneloops usually come off the charger (using a -dV termination) at over 1.50v, if I grab them right away. They'll quickly drop to around 1.43v within an hour, but it's clear that the normal method of charging gets them far higher than 1.45v.

Is trickle-charging at 1.45v (which is similar to what you're doing) going to result in a full cell?

 

[AA Cycler]

As some have pointed out, if you wanted a full charge, the final voltage in a CC/CV system would vary by brand, by generation, by cell age, and by number of cycles the cells have been subject to. More age and/or more cycles would mean higher internal resistance, which will alter voltage under charging load. I suspect that would lead to premature termination of charge cycle or extremely slow charging in the CV phase while charging older cells.
I don't think there would be any way to really know exactly what voltage to terminate at while still getting a full charge as cells change with age.
It's an interesting idea.

You are right, the higher the internal resistance the higher "the voltage under charging load". But I am not talking about this voltage

All NiMH chargers show the open circuit voltage of the NiMH cell. They turn the charge current off momentarily to take a voltage reading and that's what they display. This open circuit voltage is not dependent on internal resistance. In the OP I wrote "improved" CCCV, better wording would be "NiMH adjusted" CCCV which uses open circuit voltage for the CV phase. Does this make sense?

Cheers,
AA Cycler

 

[InvisibleFrodo]

No. That doesn't make sense. If the open circuit voltage was at the CV level, the charge would be complete. The CV phase controls the amp rate in order to hold the voltage at the level you've selected. It can't be done using open circuit voltage because again, if for example your CC/CV Target was 1.45V and your battery has an open circuit voltage of 1.45V, you would have 1.45V under no load, the charge is complete.

CC/CV is meant to bring voltage up to a predetermined level, and then manipulate the charge rate in order to get the cell to stay at that voltage. The amp rate tapers until open circuit voltage is that predetermined voltage. If you're half way through the CV phase and check the open circuit voltage, it will invariably be below your predetermined voltage level.

 

[WalkIntoTheLight]

I don't think he's trying to do a real CC/CV charge. It seems like he's trying to do something close. Perhaps a true CV phase for NiMH would just take too long, since it would end up being a trickle charge. So, he's willing to charge at 1.7v for pulses, as long as the resting phase stays at 1.45v or below. I guess with shorter pulses as the charge gets near the end, or maybe with a lower voltage (and thus current) than 1.7v.