NI-MH Batteries, am I missing something?

C

cwah

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Hello there,

I was browsing ebay on NI-MH batteries and I've found these batteries:
ViewPicture.aspx

* 100% Brand New & High quality.
* Capacity: 3800mAh
* Voltage: 1.2V
* Chemistry: NI-MH
* Dimension: CFL RECHARGEABLE BATTERY
* No memory effect
* Rechargeable Times: Up to 1000 cycle times
* For battery appliances such as emergency lighting, portable devices, power tools.
* 25 X CFL AA 3800mAh Rechargeable Battery NI-MH 1.2V
* Battery Weight: 15.6g each, Dimension 48mm in height, 13.5mm in diameter.

http://www.ebay.co.uk/sch/i.html?_nkw=3800mah+aa&_frs=1&_sop=15&_trksid=p3286.c0.m359


When I do the math, I have:
Capacity: 1.2V*3.8mAH = 4.56 Wh
Gravimetric density: 4.56Wh/0.0156kg = 292.30 wh/kg

Something is probably wrong with my math, it can't have such a high energy density.

Where did I get wrong?
 
Dont trust the unknown brand nimh cell with massive mah ratings on ebay they just make up a number.
Most of the high capacity aa cells tend to be between 2500 and 2900 mah claims of higher than that by anyone other trusted brands are bogus claims.

the best aa cells have about 100 wh per kg at best.
 
3800mAh in an AA cell? really? :p

Your calculations would indicate a higher energy density than even the best lithium available.
Pretty amazing that an eBay seller would have something for sale that exceeds even the best Panasonic cell out in the wild.

Feeling skeptical yet?

If it seems too good to be true, it probably is. If you can't resist the temptation, order a few and put them through your testing equipment to find out the truth. That's a small rip off to deal with :)

http://neptronix.org/posts/dont_buy_unknown_chinese_batteries.html

http://www.endless-sphere.com/forums/viewtopic.php?f=3&t=25904&hilit=ultrafire+ypedal&start=105
 
Right on Nep!

Maybe 800mAh from these on a good day?
 
See here:
http://www.endless-sphere.com/forums/viewtopic.php?f=14&t=35708

I'd say that basically unless you are using them for loads with only a few dozen mA at most, you won't even get a quarter of their ACTUAL capacity, much less anything even resembling the lie they printed on them.
 
Man, a new low for ebay. But they do make em better than 800 mha. I have some nice duarcell nimh for a camera that is labeled 2000 mha. Never tested em, but they do last longer than a typical 1500mha nimh cell. No telling what the real number is.

Main problem with nimh and nicad is you can't paralell cells to get the capacity up safely. So that's why the ebike nicads were big honking F cells of 8 ah. If you paralelled two strings, then you had to seperate em to charge safely. And talk about slow to recharge, hoowee!
 
You can parallel charge nimh if you use a zero peak charger or a cccv charging method.

I even built a 1s4p pack of aa cells of varying capacity and cycles and charged them with my Duratrax Intellipeak charger with its zero peak function and they charge up fine without any of the cells getting hotter than the others.

I charge my 3 12v nimh packs in parallel to 14.5v cccv and after sitting at that voltage for a while the current drops off to a trickle level and then warms them up slightly making nice and balanced.

I think the problem is some of these nimh chargers are too crude to be sensitive enough to peak detect paralleled cells properly and then they go into a state of overcharge and one packs voltage drops and thus takes power from the other packs.

Can anyone confirm problems with nimh packs paralleled at cell level?
 
chilledoutuk said:
I think the problem is some of these nimh chargers are too crude to be sensitive enough to peak detect paralleled cells properly and then they go into a state of overcharge and one packs voltage drops and thus takes power from the other packs.

Can anyone confirm problems with nimh packs paralleled at cell level?
Click to expand...

Yes, as soon as the first one get full, it's voltage drops and hogs all the charging current as well as from the other cells it is in parallel with. This creates a thermal runaway situation that generally results in explosions, fire, and destroyed cells. I had to use pliers to pull the stainless shrapnel out of the wall after discovering this issue many years ago.

With a zero peak charger, you MIGHT get away with it but I would not try personally.

The cell in the advertisement says "up to" 3800 mAh. I guess that includes when you charge it during testing or when your meter is badly out of calibration.
 
was this explosion with single cellls paralleled? any pictures?

I was saying if you parallel the cells on a cell by cell basis and then charge at a single cell rate with zero peak or cvcc you should be ok.

Theoretically if you join the cells with a good enough conductor they will act as a single cell and once settled should peak at the same time once broken in with a few cycles.

This is what i observed with a 1s4p pack i built out of some old fuji cells of varying capacity anyway.
 
I have used a hobby type RC charger on "lipo" (3.7v) mode 2S to charge Prius packs (gen 2 and gen3 6s NiMH). This takes them to a 8.4v peak (1.4v/cell), and I have done this many time both singly and when the 6s packs have been parallelled. I can't say for sure what the max amount of amps you would want to try this with, but with 3 amps or lower, you shouldn't have any problems. Of course, you can also take this further by using the 4s lipo mode for two prius packs in series or 6s lipo mode for 3 prius bricks in series. I hate to give charging advice when it comes to Nimh as I have seen some "red hot" cells in my time and I don't want to cause anybody any fires, so If anybody sees a flaw in this, don't hesitate to chime in, cheers, Gregor
 
Even with paralleled single cells, if there is enough energy in the other paralleled cells that don't drop in voltage to pour into the one that does first, it could heat the already-internally-heated cell to bursting point, or make it hot enough to ignite materials around it.

Additionally, if the charger doesn't detect the change in voltage and shutdown current supply, then it will continue to attempt to charge the cells up to the full voltage, but they will now never reach it, so it will continue pouring current into an already-overcharged cell, continuing to heat it. Because the other cells (if there are enough of them and they ahve enough capacity) may be able to hold up the voltage of the group to above what the normal delta-V detection level is set to, the charger may never see the delta-V and terminate. If the charger has no delta-V detection then it will never even know to try to shut down in this case, and again, will continue to heat the cell.

Until the other cells finally drain down equal to the first, or the charger shuts off, or the cell fails internally either quietly or catastrophically, everything will continue pouring current into it and continue heating it.


It might not happen. But it could. Especially in a pack built into something so that the pack itself is well-insulated against shedding heat (a bad idea for NiMH in general, but not that uncommon in plastic battery packs for tools/etc., even old laptops).

As long as the cells never reach the delta-V state then this isn't an issue, but once the first one does, the sequence of events from there results in lots of heat. Enough to destroy things? Maybe, maybe not.

I wouldn't want to be the guinea pig that found out the hard way. ;)


AFAICR the only ones I've actually exploded were accidentally charged in reverse, but when it happened (years ago) they destroyed a sci-fi prop I'd worked pretty hard to build and was quite proud of, when they turned into shrapnel and peppered the room with pieces of themselves and plastic casing. :(

I don't have a picture of the aftermath, only one of the device (on the left) taken by someone else at a convention, some time before the end of it's life.
Tricorders.jpg
 
the thing is a cells abilty to take charge (current) is directly proportional to there state of charge which basically means when you parallel cells on a singular level if a cell were to become more charged than the other it would begin taking less charge than the other cells.
differing internal resistance would produce a negligible effect in my opinion.

For example my sub c cells have a 2.5milli ohm internal resistance which at a charge current of 3amp equals 7.5 mv. The cells have a variance of 2mOhm to 3mOhm which means the maximum voltage difference the IR of the cells could cause between different cells charging at 3 amps is 3mv

This is a tiny voltage difference and wouldn't cause a significant current flow from one cell to the other even when charging at a fixed current with suitably sensitive peak detection.

To cccv a NIMH cell you need to know the cells peak voltage and its resting voltage after being fully charged and choose a voltage in between.
For example my 12v NimH packs peak at about 14.8v at 3 amps charge rate and they rest about 14.1 after resting from being charged.

Setting a adjustable power supply to 14.5v max voltage i can charge my cells without them getting that end of charge heat from overcharging.
Generally it takes a bit of experimentation to find the sweet spot cvcc charge voltage for your pack.

What your looking for is the current to drop down to about 0.1a but not to 0a it should after a while of being at 0.1a creep back up a little if it goes back up too high ie higher than c/20 then set you cvcc voltage a bit lower next time.

i had my cvcc voltage set to 14.8 originally and the current went back up to about c/10 which was too high for my liking so i set it a bit lower at 14.5 which works well for me and balances the cells nicely at a trickle rate at the end of the charge gently warming the cells.
 
chilledoutuk said:
the thing is a cells abilty to take charge (current) is directly proportional to there state of charge which basically means when you parallel cells on a singular level if a cell were to become more charged than the other it would begin taking less charge than the other cells.
differing internal resistance would produce a negligible effect in my opinion.

For example my sub c cells have a 2.5milli ohm internal resistance which at a charge current of 3amp equals 7.5 mv. The cells have a variance of 2mOhm to 3mOhm which means the maximum voltage difference the IR of the cells could cause between different cells charging at 3 amps is 3mv

This is a tiny voltage difference and wouldn't cause a significant current flow from one cell to the other even when charging at a fixed current with suitably sensitive peak detection.
Click to expand...

I'm not entirely sure we're communicating with each other quite right...it's not the IR voltage drops I'm speaking of, but the voltage lowering that happens once a NiMH cell reaches a certain state of charge (what is usually called "full" but might really be "over full".

It isn't the IR voltage drop that causes this, exactly, but the chemical mechanism of how the NiMH cells absorb a charge--once it absorbs enough, the voltage across it decreases, and the energy that was stored as voltage is now dissipated as heat.

It doesn't matter much what the IR of each cell is, and what that particular voltage drop is, once one of the cells in parallel reaches that state, and lowers it's voltage below the others.

This is fine as long as the charger can detect the change, either by detecting the lowering of voltage across the cell group (for instance, by momentarily shutting off charge voltage periodically like the Tenergy NiMH chargers I have do) or by detecting the increase in charge current, or by having a thermal sensor on every cell and monitoring any change in the total resistance or voltage of the paralleled sensors and shutting down when it happens (or monitoring them all individually, multiplexing them, and shutting down once any one reaches a critical temperature).


Otherwise, to be certain this will never happen, one must test the cells individually before building them into a pack, and determine the point at which each lowers in voltage, and then limit charging in total Ah or Wh or something so that it never exceeds the lowest-capacity-cell's ability to absorb a charge *before* reaching that state. You'd also need to either retest each cell periodically, or leave quite a lot of uncharged capacity to ensure that as they age they are less likely to have one exceed the limits you've got set up.


Again, there might never be a problem. But there could be.


If you only ever trickle charge, or you bulk charge partway and then trickle charge, *and* detect the *lowering* in voltage of any cell group, or the *increase* in current draw, it'd be safer. But if the capacity of the paralleled cells is sufficient, the power they dump into the cell that lowers in voltage could be enough to overheat it catastrophically.

If the charger (that is at that time in trickle mode) is capable of dropping back into bulk mode due to detected higher current draw or voltage drop, then it will also contribute to the problem by pouring in more current.


I hope it does not happen, but it could. That's why I never recommend direct parallel charging of NiMH.
 
thats interesting how can the voltage of cell A drop when its directly connected to cell B with a good chuck of copper with tiny reistance ohms law wont allow it.

The point is people are looking at the characteristics of cells as though they are individual when they are parralleled this is fine if you have paralleled strings but its not the same when you parallel cells at the cellular level.

Effectively you are creating a larger cell as long as the resistance of there parrallel connection is lower or equal the resistance of most NIMH cans.

What i am saying is that quite simply by joining the cells like this they effect each others charge characteristics the same way as making a larger cell at a factory.

When i first made a 1s4p pack with aa cells of varying capacitys one cell did get warmer than the others but since a few cycles they seem to have settled into behaving like a single cell i shal run some more tests though to confirm.
 
EDIT: added: Also, you note that the first few cycles there were cell heatings. That's the thing I'm talking about--what if you had a large enough difference in cell capacity between that cell and all the rest, and there were enough cells in parallel, that they could supply enough current for that cell to heat to catastrophic temperatures? Especially if the charger fails to cut off? ;)



*****************
Original:

I understand what you're saying...but it doesn't work quite like you'd think. Yes, the busbar-sized copper or even a theoretically perfect conductor between the cells would indeed keep all the voltages the same.

But I think it would keep them at the *lowest* voltage potential, by allowing easy current flow down to the cell with the lowest resistance (which in this case would be the one that had reached it's saturation point and gone into the delta-V stage).

Kinda like taking a bunch of resistors of different values, and paralleling them--you do get the same voltage across them all, but by that very thing you also get different current flows thru them all.

I haven't tested any of this, so I could be completely wrong. I would like to be. :)

The best experiment to prove or disprove this is to take a set of cells (preferably as identical in capacity and IR/etc as possible) and then parallel those with a cell known to have a lower capacity (to simulate a cell that has aged differently, and no longer has the capacity others do, and thus will probabably reach full charge before they do). Start the charge process, and continue beyond the point at which you know that the lower-capacity cell should become overcharged.

Monitor the temperatures of all cells constantly. It'd be nice to also monitor the current flow into each cell separately, but I think the shunts would affect the voltage drops and invalidate the experiment--so temperature is the only way to really see what is happening, I guess.




Alternately, take just the identical cell set, and deliberately pre-charge one of them to a higher state of charge than any of the others. Setup the charger on the busbars, without the cells connected. Then connect them all together with a clamping device so that you can simultaneously connect them all and not give much of a chance of cell equalization before the charge begins. This experiment might not work because the cells are all identical, which in practice won't be true later in a pack's life, which is the conditions I am most worried about. Or teh case where someone just builds a pack out of untested cells that haven't been matched first.


Another way to do it is to deliberately use cells with a wide variation in IR and do the same test, again until a cell begins to show the temperature rise that indicates overcharge begins.


If you use a tiny resistance value (like a shunt) in series with the charger up to one of the busbars, you will see a voltage drop across that resistance. If you constantly monitor that voltage as a graph/curve, you'll see it drop over time. But once any cell reaches delta-V stage, it should rise again, because current flow will rise again, too.


There are probalby other ways to do the experiment, too, but that's all I can think of in the few minutes I've been typing this up.
 
We use 100's of Powerex AA NiMh at work for wireless mics/instruments. Here's a 1 year old 2700mAh cell with about 200 cycles on it.
PowerexNiMhAA.JPG

Cycled a couple times to be certain it was well activated and then performed a top off charge:

NiMhAAtopchargeA.jpg

Next applied 1/4C load (0.6A) LVC 0.9V (1/4C was the best this cell could provide without triggering LVC)

NiMhAAquarterCdischargeA.jpg

If your device doesn't pull a lot of current and/or isn't picky about cell voltage NiMh AA's may provide a decent renewable option. However, 1V is awful low voltage for devices designed to run on Alkaline AA's.

Any thought of using them for high power and/or any eBike application will likely result in disappointment.

I'll borrow some new stock and do another round of testing in the near future.
 
Honestly the only way to test it would be to have a current monitor in line with each cell and monitor what cell gets what current during the charge process.

The cell that gets a little warmer in the pack i built is no warmer than a nimh pack should get when peak charging.
The capacity of the cells in the pack i built do vary wildly as they are old cells that haven't been used together at all.
 
cwah said:
I tested them, cycled/refreshed them to make sure them reach their maximum capacity. They all peak at 850mAH.
Click to expand...

That's about right.
Leave them the most scathing eBay feedback and cause a stink with paypal. If everyone did this, the hundreds of battery scammers out there on the net couldn't keep accounts open long enough to stay in business.
 
fechter said:
A 3x exaggeration seems almost criminal.
Click to expand...

"almost"? :evil:

That is a sad commentary on the state of things when we are so used to exaggeration that even that much of it is still "accepted".
 
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