The Six Deadly Myths of Lithium Polymer Batteries by Bob Burtis

I hope the title caught your attention. It is not my intention to scare but to inform. In the past few years Lithium Polymer batteries, due to their high power density and low weight, have become very popular with the model aviation community. As a result, manufacturers and vendors have spread an awful lot of misinformation while trying to get a piece of the action. It is my intention to give you the information that will help you find your way around these bogus claims. Lithium Polymer Batteries are no more dangerous than live hand grenades - if you know what you are handling.

Myth 1: “Failure to follow our instructions may lead to the battery catching fire.” Nonsense! Lithium Polymer Batteries don’t burn - but they may cause fires.

For something to burn requires three things, Oxygen, Fuel and Heat. (Known as the “Fire triangle”.) When a Lithium Polymer Battery “pops off”, you can remove one, two or even all three of these things without effect. It is a self-sustaining, thermal-chemical reaction, much like epoxy setting-up, but far more violent. Heat (About 130 degrees F.) triggers this reaction and the Lithium Polymer Battery “vents with fumes” until consumed. These fumes are toxic (some poisonous) and can reach temperatures of 1400 to 1600 degrees F. (760 to 871 C.). Incidentally, each cell in the pack will explode individually. Sometimes several seconds apart and the spent casings have been found as much as two hundred yards away after the fire.

Myth 2: “Our Lithium Polymer Batteries contain (Insert name of metallic compound) making them safe.” Nonsense! This is called “doping” and intended to raise the trigger point temperature. I believe some cells are now triggered around 150 degrees F. Incidentally, Cobalt is low-level radioactive. Something too keep in mind while your battery is “venting with fumes”.

Myth 3: “Always use a proper (our) balancer to charge our Lithium Polymer Batteries.” Nonsense! A properly constructed battery should never need balancing. The manufacturing process will usually insure the battery is balanced to with in 10 mV (the maximum allowance) and reputable manufactures will verify this during the Quality Control testing. I know of no balancer on the market today with a resolution of less than 100 mV. Save the money you were going to spend on a balancer and buy yourself a better battery.

Myth 4: “Our Lithium Polymer batteries have a C rating of 20 with a burst rating of 25C.” Nonsense! Very few Lithium Polymer Batteries on the market have a C rating exceeding 18 and any time the C rating is exceeded it will reduce the battery’s life expectancy by a factor of 6 if it does not trigger the reaction.

Myth 5: “Never over (under) charge a Lithium Polymer battery” or “Always use a proper (our) charger”. Well, not complete nonsense. Failure here could lead to shorter battery life or possible heating of the battery to the point of triggering the reaction.

Myth 6: “Never charge a Lithium Polymer Battery unattended.” Again, not complete nonsense. Just good common sense for any type of battery but with Lithium Polymer, you’ll want to be there when it “pops- off”. The show is spectacular and you wouldn’t want to miss it.

What do you do when your battery “pops-off” and you’re not outdoors? First, move away from the battery bearing in mind the noxious fumes and possible shrapnel. Get everyone out of the building and then call 911, Fire or Police. Do not try to move or save the battery. You’ll probably get hurt and/or just make the situation worse, just stay out of the way until the battery consumes itself. After the battery consumes itself, if you have a fire extinguisher (5ABC) and the proper training you might try to contain the damage until the Fire Department arrives. Don’t get hurt; let the professionals handle the situation.

What is the real culprit with Lithium Polymer Batteries? As a battery ages or is abused, its chemistry changes. Lithium Batteries develop Lithium Salts, which raises the internal resistance of the cell much like the sulfating of a Lead/Acid battery. When you pass a current through a resistance you develop heat and as I’ve already pointed out several times, it is heat that triggers the chemical reaction within the cell.

Checking the internal resistance of cells of any battery is easy. It only takes a “Multi-meter” and a “Load”.

Internal Resistance = (V1-V2) / I2

Here’s how:

· Internal Resistance is measured with the battery pack partially discharged.

· First V1 is the open terminal voltage and measured with no current (I1=0).

· Apply a load current (The industrial standard is 0.5 C).

· Now measure V2 and I2 after a few seconds (stabilized).

· Subtract V2 from V1 and divide the result by the current (I2).

You now have the definitive perameter of the condition of your battery.

Remember,

”Build straight to fly straight” and “Thrust is a must.”