On 06 May 2004 16:41:36 GMT, (DGManley) wrote:

Li-poly cells are better and safer than Li-ion cells:

Before you become convinced of their safety, take a gander at this warning
issued by the Academy of Model Aeronautics.

I didn't say "safe", merely safer, but I was wrong. See below.

Here's an account with pictures of what happens when a fully charged
Li-poly cell is charged for an hour at 1 amp and twice its rated
voltage...

http://www.rcgroups.com/forums/showt...pagen umber=1

I've seen it said that Li-poly cells are more stable than Li-ion but
no supporting evidence as to why this should be so. Now here's a
statement from the manufacturer of "Thunder Plus" Li-poly and Li-ion
cells. Makes interesting reading.

"The “Lithium Polymer battery” used in the hobby and in most
commercial applications is really a Lithium-ion battery that has had
the internal power generating materials folded to a flat matrix
instead of rolled up to fit in the cylindrical can, it is then
installed in a light aluminium container that was original designed
for food storage but works great. The good news is that the “lithium
Polymer battery” is a bit lighter and has the ability to support
higher loads because of it’s ability to dissipate heat under higher
loads. This is the good part; the other side of the coin is that under
the charging process the “Lithium Polymer cells” don’t include any of
the safety systems that are part of a Lithium-ion cylindrical cell, so
the cell has no way of protecting itself. This then requires control
of the charging cycle imperative. There was/is a false presentation in
the hobby that the “Lithium Polymer Battery” is completely safe and
won’t create an unhappy experience. The electrolyte used in these
cells is liquid and flammable. If the cell or pack is overcharge
(voltage) for any reason the first indication is that the cell will
start to puff up like a balloon. If you are lucky the cell pops (gas
release vent) and you cut off the charge before further problems. I
will say that lithium doesn’t like exposure to oxygen."

I guess that answers me. Li-poly are no safer than Li-ion. I've only
looked at the Kokam website in detail, but did notice that most of the
cells they sell have protection circuitry attached to them though all
are flat and contained in just a plastic membrane.

I'm not sure whether the fire/explosion problem is due to:

1) the high energy density
2) ease of electrical damage at high discharge rates
3) ease of electrical damage with improper charging
4) using a thin plastic membrane to contain an assembly
that is spontaneously flammable in air and explosive in the
presence of water.

My guess (and that's all it is) is that (3) and (4) are the most
significant causes of Li-poly fires.

There's already a direct bearing on cockpit fires because the
batteries in iPAQ PDAs (certainly in the 36xx series and probably the
others too) are Li-poly cells. They are flat, encased in a silvered
plastic membrane and don't appear to contain cell protection circuits.
The cell is stuck to the plastic backplate of the iPAQ with nothing
separating it from the PCB except its membrane. I replaced one
recently in my iPAQ 3630: that's how I know this.

BTW, I would not want to be in a closed cockpit with a lithium cell
that had even vented without burning: the 'white smoke' is almost
certainly LiOH (lithium hydroxide) and that's at least as corrosive as
caustic soda.

From the Academy of Model Aeronautics:

Emergency Safety Alert: Lithium Battery Fires
(Added 4/23/04)

Lithium batteries are becoming very popular for powering the control and power
systems in our models. This is true because of their very high energy density
(amp-hrs/wt. ratio) compared to Nickel Cadmium (Ni-Cds) or other batteries.
With high energy comes increased risk in their use.
The principal risk is fire which can result from improper charging, crash
damage, or shorting the batteries. All vendors of these batteries warn their
customers of this danger and recommend extreme caution in their use.

In spite of this many fires have occurred as a result of the use of Lithium
Polymer (Li-Poly) batteries, resulting in loss of models, automobiles, and
other property. Homes and garages and workshops have also burned.

A lithium battery fire is very hot (several thousand degrees) and is an
excellent initiator for ancillary (resulting) fires. Fire occurs due to contact
between lithium and oxygen in the air. It does not need any other source of
ignition or fuel to start, and burns almost explosively.

These batteries must be used in a manner that precludes ancillary fire. The
following is recommended:

Store and charge in a fireproof container, never in your model.
Charge in a protected area devoid of combustibles. Always stand watch over the
charging process. Never leave the charging process unattended.
In the event of damage from crashes, etc., carefully remove to a safe place for
at least a half hour to observe. Physically damaged cells could erupt into
flame. After sufficient time to ensure safety, damaged cells should be
discarded in accordance with the instructions which came with the batteries.
Never attempt to charge a cell with physical damage regardless of how slight.
Always use chargers designed for the specific purpose, preferably having a
fixed setting for your particular pack. Many fires occur in using
selectable/adjustable chargers improperly set. Never attempt to charge lithium
cells with a charger that is not specifically designed for charging lithium
cells. Never use chargers designed for Ni-Cd batteries.
Use charging systems that monitor and control the charge state of each cell in
the pack. Unbalanced cells can lead to disaster if it permits overcharge of a
single cell in the pack. If the batteries show any sign of swelling,
discontinue charging and remove them to a safe place—outside—as they could
erupt into flames.
Most important: NEVER PLUG IN A BATTERY AND LEAVE IT TO CHARGE UNATTENDED
OVERNIGHT. Serious fires have resulted from this practice.
Do not attempt to make your own battery packs from individual cells.
These batteries cannot be handled and charged casually such as has been the
practice for years with other types of batteries. The consequence of this
practice can be very serious and result in major property damage and/ or
personal harm.

—AMA Safety Committee



--
martin@ : Martin Gregorie
gregorie : Harlow, UK
demon :
co : Zappa fan & glider pilot
uk :