overcharge/overdischarge reserve in ni-mh batts

[davidefromitaly]

i have try to search if there is some old thread about this but i haven't find anything so i have decided to start this thread

ni-mh batteries manufacturers declare that some series of their ni-mh batteries (i think the strongest series, for heavy applications) have the catode oversized respect the anode for tolerate a bit of overcharge and overdischarge

the difference between the 2 electrodes is called "reserve", the only problem is that they don't specify in what kind of batteries they made this reserve, can we deduct it by testing batts?

 

[Mr Happy]

i have try to search if there is some old thread about this but i haven't find anything so i have decided to start this thread

ni-mh batteries manufacturers declare that some series of their ni-mh batteries (i think the strongest series, for heavy applications) have the catode oversized respect the anode for tolerate a bit of overcharge and overdischarge

the difference between the 2 electrodes is called "reserve", the only problem is that they don't specify in what kind of batteries they made this reserve, can we deduct it by testing batts?

All NiMH batteries are designed this way, not just some of them.

They have to be designed like that since a certain amount of overcharging is inevitable in normal application (for instance, the typical -dV charge method leads to overcharge at the charge termination condition).

If the negative (hydrogen producing) electrode did not have an excess capacity compared to the positive electrode then free hydrogen gas would be produced at the end of charging, which would raise the internal pressure inside the cell and damage it.

By making the positive electrode smaller it generates free oxygen at the end of charge, which diffuses across to the negative electrode and recombines with hydrogen to form water. This stabilizes the cell and keeps the internal pressure under control.

 

[SilverFox]

Hello Davidefromitaly,

As you have indicated, the reserve minimizes the damage from over charge and over discharge conditions. To test for this you need to subject the cell to those conditions and monitor the amount of damage done to the cell.

As the capacities of cells has increased, the reserve has been "refined" in order to allow for more material inside the cell. The exception to this is the low self discharge cells, which do not tolerate over charging very well at all. Another method used to increase capacity is to reduce the thickness of the separator material. The downside to this is increased self discharge rates.

In addition, the chemical mix of the electrolyte can be, and is, adjusted frequently. An adjustment in the chemistry can tend to minimize the oxidation that occurs at the extremes, and can reduce the amount of reserve needed.

In general, cells that are capable of high drain also handle 1C charging very well, and are tolerant to some degree of over charge.

For example: Here is a data sheet for a GP SubC cell. It is rated as a high drain cell, and as you can see it is capable of being discharged at rates up to 50 Amps. I would expect that cell to have more reserve than the high capacity version of the same cell.

Tom

 

[drmaxx]

The exception to this is the low self discharge cells, which do not tolerate over charging very well at all.
Tom

I missed this point for LDS cells completely. What is the effect of overcharging? Do you loose the LDS functionality or capacity or both?

And: What does overcharging mean? Is it simply the amount of electrons I pump into the cell or does rate matter?

 

[Kestrel]

Good question & good answers. Thanks, I learned another new thing today.:huh:

 

[davidefromitaly]

for what i have read in the past, the catode is 40% bigger in the batts designed for heavy use and 20% bigger for standard batts. now in the latest 2700mAh batts i don't know how is bigger, maybe less than 20%

another thing is the alloy of the catode: if is used the AB2 alloy, we can obtain more capacity but the alloy degrade faster and after few cycles the catode can be smaller than the anode, at this point the deltaV recharge can be quite impossible

duracell and GP declare to use the AB5 alloy for their heavy-use batts, the AB5 last longer but have less capacity, don't know if this alloy is used also on standard and high capacity batts

when the anode reach the full charge the voltage raise more quickly cause the anode start to produce oxygen that is assorbed by the catode (recombination? i'm not expert in this stuffs)

when also the catode reach the full charge start a recombination that produce a lot of heat, this lower the internal resistance and the voltage under charge, so we can stop the charge by detecting the -dV

at this point the anode is in a state of "overcharge" (don't know how i can call it correctly) that disappear in few hours, so just after the charge the battery can output more mAh while 24hr. later much less. now would be interesting if the declared capacity of batts are referred to the capacity just after the charge of after few hours

about the overdischarge, the bigger catode help the anode to not oxydize during deep discharge

please correct me if i have write something wrong

 

[SilverFox]

Hello Drmaxx,

The first dead Eneloop cells I checked out died because of leaving them on the charger to trickle charge after charging. I believe it took 3 - 4 months to kill them off.

They developed high internal resistance (which results in capacity loss) and lost their low self discharge properties.

Tom