Microsoft word - roar_lipo.doc

8.3.1.6 General information about Li-Poly batteries

8.3.1.6.1 Lithium Polymer packs must be charged with chargers capable of the industry
standard CC/CV (Constant Current/Constant Voltage) charge profile.
8.3.1.6.2 Li-Poly batteries may be charged to a maximum of 8.40V +/-0.04V. Overcharging is a
serious safety hazard and will not be tolerated.
8.3.1.6.3 All Lithium Polymer packs used for motor power must be charged inside a “Lipo Sack”
or similar fire mitigation device proven to withstand a minimum of an 8.4v 5000mah
Lithium Polymer pack failing destructively without showing external flame.
8.3.1.6.4 A Lipo battery pack is damaged when any of the following rules are broken. The
damage is cumulative and cannot be reversed. These rules provide the safest
operation and longest pack life. Going outside these rules may result in a destructive
pack failure.
8.3.1.6.4.1 Do not over discharge Lithium Polymer battery packs and use a Proper ESC cutoff
voltage.
Some newer speed controls give you the option to set a cutoff voltage, and
some do not. The cutoff voltage setting is working properly when the ESC does not
allow the motor to spin anymore when the pack voltage reaches this set cutoff. A
Lithium Polymer battery is damaged when it goes below a set voltage whether under
load or not. The lower the voltage and the longer it stays low, the more damage is
occurring to the cells. If your ESC doesn't have a setting for cutoff voltage, we strongly
suggest not using any Lipo pack with it unless you have a secondary device to cut off
the motor at the correct voltage. By the time the pack “feels soft” at the end of the run
or you notice any decrease in power, the pack has already been damaged. Consult
your Lipo pack manufacturer for the proper low voltage cutoff since this value varies
based on manufacturer.
8.3.1.6.4.2 The maximum safe temperature of a Lithium Polymer pack is 140degF. Generally the
pack temp will INCREASE for about 5-10mins after the run is over, so measure the
temperature of the pack immediately after the run and then again about 10 minutes
later. The faster the car is geared, the more amps the motor is drawing and the
battery is delivering. The less capable of outputting high current (amps) the pack is,
the more it will heat up with the same load (think IB4200's vs. NiCad 2400's on a mod
motor) Exceeding 140degF pack temperature causes damage, and the pack is also
less efficient at near critical temperatures.
8.3.1.6.4.3 Only charge Lithium Polymer packs with a charger that uses the industry standard
CC/CV charging algorithm for Lithium based batteries.
There are two settings you will
need to either set or verify on your charger each and every time before you begin
charging a pack. The first is the pack voltage or cell count (each charger uses
different nomenclature). If your charger is asking for the voltage of the pack, the
choices are 3.7v (one cell), 7.4v (two cell), and 11.1v and beyond (3+cells). ROAR
legal Lithium packs are all two cells, or 7.4v packs so set your charger accordingly.
Some chargers ask for the cell count of the pack (one cell, two cells, and etc.) so you
would set it for a two cell pack. The next setting is the charging rate. Lithium Polymer
battery packs not only show no performance benefit from charging at higher than
recommended rates, but they can be damaged by charging rates that are too high.
The standard charging rate is “1C” which means the actual capacity of the pack in
Milliamp hours. We charge in Amps not Milliamps, so divide the Milliamp Hours (Mah)
of your pack by 1,000 to get your proper charging rate. For a 4800mah pack,
4800mah divided by 1,000 = 4.8 Amp charge rate. For a 3200Mah pack = 3.2 Amps,
and a 5000Mah pack = 5.0 Amps. Unless specifically recommended by the
manufacturer with no loss of cycle life, a maximum of 1C charge rate should always
be used.
8.3.2.7.4 Lithium Polymer packs that will not be run for more than a month or two should be
stored approximately half charged. Do not store them fully charged and do not store
them near fully discharged (down to 6.0v) or damage will occur.
The best way to know
the charge state of a Lipo is to use the Mah displayed on your charger when charging
from fully discharged. For a 5000mah pack driven all the way to cutoff, charge it until
you have 2500mah back into the pack and disconnect it from the charger for storage.
Or use the discharge function on your charger, and discharge a fully charged pack to
1/2 of its capacity. So for a fully charged 5000mah pack, discharge 2500mah from it
before long term storage.
8.3.2.7.5 There are six main root causes for lithium ion/polymer battery fires.
8.3.2.7.5.1 External Thermal Damage – Lithium Polymer cells will get damaged by external heat.
Most manufacturers recommend keeping the cells under 60 deg C or 176 deg F. At
about 90 deg C (194 deg F), the cell will start to balloon up as the electrolytes starts to
break down and the internal layers start to delaminate. If the temperature is
extremely severe (approx 190 deg C or 375 deg F) – the cell will go into thermal
runaway and you will have a flaming mess. The thermal volatility is directly related to
the cell chemistry used by the manufacturer.
8.3.2.7.5.2 Overcharge – Lithium Polymer cells are extremely non tolerant to an overcharge
condition. A standard charge profile is CC/CV to 4.200V. Drastically overcharging a
cell just once is a sure way to send a cell into thermal runaway. Overcharging a cell
slightly but repeatedly is also extremely detrimental for a cell. For example, it you
charge a cell to 4.300V, the lithium ions start plating on the electrodes forming lithium
metal. Lithium ions are not flammable, but lithium metal is. Every slight overcharge
cycle will plate more and more lithium metal resulting in a battery that is very prone to
igniting. The best way to prevent overcharging is to charge through a balancer and to
avoid chargers that do not charge with the standard 4.200V CC/CV charge profile.
8.3.2.7.5.3 Over discharge - over discharging by itself is not dangerous, but it will destroy the cell.
Over discharging below the recommended cutoff voltage will cause the copper to start
dissolving in the electrolyte. The dissolved copper will then start plating on the
electrodes which may start an internal short circuit within the cell. The safety of the
cell is compromised once the plating action starts and the next charge/discharge
cycle will be of concern since there is now an internal short circuit. Don’t store you
cells completely discharged. All cells have a small self discharge when left alone and
if the self discharge takes the cell down below its minimum voltage, then the cell will
be destroyed. It is recommended to disconnect the battery from all electronics
(remove from speed controls, disconnect lithium polymer receiver packs from
regulators etc) since most electronics have a small current drain even in the “off”
position.
8.3.2.7.5.4 External Short Circuit – Lithium Polymer batteries have extreme current capability.
When these cells are shorted out, the excessive current drain will cause the battery to
overheat and possibly cause the cells to go into thermal runaway resulting in a
possible fire.

8.3.2.7.5.5 Internal Short Circuit - this is mostly caused by contaminants getting into the cell at the cell manufacturing level. Contaminants can poke through the separator over time causing an internal short where one of two things can happen. An internal short result in the cell having a high self discharge rate. Or an internal short can cause localized heat buildup and initiate a thermal runaway condition – and thus another possible fire. Another source of internal shorts is the punching process the manufacturer uses to stamp out the anode and cathode electrodes. Some manufacturers use a low cost steel rule die and others manufacturers use a die that costs a couple orders of magnitude more. The lower cost steel die punches tend to leave burrs on the electrodes, while the higher cost dies do not. Burrs have a tendency to puncture the separator and create micro-shorts. This micro-short will create an area of localized heat. In most cases, this will cause the cell to expand (puff up). In bad cases, this localized heat may be enough to ignite the cell. Every time you charge a cell, the cell will expand about 5% in the thickness dimension. This expansion/contraction may cause the burr to eventually rub through the separator. The vibrations and shock from RC use also causes the burr to rub against the separator. The infamous Sony recall was largely attributed to burr type contaminants. 8.3.2.7.5.6 External Mechanical Damage - A lithium polymer battery is made up of 20-30 layers of a very thin sheet copper anode, a thin plastic separator and a thin aluminum cathode. The vacuum sealed aluminum pouch keeps even pressure on the anode/cathode pairs. A dent can create a micro-short by making the stiff metal anode or cathode poke through the soft plastic separator. This microshort will create an area of localized heat. The cell will expand and then becomes a possible fire hazard. Another repercussion of a dent is that some layers of the cell will become delaminated and thus inactive. This means that the working layers will need to work harder to provide current and thus generate more heat in a localized area. ROAR believes that hard cases will greatly minimize the chance of external mechanical damage to the cells. http://www.roarracing.com/downloads/2008_ROAR_Rule_Book.pdf?bcsi_scan_E27F1388B30D28C8=1

Source: http://www.ymr.no/bilder/Tips/LiPo/ReglerROAR/ROAR_LiPo.pdf

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