[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: (ET) battery drainage question

David,

Good point, all my NiCad experience is with battery packs, not individual cells and is from about 20-30 years ago.  When we let our packs go all the way dead, sometimes one ore more of the cells would fail and need to be replaced.  I defer to your greater experience with NiCads.  Personally though, I am happy to move away from NiCads.

The lithium cells we've been playing with are very robust.  They accidentally wired a third of the truck pack backwards, 108 volts against 36 volts, without causing any serious damage to the batteries or electronics.  (There were definitely sparks when they attempted to drive the truck.)  Lithiums do require specific charging methods, but if those are followed they have excellent life expectancies.  If I can believe what I heard, they have tested their batteries to 7000 cycles with the pack still at 75% of capacity.  The energy density they possess is amazing and they have the ability to deliver incredible bursts of current without damage to the pack and only minimal voltage drop.

There are certainly places for NiCads, but in an EV lithiums are the future.  They are beginning to automate some of the assembly of the new packs.  Hopefully prices will drop as they develop better manufacturing methods.

George

"David Roden (Akron OH USA)" <etpost drmm net> wrote:
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.



_______________________________________________
Elec-trak mailing list
Elec-trak cosmos phy tufts edu
https://cosmos.phy.tufts.edu/mailman/listinfo/elec-trak