Re: (ET) Snow at last (blower data)

["David Roden" <etpost drmm net>]

A few comments on Christopher's nicad posts.  

He exaggerates a bit, but not much.  Nicads are not quite "totally 
unaffected" by cold.  Their capacity does decline, but far less than lead 
acid batteries' does, and at lower temperatures.  They are unquestionably 
a 
good choice for cold climates.

On the other hand, they're typical Northerners; heat makes them cranky.  
Their cycle life will start to take a hit at temperatures in the 40-45 deg 
C 
range.  If you see 60 deg C internal cell temperatures, you had better 
just 
stop right there because those batteries are really hurting.

One interesting aspect of this aversion to heat is that a nicad charged at 
cell temperatures over 35 deg C won't reach full charge. Hot charging also 
has a deleterious effect on cycle life, though exactly how much it hurts 
isn't entirely clear to me.  Of course both discharging and charging 
causes 
some internal heating (their resistance isn't zero), so after discharging 
them for a while, it's best to hold off charging until their internal 
temperature falls below 35 deg C if at all possible.  

You always, always have to cool them!  Typically this means mounting the 
blocks or cells with space between the long sides and blowing air between 
them.  In hot climates such as the southern US (AZ, TX, etc.) you're 
probably going to need some fairly aggressive cooling measures.  You also 
shouldn't do without some means of measuring their internal cell 
temperature.  It's very possible that you'll have some periods where you 
simply can't or shouldn't use them because they've gotten too hot.

Nicads in general are longer-lived than lead batteries because the primary 
means of lead battery depreciation, sulfation and grid corrosion, don't 
affect them.  But they're NOT indestructible!  I already mentioned the 
negative effects of heat.  Nicads tolerate overcharging well but if 
carelessly charged they can go into thermal runaway, just as lead 
batteries 
can.  I know of a person who literally melted the tops of his flooded 
nicads 
with an unmonitored and unregulated charger.  He was lucky - they could 
have 
caught fire.  Needless to say he turned his expensive batteries into junk.

Nicads are also negatively affected by the absorption of CO2 from the air 
into their electrolyte.  I believe this can be fixed by changing the 
electrolyte, but I'm not sure.

Nicads' electrolyte is a base.  Neutralize this even a bit and you've 
murdered them.  This means you can NEVER use a battery filler or 
hydrometer 
which has been in contact with the acidic electrolyte of a lead battery.  
Some people are so concerned about this cross-contamination of nicad 
electrolyte that they suggest not housing nicads in the same room or even 
the same ^building^ as lead batteries, because of the acid mist evolved by 
most flooded lead batteries during charging.

In a lead battery the electrolyte changes chemically during cycling, but 
in 
a nicad it serves only as an ion carrier.  This means that you can't 
measure 
nicads' charge by reading their specific gravity.  Voltage is also fairly 
stable until the cell is almost discharged, and then it falls with a thud. 
 
Thus voltage can give you something along the lines of "Something's left" 
vs. "I'm empty," but if you want an accurate indication of SOC probably 
your 
best bet is to monitor the number of amp hours withdrawn from your 
battery.  
(That's also a pretty good way to control charging.)

Other considerations: Nicads gas more than lead batteries, and have higher 
self discharge.  Their charge efficiency is thus somewhat lower than lead 
batteries' (meaning charging is less efficient with more losses to self-
discharge, heat, and electrolysis).  OTOH they're not damaged when left 
partly or fully discharged for prolonged periods.  This is a VERY big plus 
in my book.

Properly maintained and charged, nicads can indeed last for between 1,000 
and 10,000 charge cycles depending on design and depth of discharge.  
(Christopher's batteries are lower in specific energy than many nicads, 
and 
this usually means longer cycle life, though I don't know why.)  

Carelessly maintained and charged, they can be killed in a few dozen 
cycles, 
just as lead batteries can.

Christopher seems to be doing rather well with his so far, but I have to 
say 
that he has more confidence in the ET charger's ability to treat nicads 
right than I do. ;-) Charging nicads in parallel is also an iffy prospect, 
though he seems to be dodging that bullet so far too.  I wish him luck, 
and 
would caution anyone trying to duplicate his setup that other apparently 
similar aircraft nicads may not be as forgiving as his seem to be.


David Roden - Akron, Ohio, USA

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