Re: (ET) battery drainage question

["David Roden (Akron OH USA)" <etpost drmm net>]

On 14 Apr 2008 at 10:48, George Stoneberg wrote:

> As far as NiCds go, I ran them with R/C model airplanes years ago and if 
> I
> remember right it was bad to let them sit at a discharged state as they
> could reverse polarity.  

I've never heard this, and I can't see any mechanism by which it would 
occur.  The standard with NiCd *cells* is to store them fully discharged - 
in fact it's preferable to store them shorted!  (Again, this isn't 
recommended with NiCd batteries, as the discharge to zero can reverse a 
cell.)

> I would think the best approach with NiCds would be to cycle them
> monthly, ie dishcarge them fully and then fully recharge them.  

I don't agree.  In spite of what you may have read, full discharge of a 
NiCd 
battery is not beneficial.  As mentioned above, it can cause cell 
reversal.  
Like any other battery, NiCd batteries last longest when not deeply cycled.

> I did this occasionally even while they were getting good use to try
> and keep them from memory problems.  

True memory effect is very rare and occurs only when a cell is cycled to 
EXACTLY the same point regularly.  The canonical example was a 
solar-charged 
satellite which got exactly the same amount of sun each day and used 
exactly 
the same amount of energy each night.  Normal cyclic use will not cause 
memory effect in NiCd batteries.

What is sometimes called "memory effect" is actually voltage depression.

Normally a NiCd cell holds close to its nominal voltage until it is almost 
100% flat, then voltage falls rapidly.  However, a cell with voltage 
depression will drop in voltage shortly after being placed into use, then 
remain close to that plateau until it is nearly flat.  

Many electronic devices aren't designed to recognize this, and behave as 
if 
the battery is flat when its voltage has merely plateaued a bit lower.  
The 
device shuts down with a "dead battery" alarm.  The user thinks the 
battery 
is flat, when in fact it has far more capacity still available that the 
gadget can't use.

>  I'd buy lithiums over NiCds now every time.

Not me!  Good NiCd cells have a longer service life, cost less, and are 
far 
more tolerant of careless charging.  If their lower specific energy isn't 
a 
problem, they're a superior choice in terms of usability.

Regrettably, the trend toward tinier and tinier electronic devices almost 
forces the use of lithium's higher specific energy.  You probably wouldn't 
want to have a NiCd battery in your cell phone, for example.