Re: (ET) ni-cads

["David Roden" <etpost drmm net>]

On 15 Feb 2006 at 7:44, tbamc wrote:

> I'm curious as to how much of your description of ni-cads appplies to 
> AAA, AA, C, D size batteries?

I welcome comments from those more familiar with semisealed nicads, but as 
I 
understand it the chemistry is pretty similar.     

There are some changes to reduce gassing as you might expect expect (the 
cells are sealed but have pressure release valves).   They design the 
negative electrode (cadmium) with extra capacity.  As in any cell once it 
reaches gassing voltage during recharge, the water is electrolyzed into H2 
and O2. The oxygen is evolved at the positive electrode.  Because its 
effective capacity is lower than the negative's, it reaches gassing 
voltage 
sooner, or so I've read (how this works is a mystery to me, as I'd've 
thought that if H2O was electrolyzed you'd have to get both gases).   

Anyway, assuming this makes chemical sense and following the good old 
Everready Nicad applications manual, as long as the overcharge current is 
kept low (either 10% or 1% of capacity sticks in my mind), the negative 
electrode can dissipate the oxygen by literally oxidizing.  I would assume 
that this cadmium overcapacity (and consequent actual undercharge) is a 
factor limiting the cell's specific energy.  

I've always read that consumer semisealed nicads were designed to 
withstand 
low current overcharge fairly well.  However, if my understanding of this 
design is right (and I warn you that I am NOT an electrochemist), 
prolonged 
low current overcharging of a semisealed nicad may not immediately damage 
it, but it would have to eventually degrade its negative electrode.  This 
is 
somewhat similar to the way overcharging of a lead cell will corrode its 
positive grids.  Again though I admit that I am NOT an expert on this.  

Certainly if the overcharge current is too high and gas evolution gets out 
of hand, the cell will open its pressure release valve so as not to 
explode. 
This is a safety feature and if it happens it means a significant hit on 
cell life, just as happens when a lead AGM or gel battery vents gases.  

Anyway, I'd say that the advantages and limitations are similar.  In 
particular there's no harm at all in storing semisealed nicads in any 
state 
of charge, so unlike lead batteries there is no need to always recharge 
after use.   

FWIW, sealed nicads have better cycle life and high current performance 
than 
the now more popular NiMH, but they don't quite match NiMH for specific 
energy.   

Nicads also pose more of a disposal problem.  Cadmium is nasty stuff.  
There 
are certainly sites for recycling nicads, and in the EU there have long 
been 
laws about manufacturers making efforts to reclaim them.  But it seems 
that 
too many people don't take the responsibility for this and just landfill 
appliances with integral nicads.  (Think of all those Dustbusters out 
there 
with dead nicads.) Thus I believe they are or will be soon quite 
restricted 
in the EU.   

The US is rather more cavalier about such things and AFAIK there are so 
far 
no laws restricting nicad use here, but NiMH is the more fashionable 
chemistry (with Lithium hard behind it) so it's becoming less of an issue 
anyway.  

The above is the best of my rather meagre knowledge, and as always 
corrections of my ignorance are welcome!  


David Roden - Akron, Ohio, USA

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