Plenty of Capacity - but voltage drops quickly?

What (in general) does it mean if you have a NiMh cell that delivers it’s stated capacity, but seems to drop voltage too quickly?

For example, on a discharge testing hobby charger, 1.2v NiMh cell goes from 1.40v to 1.1v within 5 minutes, then takes almost two hours to go from 1.1v to .90v

So, it can supply CURRENT for a long time, but not Voltage?

The battery in question is an AA and has an IR of 71 mOhms

From my testing on a Duraloop several years back that seems about normal to me. Discharged at 1 amp.

Yes, thanks
Your graph does seem to agree.

One difference is I discharged at 500mA

in my graph below, the Left side shows the Discharge and the Right side the Re-Charge (the flip took place at the 3.0 hours mark)
Just as in your graph, it dropped from 1.5v to about 1.10v in about 30 minutes.then held there for about 2 hours then began to drop sharply to .92v

One remarkable notation……
The battery in question had previously been in the trash bin. On several different chargers the Capacity measured only approximately 163mAh
I ran it through 4 different tests to be sure. Then, before putting it outside with the garbage bin, I decided to do something I felt was a waste of time.
I connected it to a CC/CC DC Power Supply at 10v and 1.2Amps for 5 30 seconds bursts. I was flat amazed to see it recover like this.
I did this with a few other NiMh batteries with the same result. batteries that had tested to have nearly no significant Capacity remaining, now were back to approximately 80% of their new capacity. Too bad LiLo’s don’t work like that.

Interesting discovery, wondering what brand of cells they are. Charge one up and let it set for a few weeks or a month and run the test again. That should give you A idea on how usable they are with their self discharge.
Did the cells have any kind of marking?

Yeah, NiMH are rated 1.2V. Probably gets to 1.4V right off the charger (overstuffed), then drops to what it should be, and is essentially flat ’til it’s about to give it up.

Yeah, I used to blast the Hell out of NiCd cells that grew “whiskers” and would self-discharge to almost nothing. Charge up a huge cap with and hit it good, enough to almost spot-weld the wire to the cell.

Would sometimes go from 0V (dead short, wouldn’t charge at all) to start charging. After all, that “blew the fuses” that were shorting (+) and (-) together.

Didn’t think NiMH cells might have anything similar.

They are HarborFreight 2200mAH AA’s. About 3 years old.
I’m trying it now with some Old, Ray-O-Vac Platinum 2100mAH NiMh batteries.

These have had a significant reduction in IR after the blast treatment so I guess it could have been those shorting tentacles here also.
No idea how long they’ll go but for now they’re back in the Good-To-Go Bin.

I go overboard keeping track of my batteries and their performance and history so I’ll watch these and report back down the road. lol

IR in “general” is decent for getting a picture of health for NiMh. If it’s under 100, they tend to be OK. Once they get over 200 they are going downhill, over 500 is lousy, higher than this is really poor. As IR goes up, current ability goes down, but sometimes capacity will seem to hold up anyway, but ONLY for really low draw.
BUT, IR doesn’t seem to be stable on NiMh. If you cycle a poor one a bunch it goes down. Then use it in a low draw LED for or let it sit awhile and it goes right back up.

I have had some NiMh that pretty consistently show low IR, but they are still lousy batteries. I have no idea why. They are definitely outliers though.

As stated, completely full is 1.4v. Nominal is 1.2v. On a VERY low draw test they will drop to 1.2v or a bit below and hang in until depleted and then drop fast. As you up the current draw the voltage drop is more severe and the capacity will appear as less. HJK’s NiMh draw graphs show this clearly. A really poor battery will fail in minutes if asked for a current draw it cannot supply. I tend to sort my battery quality based on the requirement. I have a bunch of motion controlled LED’s around the house and they are quite happy with batteries that are pretty thrashed, though I do have to charge them more often.

The “blast/zap” treatment works, but I’ve found it temporary at best. Once you need to do that the cell is on it’s last legs. I don’t bother anymore. Good cells are easy to come by.

Old Harborfreight tend to be junk. Lenmar are awful. High capacity Annsmann go downhill fast, as do the pale blue Tenergy. I have some old Rayovac 4.0 (blue and silver wrap) that I cannot believe how well they’ve held up. I use them continually in the motion lights because they are just a tad smaller so less difficult to get in and out.

This has me thinking. I usually do my discharge tests at 500mA because most of my applications are low drain.
But….as you suggested, I need to do some high drain discharge tests just to see where the battery is internally.

Mist of my 18650’s are used for flashlights, Headlights (worn on your head) and portable USB Power packs for cell phone recharging when I’m away from power. AA’s are mostly used in low drain LED motion lights and some convenience LED lighting (very low drain)

I guess I don’t have many high drain applications for my round cells except my Convoy L6

Specification for capacity testing is to do 1/5C (capacity). That’s what manufacturers are suppose to do. Sometimes I think they use 1/20C to get the highest capacity they can publish.
One of the problems with most of the home grade analyzing chargers, and similar hobby chargers, is their limited discharge ability. It’s OK for small, low draw type cells but can’t really test higher draw.

I purchased a bunch of these 5 years ago from this thread. Target Clearance - White-top Duracell battery 4-packs (EDIT) now $3.65 (Duraloops)
They were already a year or so old by the date code so 6 years since made.
They are ever single one still performing well. They have not seen heavy use but occasional use in remotes, scales and toys.
I have used a few in flashlights as well, they are very good batteries and I will be buying more when they crap out.
Rebadged Eneloop’s. I can confirm they have lived up to their hype and that is one of them in battery discharge graph I posted above.
I have also had good luck with the older LSD Tenergy brand.

You know who also has EXCELLENT NiMh AA and AAA batteries?


LADDA. Some of the best I’ve had / have.

Don’t be worried about the big voltage drop (yet still get high capacity ratings)…nimh have basically solid high draw chemistry that even with high amp draw devices, the voltage just stay basically flat at 1.2v for a very long time, allowing to stay in Turbo mode much, much longer. I noticed this in my Logview data using my iCharger, which can draw up to 7A.

My Fenix TK70 ( 4 x D cell) reportedly draws 9 amps and my Tenergy/Imedion Ds can sustain this light in Turbo until the cells no longer have enough juice to power it in that mode…Remember, the TK70 is one of the very few lights that DOES NOT drop to High from Turbo.

For comparison, Energizer alkalines cannot even last a minute on Turbo in my TK70, it just drops to High that short.

I’ve had the opposite experience with 8 AA I bought years ago. Some were at 0 volts out of the package and all of them discharged much faster than my Enloops. I’ve also had hit or miss experiences with the Tenergy AA and AAA lsd batteries. And the Duracell 2400 Enloops Pro clones were awful for me. Most of them won’t charge on a number of my chargers because of high IR (over 2000 according to my Opus 3100 but I’m highly doubtful of the IR test accuracy).

These days I just stick to Enloop and Fujitsu for their quality and consistency

Yes, my Tenergy and Imedion D cells are trash, but unfortunately Eneloop does not make D cells, so…

Ladda, Ikea, duraloop, Fujitsu, Eneloop, all made in the same factory at least specific ones.
Made in japan by FDK.

Cool. Good to know.
Yeah, I haven’t had the problems with ANY of my batteries like SIGShooter has.
Even my HarborFreight NiMh batteries have done an excellent job and last years and years.

Maybe he stresses his (high drain applications) a lot? Or maybe his charger(s) are defective?

I don’t use lower priced cells for high drain applications. If I need a high drain battery I go to IMRBatteries and buy some high quality cells rated for high drain.
Only thing I have that is really high drain is my Convoy L6 flashlight.

Some of my NiMh’s are 7 years old or more (Ray-O-Vac, Duracell and HarborFreight) and still have 75% of their original capacity.

It could be that I don’t use my batteries enough. I mostly use 18650’s so my NiMh batteries are rarely used, many times sitting in my closet for years at a time. I run discharge tests on them every so often and the Eneloops always perform well but the Ikeas/Duracells/Tenergys are at the other end of the spectrum.

I’ve charged the batteries on a number of chargers so I doubt they’re all defective :slight_smile: