Longevity of NiMH cells

[saturation]

There is actually very little data on calendar life, meaning the total number of years NiMH cells can still be used.  This assumes you have optimized charging methods, cycles and ambient conditions.

I'd like to hear what end users are able to achieve with NiMH.  This includes individual cells to battery packs.

A paper by Cobasys is one of the few to suggest lifespan can easily be over 10 years, bottom of page 5, where the pack is rated to 80% of its original mAH.

http://www.cobasys.com/pdf/tutorial/Considerations-for-NiMH-in-Stationary-Apps-battcon05paper.pdf

Li ion or polymer batteries degrade whether used or not, and by 4 years, most are unusable.  Worse, most users don't realize by 2 years, mAH is often down 50% in 2 years.  A new Li secondary chemistry, Li iron phosphate, claims to have a 10 year calendar life.

As Li secondaries have high power densities and are preferred when necessary to reduce size, such as cell phones, its less of an issue in devices that use traditional sized batteries like AAA, AA, etc., and have low power consumption: MP3 players, clocks, LED flashlights, electric toothbrushes, shavers, external camera strobes,  etc., and are far more economical to run, particularly with the low self discharge variety.

[Zero999]

It depends on what you mean by energy density?

Li-ion beats NiMH mass wise but volume wise there's not much difference. I thought cost was also a factor, NiMH is more expensive due to the high cost of nickel.

[NiHaoMike]

The main problem I have seen is internal resistance increasing over time. In low current applications, that is not much of an issue and the batteries end up lasting a long time. The NiMH batteries in my TI-83 are approaching 7 years old and they're still working great.

[saturation]

What I experience is that over time, where the power consumption is applicable, NiMH cell are more economical for end users to operate over Li ion or polymer because its still has usable life for many years far over Li ion or polymer.

Here's a chart that clarifies your questions:

http://en.wikipedia.org/wiki/Rechargeable_battery#Table_of_rechargeable_battery_types

In graphic form:

It depends on what you mean by energy density?

Li-ion beats NiMH mass wise but volume wise there's not much difference. I thought cost was also a factor, NiMH is more expensive due to the high cost of nickel.

[Zero999]

What about efficiency?

NiMH is poor compared to Lipol or even lead acid so is a bad choice in an application where low energy consumption is important such as an electric car or UPS.

[Mechatrommer]

i bought many AA NiMH ranging 1800-2700mAH when i started photography few years back:
1) Sony (immitation )
2) GP
3) Energizer
4) PowerEx
4) PowerEx Imedion

after 2 years of usage, they start to self discharges within just few days. usable, but need immediate charging before use. usage frequency: 1-3x in a month for camera flash, camera, anything camera. i never trust any brand anymore that can do >3years that can retain charges for more than a week. 3 vote for 5 years? what are you talking about, people?!

edit: similar to LA pack, ~2years lifetime... my usage, not too severe, just normal. buy new every 2 year.

[saturation]

Sorry to hear mecha, its probably your charger or the batteries are made poorly.  If its constant current, it will kill your batteries far before their rated charge cycles.  Better chargers are pulse or the most recent is burp chargers.  Make sure your batteries stay under 35^oC during charging, and if it exceeds it you have to reduce the charge current or insure its properly cooled.

http://www.crypto.com/chargers/

The maximum charge temperature is 45^oC, if you exceed it it will shorten cell life, a good charger should automatically stop charging until the cells cool down.  The absolute cutoff temperature is 60^oC.

Other than that, assuming you have good chemistry NiMH cells, those are the key issues.  Optimal NiMH charging is a bit involved, some of your cells may only need to be 'reconditioned'. Only recently has a charger of this quality come to market:

http://www.amazon.com/Maha-Powerex-MH-C9000-WizardOne-Charger-Analyzer/dp/B000NLUSLM

i bought many AA NiMH ranging 1800-2700mAH when i started photography few years back:
1) Sony (immitation )
2) GP
3) Energizer
4) PowerEx
4) PowerEx Imedion

after 2 years of usage, they start to self discharges within just few days. usable, but need immediate charging before use. usage frequency: 1-3x in a month for camera flash, camera, anything camera. i never trust any brand anymore that can do >3years that can retain charges for more than a week. 3 vote for 5 years? what are you talking about, people?!

edit: similar to LA pack, ~2years lifetime... my usage, not too severe, just normal. buy new every 2 year.

[Mechatrommer]

Sorry to hear mecha, its probably your charger or the batteries are made poorly.

i'm using energizer http://data.energizer.com/PDFs/ch1hr.pdf and currently powerex mh-c801d charger. guess those are poorly made too?

[saturation]

If charged properly, the inefficiency can be compensated for, the advantage in doing so would be to extend the calendar life of the cells. 

For example the original Prius of Toyota uses NiMH packs, and some old packs have been tested.

http://en.wikipedia.org/wiki/Toyota_Prius#Battery

For UPS case studies, the Cobasys paper linked below describes its usage. 

or

http://ieeexplore.ieee.org/Xplore/login.jsp?url=http%3A%2F%2Fieeexplore.ieee.org%2Fiel5%2F4018071%2F4018072%2F04018103.pdf%3Farnumber%3D4018103&authDecision=-203

What about efficiency?

NiMH is poor compared to Lipol or even lead acid so is a bad choice in an application where low energy consumption is important such as an electric car or UPS.

[Mechatrommer]

http://en.wikipedia.org/wiki/Toyota_Prius#Battery

The cost of replacing the battery varies between US$2,200 and US$2,600 from a Toyota dealer

in 10 years? well, 2200 / 5 = $440/2 years. i guess my AA batt cost still alot cheaper

[Russel]

I almost gave up on NiMH rechargeable cells years ago because the self-discharge rates were so high. Since then, I've am really enjoying Eneloop LSD (Low Self Discharge) cells, in both AA and AAA sizes. At this point I've almost eliminated every alkaline cell in the household. I find that good NiMH cells don't leak, whereas using alkaline cells is like playing Russian roulette with your electronics.

I've got Eneloops and AccuEvolution LSD cells that are three years old and still retain about 95% of their advertised capacity. I'm wondering how long they will last under normal use. I usually cycle each cell about once every three to six months, if it doesn't get discharged through normal use, such as cell used in TV remotes. Using the MH-C9000 charger, I run each cell through a break-in cycle once a year. So far, I'm happy with the results.  I find the Maha MH-C9000 charger to be very useful for charging cycling and evaluating AA and AAA cells. I also use it for C and D cells with a homemade adapter.

I keep extra unused cells at about 10% state of charge in a sealed zip-lock in the refrigerator. I've had good luck refrigerating unused cells to preserve them, NiMH and Lithium-Ion and Lithium-Polymer. One Lithium-Ion notebook battery pack that I have still provide good performance after six years, although it has spent most of it's life at about 40% state of charge, in the refrigerator.

In my experience, the combination of Eneloop AA cells and the Maha MH-C9000 charger is just about the most ideal for applications that use AA cells.

[saturation]

They are well made products, I know something off them but a lot depends on the charging algorithm.  The 1 hr charge mode should be used only for emergencies, as it reduce your cell life quite a bit.  If its constant current, it will overheat easily.

The energizer charger seems not well designed, it claims a cutoff of almost 60^oC, its guaranteed to shorten the cell life. 

Here is a test on the Maha C9000 charger made in 2007, comparing charge temperature and charge voltage.  You can see the pulsing at the top.  You can easily check the output voltage of your chargers by putting a cell on it and the Rigol across the battery as it charges, and view the charging waveform.  Likewise, put a thermocouple from your DMM to monitor charge temperature.

Sorry to hear mecha, its probably your charger or the batteries are made poorly.

i'm using energizer http://data.energizer.com/PDFs/ch1hr.pdf and currently powerex mh-c801d charger. guess those are poorly made too?

[Russel]

It is said that a picture is worth a thousand words...

OK, everybody like illustrations, right?

Attached is a photograph of the D cell adapter I made, as well as the MH-C9000 waveform charging at 1000mA (2000mA pulsed at about 50% duty cycle) and also a closer look at the wave form of the charge current.

[saturation]

Wow.  Thanks a million for those Russell, your waveforms and battery adapter is the best I've seen for this model.  You adapter is brilliantly effective and simple, I will call that the Russell adapter and make one for myself, it won't look as nice though!

The C9000 is, IMHO, the best commercially available NiMH charger today.  The burp chargers are very expensive and its unclear if its better than the pulse charger, as you pictured below.

I own a C9000 too, its initially simple to use, but the options can make charging and revitalizing old NiMH fairly involved if you avail of the options.  A close competitor is the LaCrosse charger, but production models have a habit of overheating and poor build quality, and should be avoided if the Maha is available.

http://www.amazon.com/Crosse-Technology-BC-9009-AlphaPower-Battery/dp/B00077AA5Q/ref=cm_cmu_pg__header

[Russel]

I don't mean to drag this thread off topic. Well, I guess that an adapter for charging NiMH cells isn't too far off the subject...

Anyway, here are a few more photos of the adapter with a little more detail. I recessed the positive and extended the negative terminal so that there is no electrical connection if the battery in installed backward. The connectors on the AAA adapter that fits in the cell bays of the charger is made of brass that I machined to thread into the center plastic section. The hole in the inner end of the connectors allows easy wire soldering. The connectors in the D cell battery holder are similar, but no threading was used, they are just pressed into the plastic battery holder.

[Mechatrommer]

my maha c801d charger trickling voltage profile... i dont know what mode it is... charging a weak battery. some weird pulses there.
i just got electric shock while poking +ve terminal! what the hell!

[Russel]

The Maha MH-C808D (big brother of the MH-C801D charger) waveform looks similar. It charges with a 2000mA pulse, here show charging at 2000mA and at 1000mA

[Mechatrommer]

so. is that not good? i think i should try the LSD eneloop. any recommended model/link russel?

[Mechatrommer]

sorry my ignorance. this is c801d slow/softcharging voltage. quite nasty 2.5Vpp spike there.

[saturation]

Nice.  Do you have a lathe?  This is so well made you can sell these as an accessory.  I build using junky parts, so it tends to look far worse than its function. 

I don't mean to drag this thread off topic. Well, I guess that an adapter for charging NiMH cells isn't too far off the subject...

[saturation]

Those are interesting waveforms for a charger, shows the Maha engineers have really kept up with the charging literature.  Its like a burp charger; a burp charge is a negative going pulse interspersed between the positive charge currents, magnitude, frequency and shape vary quite a bit.  It was patented in the 1970s   and couldn't be used until the patent expired, and when rediscovered folks weren't sure what the value or application was.

Narrow pulses give the equivalent of lower amps, so its the same magnitude but with longer rest periods, and in pulse chargers its only positive going.

Here's a NASA algorithm for burp charging NiMH cells for space missions.

http://ntrs.nasa.gov/search.jsp?R=19970013743&qs=Ns%3DLoaded-Date|1%26N%3D4294724820

sorry my ignorance. this is c801d slow/softcharging voltage. quite nasty 2.5Vpp spike there.

[saturation]

The poll distribution is interesting, I presume EE trained folks can optimize their NiMH cells and there is some suggestion its being done.

[Russel]

Nice.  Do you have a lathe?  This is so well made you can sell these as an accessory.  I build using junky parts, so it tends to look far worse than its function. 

I don't mean to drag this thread off topic. Well, I guess that an adapter for charging NiMH cells isn't too far off the subject...

Just a little Sherline lathe that I can store out of the way on a self in the garage when not in use.

[saturation]

Very nice.  I didn't realize they made them so compact,  ... can be on most home lab's bench, if you have the room!  New ideas for my tools.  You can make very nice custom cases, knobs, even rare spare parts with a lathe, but I always found them fairly large and noisy, but its the right answer to make accessories, such as for the Maha C9000 charger.

Just a little Sherline lathe that I can store out of the way on a self in the garage when not in use.
[/quote]

[Mechatrommer]

Those are interesting waveforms for a charger, shows the Maha engineers have really kept up with the charging literature.  Its like a burp charger...

yea the nasa pdf is 1997. but now its gone in maha c9000. maybe they found out its not a good thing? maybe i should open up my charger if i can get rid of the "burp".
very very nice machine you have there russel! i wish there is such a small size sold locally here.