| > of events happens. The pack starts to get hot, then hotter, then
> even hotter. Sooner or later the heatshrink fractures. This can
> result in the metal case of one cell touching the metal case of
> the other, a dead short resulting in more heat, much much more heat.
> At this point the pack will probably explode, rather like a hand
> grenade. Not a good idea, particularly in the confines of your
> car trunk!
Not exactly true. the metal case of the NiCad cell is the
negative electrode (commonly refered to as a Cathode) If you
connect negative to negative, NOTHING will happen, unless there is
a short someplace else in the pack. The real reason you want to
discontinue charging relates to the way NiCads work. As the cell
is charged, gas is produced in the cell. The cell of common fast
charge NiCads is designed to accomodate some amount of internal
pressure as a result of the gases created during the charging
process. When really overcharged, a cell could well Explode, so
manufacturers (always wary of the potential for a liability suit)
manufacture the cells so they won't blow up. To do this, all
NiCads are produced containing a vent in the cell which allows
excess pressure to escape when the cell is overcharged. When this
vent is forced open, some of the chemicals neccessary for the
proper operation of the cell can escape as both gas and liquid.
When this happens, your cell loses efficiency, if enough escapes,
the cell stops functioning altogether.
> WAys of detecting when to shut off the fast charging vary. Some
> chargers detect a "peak" voltage, but SCRs peak lower than SCs and
> after peaking the voltage goes down. In the UK there is a charger
> that detects this voltage change turn about (Schumacher Peak Charger).
> Another method is a timer to time limit the charge. Probably the
> safest method is a temperature detecting charger, as heat is the
> danger that we are trying to avoid. The problem is that your cells
> have to be cold before you start, OK if you have enough good packs
> for a meeting, but a problem if you don't.
There are three common type of chargers. Timed, Delta-Peak, and
thermal. Timed chargers are the types with a current limiting
resistor and a simple mechanical or digital timer, which cuts the
charge off, or switches to a trickle charge, after a specific
amount of time, the charger current is usually calibrated so that
given a discharged cell of specific rate (typically 1200MaH), you
will end up with 80 percent or so of a full charge after 15
minutes. Delta-Peak chargers rely upon a characteristic that
NiCads exhibit when charged, this being that the voltage across
the cell will increase under charge, until the cell is fully
charged. After this point the cell internal resisitance rises
quickly (as does the temperature of the cell) and the voltage
across the cell starts to drop. The "Peak" voltage is detected by
a sample and hold circuit, or a differentiator, that "sees" the
drop in voltage after the peak and shuts off the charge, resulting
in a battery that is nearly 100 percent charged. Thermal cut-off
chargers rely upon the same characteristic as Delta-peak chargers,
only they look for the temperature rise associated with full
charge, instead of the voltage drop. They monitor cell temperature
through the use of a thermistor placed in contact with the pack.
The problems with thermal detecting chargers are these : The pack
must be cool before you can charge it and the pack MUST be in contact
with the detector. I think that delta-peak chargers are the best
choice, they give the most consistant charge, and you never have
to question whether or not your cell is in good contact with the
detector.
> Personally we use 4 peak voltage detecting chargers, and one voltage
> turnaround detecting charger to top the cells up. We always fully
> discharge cells after use and store them with a shunt across the
> terminals.
I believe that NiCads should be stored with a slight charge, This
is the way manufacturers ship them. "Cold" dead NiCads may
develop internal shorts that must be "burnt" out with a short,
high voltage pulse, before the cell will operate properly again.
> As a footnote there is a way of giving your cells a "kick" if they
> are a little tired. Put them in the frezer for 24 hours (the colder
> the better) then pump them with power till they get to 60 centigrade
> then discharge a quickly as possible, auto headlamp bulbs are the
> cheapest. If you graph the voltage drop into a 1 ohm resistor before
> and after you will find as much as .2V improvement during the first
> few minutes. Do take care during the pumping phase as at 4 amps
> they pass quickly through 60!
NiCads cells develop a "Memory" when used. If you constantly
charge a cell to full charge, and then discharge it only 50%,
after a while the cell behaves as if it only has 50% of its rated
capacity. "Erasing" this memory is as simple as fully charging the
cell, and then discharging it fully, once or twice. The point at
which a NiCad is considered discharged is when it is only
producing 1 volt (it nominally produces 1.2 volts). A more
significant problem with NiCad packs involves the fact that all
cells are not created equal. This means that some of the cells
discharge, and also charge, at different rates than others in the
pack. After a while, the inequality of an individual cells gets
to the point where some cells interfere with the operation of
others (for instance, if one cell is 50% discharged while the
others are 75%, when charging, the 50% cell will "fool" the
charger into thinking the pack is charged, when really only the
one 50% cell is charged. The other cells in the pack will still
only be 75% of full charge). Fixing this problem is called
"Equalizing" the pack, and is simple as trickle charging the
discharged pack until it is fully charged. Most serious NiCad
users (for cars, or in my case planes) Equalize their packs once
every 5 or 6 charges.
|
| Hmm, now I know why I like Gates Cyclon cells so much, you don't
have any of these problems....
> Not exactly true. the metal case of the NiCad cell is the
> negative electrode (commonly refered to as a Cathode) If you
> connect negative to negative, NOTHING will happen, unless there is
> a short someplace else in the pack. The real reason you want to
Not exactly true.... :+} The metal case of the first NiCad cell
is at (let's call it) 0 volts, but (in a 7.2 volt pack) the metal
case of the last cell in the pack is at 6.0 volts. Shorting these
metal cases together results in a shorting out of all the cells
between, which causes lots of warmth....
>> We always fully
>> discharge cells after use and store them with a shunt across the
>> terminals.
>
> I believe that NiCads should be stored with a slight charge, This
> is the way manufacturers ship them. "Cold" dead NiCads may
> develop internal shorts that must be "burnt" out with a short,
> high voltage pulse, before the cell will operate properly again.
NiCads stored with shorts across the terminals will not only attempt
to reverse charge any unequally charged cells, but will also develop
internal shorts. While these internal shorts can be blown away
with high current pulses, the lifetime of the battery is going to
be shortened, perhaps drastically...
This is why I avoid NiCads like the plague, you can't store them
charged, you can't store them discharged, you can't charge them
fast, you can't overcharge or undercharge them, you can't read the
state of charge with a voltmeter, you can't avoid memory effects....
Does anyone have a car with a 6-cell pack that would be willing
to try a Gates Cyclon Monobloc battery (6 volts, 2500 mAh) for a
comparison? I've got a spare from Tycho's battery change....
Willie
|