Interesting... but not recommended...

[45/70]

Good Information, Ray

Regarding the decay of LSD capabilities with overcharge, I can already tell that it is the most likely LSD killer, as proved from the batteries left under "maintenance charge". I use quotes because I want to emphasise that "mainteinance" in this case is sinonimous of "killing"...

This seems to further confirm what SilverFox and others have seen. LSD cells don't like to be trickle charged!

Dave

 

[Ray_of_Light]

I just found a link to a document I had, which is very explicative about the stray deposits on the separator:

http://www.electrochem.org/meetings/scheduler/abstracts/216/0268.pdf

Look at the picture of the separator. The black deposit is manganese and cobalt coming from the lattice alloy. An Energiser AA 2500 mAh, and after 10 cycles had a similar situation.
Trickle charge produces such deposits even in absence of cobalt and manganese.

Regards

Anthony

 

[Bullzeyebill]

So use a charger that does not trickle charge, and turns off when fully charge is reached? Which chargers do this? What voltage would be fully charged, but not overcharged? I have noted, from posts, that sometimes Eneloops will come off of a charger at almost 1.5 volts. Is that to much charge?

Bill

 

[45/70]

Trickle charge produces such deposits even in absence of cobalt and manganese.

Well, I've known since I started using NiMH cells in the late 90's, that they were less tolerant to trickle/overcharge (and thus shouldn't be charged on NiCd chargers). That is likely a major part of the reason why.

Dave

 

[lyyyghtmaster]

Yes, I seem to remember that intolerance to constant trickle charge being trumpeted by the big manufacturers right from the beginning. Now that we have LSD cells, where are the chargers that shut off completely and don't trickle charge at all?

On an earlier note, I run Eneloops in my D20 and a while back ran a pair to the completely dark point when I didn't notice it was on (full brightness) in my holster. I immediately marked the lower voltage of the two cells and have been tracking it over several complete cycles since. It does not seem to have suffered any ill effects from that mistreatment compared to the other cell of that pair! That is quite amazing! Maybe the flashlight acts to reduce the damage? This torch draws enough current at full that it dims way down when the cells still seem to have about a hundred mAh left in them, but I haven't measured how this observation relates to the discharge point where there is no light left at all.

 

[Ray_of_Light]

The sealed Ni-CD batteries reforms water with an indirect chemical reaction, which requires the presence of oxygen under pressure (about 4 bars). This is a chemical reaction that, provided the Nickel and the Cadmium are of high purity and the seals of the battery canisters are of good quality, can go on indefinetively.

The Ni-MH battery reforms the water based on the catalytic properties of the lattice alloy. The main advantage of this is the fact that the positive nickel electrode doesn't need to be half-capacity of the capacity of the negative electrode, as it was in the Ni-CD to avoid formation of gaseous hydrogen. With the same size positive electrode, a Ni-MH battery has double capacity of a Ni-CD battery.
The negative side of using the negative electrode of Ni-MH battery as catalyst is the stress placed upon it. In a 2000 mAh AA Ni-MH cell, if you overcharge it of 2000 mAh, (even very slowly) it is like you detracted one cycle from its cycle life.

Regarding the end voltage of a Ni-MH battery, things are a bit complicated. The nickel has different oxidation states, which influences the voltage in manner which is not proportional to the state of charge of the battery. Also, the purity of the chemicals used, the concentration and composition of the electrolyte, the temperature of the battery, its state of conservation and the number of cycles influences the end voltage. Therefore, a charger stops the charge when the relative voltage (during the charge) stops climbing and starts a slow descent (from 2 to 10 millivolt, the famous "- delta V").
To add to this, some off brand or old batteries do not provide this voltage drop, further complicating the issue of charge termination. For these batteries, you need to use a timed charger or use a programmable charger set to "zero-delta-V" termination.

If you use any common, good single channel charger, and quality LSD batteries, just remove the batteries when the charger indicates end of charge. Voltage can well be 1.5 V off the charger, and setting to 1.4 V after 24 or more hrs. LSD batteries will stabilise around 1.3 volts in the months to come, while non-LSD will have a steady decline, which can last from few days up up to two months, depending on their charge retention capabilities and the ambient temperature.

Hope this helps

Anthony

 

[Battery Guy]

Greetings Everyone,

So here's the deal with respect to over discharge of NiMH cells.

There are three stages of discharge, as shown this plot copied from the 3rd edition of the Handbook of Batteries:

The three stages of discharge are described as follows:

Stage 1: Normal discharge

No gas products are generated or consumed.

Negative electrode: MH + OH- --> M + H2O + e-
Positive electrode: NiOOH + H2O + e- --> Ni(OH)2 + OH-

Stage 2: Hydrogen evolution at the positive electrode (cathode)

Stage 3: Oxygen evolution at the negative electrode (anode)

As you can see from the graph above, the cell voltage must go negative in order to get to Stage 2 discharge. As long as the cell stays at a positive voltage, there is very little or no damage that should occur in the cell.

The real damage comes when the cell goes into Stage 3 discharge, and oxygen is evolved at the negative electrode (which is actually the positive electrode during reversal, but let's not confuse the issue) simultaneously with hydrogen at the positive electrode (which is actually the negative electrode during reversal). During this stage of discharge, several side reaction occur in the cell that cause irreversible damage to the cell (e.g. corrosion of the metal hydride alloy). You are also electrolyzing your electrolyte during this stage, and will therefore loose water in the cell in the form of hydrogen and oxygen gas.

Based on my experience with NiMH cells, you won't actually overdischarge them and cause damage unless you get into Stage 2 or Stage 3 discharge. Of course, in order to get to Stage 2 or to Stage 3, you need to drive the cell into reversal. This means that you should not get noticeable degradation in performance by simply shorting a cell and clamping it between conductors in order to drive the voltage to 0V.

Cheers,
Battery Guy

 

[45/70]

Real quick, as I have to go, I read somewhere where when a NiMH cell was discharged to a point below ~0.9 Volt, that the separator was weakened. As I remember, any damage done because of this, was dependent on how long and how far below this voltage the cell was discharged. This was in reference to regular NiMH cells, not LSD.

Dave

 

[march.brown]

Based on my experience with NiMH cells, you won't actually overdischarge them and cause damage unless you get into Stage 2 or Stage 3 discharge. Of course, in order to get to Stage 2 or to Stage 3, you need to drive the cell into reversal. This means that you should not get noticeable degradation in performance by simply shorting a cell and clamping it between conductors in order to drive the voltage to 0V.
Cheers,
Battery Guy

Does this mean that on a single cell torch , the battery will not be damaged by totally discharging it , whereas with a multi-cell torch it would be possible to drive one of the cells into reversal and thereby damage it ?

If this is so , it is a good reason to use a single cell torch in preference to a multi cell torch.
.

 

[Battery Guy]

Real quick, as I have to go, I read somewhere where when a NiMH cell was discharged to a point below ~0.9 Volt, that the separator was weakened. As I remember, any damage done because of this, was dependent on how long and how far below this voltage the cell was discharged. This was in reference to regular NiMH cells, not LSD.

Dave

I have never heard of this, and cannot think of a reason why it would be true. That being said, I am always learning new stuff about batteries, so I cannot exclude the possibility. If you can find the reference that you read this in, I would love to take a look at it.

While Stage 2 is certainly not healthy, the real damage to the cell occurs during Stage 3. NiMH cells are designed to have an excess negative electrode capacity to make Stage 2 sufficiently long that you are unlikely to get to Stage 3.

So the observations that Silverfox has made on the LSD cells are consistent with what one would expect from any NiMH chemistry. As long as you don't go to negative cell voltages, you should be fine.

Cheers,
Battery Guy

 

[Battery Guy]

Does this mean that on a single cell torch , the battery will not be damaged by totally discharging it , whereas with a multi-cell torch it would be possible to drive one of the cells into reversal and thereby damage it ?

Yes, that is exactly what it means.

If this is so , it is a good reason to use a single cell torch in preference to a multi cell torch.

Well, you could use multiple cells in parallel. Then you never have to worry about imbalance.

Of course, the more cells you have in series the more careful you need to be. With two cells in series, you will never get to Stage 3. With eight cells in series in a high power light, you could easily push a cell into Stage 3 discharge.

Another conclusion you might draw is that when running multiple cells in series, be sure to use high quality, well matched cells. Also, don't run your battery pack to complete depletion. In other words, use the same good battery management practices that have been recommended for years.

You should note that NiMH cells are designed to have an excess of anode capacity. This excess anode capacity provides the Stage 2 plateau, and gives you some amount of breathing room. So if you have a low capacity cell in your series string, it will be driven to Stage 2, but probably won't make it to Stage 3. The excess anode capacity is significant, typically 150% of cathode capacity. So you do have quite a bit of buffer before you do real damage.

That being said, one of the things that cell manufacturers do to get to higher cell capacity is reduce the amount of excess anode. So, in general, high capacity cells are not going to have a shorter Stage 2 plateau than low capacity cells. Some manufacturers may also reduce the amount of excess anode to save $$$.

Cheers,
Battery Guy

 

[45/70]

I have never heard of this, and cannot think of a reason why it would be true. That being said, I am always learning new stuff about batteries, so I cannot exclude the possibility. If you can find the reference that you read this in, I would love to take a look at it.

Well, I think it's already a pretty well established opinion that NiMH cells are less tolerant of being deep cycled than NiCds. However, at the moment all I can find are a couple vague references from Battery University ....... here,

[FONT=Verdana, Arial, Helvetica, sans-serif]Among battery chemistries, nickel-cadmium is least affected by repeated full discharge cycles. Several thousand charge/discharge cycles are possible. This is why nickel-cadmium performs well on power tools and two-way radios that are in constant use. nickel-metal-hydride is less durable in respect to repeated deep cycling.[/FONT]

and here,

The NiCd battery is least affected by repeated full discharge cycles. Several thousand charge/discharge cycles can be obtained with this battery system. This is the reason why the NiCd performs well on power tools and two-way radios that are in constant use. The NiMH is more delicate with respect to repeated deep cycling.

I'll try to find reference to the actual separator weakening, but am a bit busy this weekend.

Dave

 

[45/70]

I haven't had a lot of time to look for a better explanation of what happens to the separator when standard NiMH cells are over discharged. I did look through a GP NiMH Handbook that I have (here is a link to a newer one) and they have this to say:

2.4.4 Deep discharge
The cycle life is also affected by the depth of
discharge. The number of charge/discharge cycles
will decrease if the battery is repeatedly subjected
to deep discharging below 1V, or to a status of
polarity reversal. Considerably more cycle
numbers can be obtained if the battery is cycled
under shallower cycling conditions.

I am fairly certain that weakening of the separator results from over discharging standard NiMH cells, and I will keep an eye out for a link that better explains, what actually happens. With the advent of LSD technology, which uses a more robust separator with different chemical compounds, it may very well mean that over discharging these cells has less, or possibly no effect. The jury seems to still be out on this.
​

Dave

 

[Ray_of_Light]

That being said, one of the things that cell manufacturers do to get to higher cell capacity is reduce the amount of excess anode. So, in general, high capacity cells are not going to have a shorter Stage 2 plateau than low capacity cells. Some manufacturers may also reduce the amount of excess anode to save $$$.

Cheers,
Battery Guy

I can confirm.
Some cheap chinese Ni-MH I have analised had no buffer area at all.
The problem is that, in these condition, the end of charge -dV signal is pretty inexistant.

Anthony

 

[TinderBox (UK)]

The Maha C9000 seems like a great charger , can somebody answer this question.

I know that the four charging bays are independent and can charge and discharge independently.

But if i am going to charge four identical battery's do i have to adjust the setting of all four bays or can i just set the first one and the other three will have the same profile.

thanks for the help.

 

[Mr Happy]

You have to set all four bays individually, but in practice it is not much of an issue. The user interface is fast and easy to manage.

 

[TinderBox (UK)]

Thanks for that, are their any know problems with the C9000 , I bought lacrosse BC-900 but they had a problem of overcharging and melting down so i binned it. :ohgeez:

 

[Battery Guy]

I haven't had a lot of time to look for a better explanation of what happens to the separator when standard NiMH cells are over discharged. I did look through a GP NiMH Handbook that I have (here is a link to a newer one) and they have this to say:

I am fairly certain that weakening of the separator results from over discharging standard NiMH cells, and I will keep an eye out for a link that better explains, what actually happens. With the advent of LSD technology, which uses a more robust separator with different chemical compounds, it may very well mean that over discharging these cells has less, or possibly no effect. The jury seems to still be out on this.
​

Dave

Hey Dave

Ok, you have convinced me that I need to do some cycle testing. I simply cannot understand why discharging to 0.9V would be any different than discharging to 0V. But you have put forth enough references that indicate that it does make a difference, so I think I need to put it to the test.

Stay tuned!

Cheers,
Battery Guy

 

[spencer]

This is a great thread. Interesting discovery.

 

[Battery Guy]

Hey Dave

Ok, you have convinced me that I need to do some cycle testing. I simply cannot understand why discharging to 0.9V would be any different than discharging to 0V. But you have put forth enough references that indicate that it does make a difference, so I think I need to put it to the test.

No test results yet (sorry!), but I have been giving this some thought and I think I might have an answer.

I have been thinking about the Ni-MH packs in the Toyota Prius and other hybrid vehicles. These have proven that they can last over 10 years without replacement. The way that Toyota and others get this kind of longevity from their Ni-MH packs is to significantly limit the state-of-charge (SOC) window that they are cycled. My understanding is that these cells are only cycled over approximately 40-60% SOC. This is important for the following reasons:

1. No gas generation because the cells are never overcharged or overdischarged into reversal
2. Minimal strain on the metal hydride alloy because the hydrogen content does not change significantly

The latter is important, because "pulverization" of the metal hydride alloy is one of the important mechanisms for capacity loss in Ni-MH alloys.

So I think this is why people generally see more cycle life from shallow cycling of Ni-MH cells. However, if you discharge repeatably down to 0.9V, I don't think that will be substantially different from repeated discharges down to 0V because the difference in capacity (and therefore hydrogen content in the metal hydride alloy) will not be significantly different.

That is my hypothesis based on the information I have been able to dig up. Hopefully I can test this in the next few weeks.

Cheers,
Battery Guy