Re: (ET) T105 capacity vs BB600

["David Roden" <etpost drmm net>]

The following response is based on my experience with Saft STM5-100MR 
nicad 
modules (specially designed for EV use).  They're not the same as the 
aircraft (?I think?) nicads that Christopher is using, but the principles 
are similar.

On 10 Jul 2005 at 23:38, Ralph G Vogan wrote:

> They gas when charged.  

So do lead batteries.  However, nicads usually gas more during the 
absorption phase of charging because their gassing voltage is lower.

FWIW, this characteristic makes their charge efficiency lower than lead 
batteries'.

> They also gas when discharged.  

I have not observed this to be the case with my nicads.  I have heard that 
it occurs with nickel iron batteries, however.

> They need to be vented.  

So do lead batteries.

> They should be fan cooled when charged.  

This is certainly true for my nicads, because they have relatively high 
specific energy.  It's also generally true for any advanced chemistry 
battery with high specific energy and/or high specific power.

The really good nicads are liquid cooled, IMO.

> They should be charged at 3
> times their discharge rateing to get the maximum life out of them.  

I've never heard this.  For the STM range, Saft recommends constant 
current 
charging at C/5 (one-fifth the amp-hour rating).  That's a pretty typical 
recommendation.

According to Saft and several EV users, the most crucial considerations 
for 
long life are: 

1. Avoid contaminants.  Keep acids and carbon dioxide out of the cells.  
Don't even think of using the same watering equipment or water jugs you 
use 
on your lead batteries; traces of acid are the quickest way to murder your 
nicads.  You'll want to buy a completely separate second set of watering 
tools and water jugs.  Keep lead batteries far, far away if you possibly 
can.  (Some people say don't even keep them in the same building, though I 
don't go that far.) 

Don't take the cell caps off unless you absolutely have to; they're 
designed 
to keep atmospheric CO2 out of the cells.  CO2 reacts with the KOH in the 
electolyte to make K2CO3, and with the cadmium to form CdCO3.  You can 
change the electrolyte but not the cadmium electrodes, so keep the caps on.

2. Avoid heat. Try not to charge or discharge at temperature above 40 deg 
C. 
Limiting temp to 35 deg C is even better, if you can do it.  Consistent 
operation at  50 deg C will cut your cycle life in half!  

Nicads don't much like heat but they lose very little capacity at low 
temperatures, so they make excellent cold-weather batteries.

Nicads are very forgiving of overcharging as long as they are kept cool. 
The 
person who charges nicads without temperature sensors, proper temperature  
compensation for EOC voltage, and overtemp shutdown, is apt to murder his 
batteries.  A smart charger is mandatory!

Nicads can be discharged to 100% repeatedly.  Even cell reversal, while 
not 
recommended, isn't fatal.  That kind of use will have a deleterious effect 
on life, but won't mean nearly instant death as it often does with lead 
batteries.

They also aren't subject to sulfation.  They can be stored in any state of 
charge without damage.  They don't freeze at any human-tolerable 
temperature 
(freezing is somewhere around -50C to -60C).

Their voltage curve as they discharge is much flatter than lead's.  
However, 
when the voltage does finally fall, it plummets and the game's over!  If 
you're not near a power source, you'll probably be calling for a tow.  
Unlike lead batteries, they don't have much "bounceback."  

> Their fill hole is very small, 1/4 to 3/8 inch.  

I believe this is true of at least some of the aircraft nicads.  Seems 
inconvenient, no?  Some pocket plate nicads have a huge excess of 
electrolyte so as to minimize maintenance, and I wonder if this might be 
the 
case with Christopher's cells.

Mine don't have fill holes; the tops are sealed.  There are water / gas 
nipples, and they have an automatic central watering system.

> They need to be mounted with the fill hole up.  

So do flooded lead batteries.

> They need to be mounted so you can reach the fill hole because
> you have to fill then often.  

You bet!  As I said above, cell design influences how often you have to 
water.  But all other things being equal, yeah, you have to water them 
more 
because they gas more.

Mine have catalytic recombination to reduce water usage, but most flooded 
nicads don't.  Honestly, I'd rather not have the recombination.  It makes 
them more fragile.

> The chemical in them is a poison.  

Well, so is the electrolyte in lead batteries.  You keep baking soda 
around 
to neutralize sulfuric acid; you should hve boric acid next to in case of 
a 
KOH spill.

And yes, cadmium is a poisonous heavy metal.  So is lead.  In spite of 
what 
the naysayers claim, nicad disposal is not really an issue, not for these 
guys.  You don't discard big nicads - they're too valuable to junk.  You 
send them back to the manufacturer and have them rebuilt, and then they're 
good for another 10-20 years of use.

> I don't know
> if its true but,  I heard that they need to be mounted in a strong metal 
> box
> to keep them from expanding when they get hot.  

The nickel electrodes do expand and contract quite a bit.  Some modules 
have 
reinforcement built into their case to keep this under control, but others 
need external restraint.  If they do, usually the manufacturer supplies 
the 
necessary hardware.  On some Saft EV nicads, a steel plate is required on 
each end of the row.   

> If they expand to much they
> will loose the seal around the contacts and leak.  

I don't have any experience with that.  I don't know what happens if 
nicads 
that have to be restrained aren't.  I suspect that the positive electrodes 
will deteriorate and maybe disintegrate, but I'm not sure.  My batteries 
are 
of a design that doesn't require restraint, so I haven't looked into it 
very 
much.

On 10 Jul 2005 at 22:15, Christopher Zach wrote:

> In NiCD and NiFE (Nickel Iron) type batteries the positive and 
> negitive plates never change state ...

It's not really accurate to say that the electrodes undergo no physical 
change during cycling.  Again, I'm no electrochemist, but this is what 
I've 
read: during discharge, the cathode oxidizes from cadmium to cadmium 
hydroxide, and the anode reduces from nickel hydrate to nickel hydroxide.  

You have it right that unlike lead batteries' electrolyte the KOH 
electrolyte DOES NOT take part in the reaction.  It's strictly an ion 
carrier.  This is a Good Thing.

KOH concentration runs typically 20% to 35%.  Usually the electrolyte is 
doped a bit with lithium hydroxide, which improves cycle life.

> The plates don't really degrade ...

I'm not so sure I'd say the electrodes absolutely don't degrade, but they 
do 
seem to last longer than the electrodes in lead batteries.

In any case, the "plates" aren't really in plate form.  The active 
material 
is usually powdered, and held either in what they call pocket plates, or 
gelled and placed in a sintered plate.  Pocket plates are longer lasting 
but 
have a lower specific energy than sintered plates.  

There seem to be other methods of holding the active material too - Saft 
uses what they call a "bonded plastic" negative electrode - but don't know 
how these work.  

The separators may have been pig intestine a LONG time ago, but not any 
more; now they are various plastics.  Saft uses a 3-ply material.  And, I 
might add, a few years back it was a weak point for some of their 
batteries, 
though they were, and are, not too keen to admit it.

The electrolyte WILL degrade over time, because (as I mentioned above) it 
inevitably takes up CO2 from the atmosphere. Eventually it may need to be 
replaced.  This is probably best handled by sending the battery back to 
the 
manufacturer for evaluation and servicing.

I hope this clears things up a bit.  As I'm a relative neophyte with 
nicads, 
having been tinkering with them only about 5 years, emendations and 
corrections (or at least divergent opinions ;-) from those with more years 
of experience are welcome.


David Roden - Akron, Ohio, USA

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