Electronics > Beginners
Theory about NiMH battery recovering voltage in between discharges
atmfjstc:
Hey there, sorry for resurrecting this, I had some issues and got distracted.
JS's answer makes a lot of sense. Is this somewhat similar to the phenomenon of 'surface charge' in lead-acid batteries by any chance?
Can you point me to some specific books and papers showing a model of this phenomenon in action?
KL27x:
I remember reading everything I could to understand NiMh, back in the day. The understanding I end up with is that they are different than all of the other popular battery chemistries.
In every other battery type, you will get some "recovery" for the reason that JS stated. Give the battery a rest, and the float voltage goes up. NiMh has some other effect going on in addition to that.
My memory is notoriously bad, but what I inferred and internalized from all my research was that in NiMh, the voltage is not a function of charge, the way it is in other batteries. As the cell gets more charge, there are places in this process where the voltage will go down. So you can't even charge these batteries consistently when hooking them up in parallel. It is preferred to charge them in series. Even within one cell, there are some parts of the cell that will charge to a higher energy state than other parts, and they will not equalize. These two different energy states in the cell are both happy at the same voltage.
KL27x:
I remember reading what appeared to be official NASA documentation for maintenance procedures for NiMh batteries, including use in satellites and whatnot. The thing read like a voodoo handbook. It appeared to be as much superstition as science.
digsys:
--- Quote from: JS ---The available ions depend on the charge, the ones in solution show the voltage, the equilibrium when the reaction is stopped you see about 1.4V. The speed of the reaction is much slower than the discharge to keep up with the consumed ions, so the concentration in the solution drops down till a point the ions jumps faster to solution and reach an equilibrium, that 1.2V level. When the ions are in this concentration the reaction is faster and the voltage is stabilized.
Once you are at that 1.2V and stop the discharge the reaction continues till gets to the steady voltage, when you start the discharge again, the total available ions are less so the reaction is slower than at full charge so it reaches equilibrium faster.
Something similar is why at higher discharge rate usable charge is less, that and the ESR.
--- End quote ---
Well written explanation ... As also noted "surface charge" , "electron migration" and others also have an influence. Throw in some "deposition" etc and it
really becomes fun :-)
I make up large LiIon packs for EVs and those bastids are just as difficult to quantify !! I run a few load lines / settling times etc, plot curves .. yadda yadda.
In saying all that, if you don't do all that, you may only be 5-10% out in capacity. What it is VERY useful for is - predicting EOL / intervening for better EOL.
KL27x:
--- Quote ---I make up large LiIon packs for EVs and those bastids are just as difficult to quantify !!
--- End quote ---
This would not even be practical with NiMh. Until li ion is developed, we are still using lead acid powered golf carts.
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