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The Characteristics of Li-Ion/Polymer Battery

The nominal voltage of a Li-Po battery cell is 3.7V (about 4.23 V when fully charged). Two and three cell batteries are available giving us a choice of 7.4 or 11.1 volts. Li-Po batteries can provide substantial current, 5000mah lipo 2s continuously and 12A for short (30-second) bursts. Li-Po cells have a flexible, foil-type (polymer laminate) case. Since no metal battery cell casing is required, Li-Po batteries are very light. Because of the absence of metal casing and less space utilized in intercell spacing, the energy density of Li-Po batteries is over 20% higher than that of a classical Li-ion battery and store more energy than nickel-cadmium (NiCd) and nickel metal hydride (NiMH) batteries of the same volume.

Care must be drawn in using Li-Po batteries. Their capability to provide high burst currents, lack of a rigid case, and overcharging sensitivity presents both electrical and thermal hazards. Intelligent chargers designed specifically for Li-Po batteries must be used to recharge these batteries. Overcharging a Li-Po battery will likely lead to explosion and/or fire. Li-Po cells must not be discharged below 2.75V per cell, or else the battery will subsequently no longer accept a full charge and may experience problems holding voltage under load.

Very little battery performance data seems to exist regarding the current generation of low-cost Li-Po batteries. Since we were unable to locate discharge or charge rate charts for the batteries, we decided to perform our own characterization.

Since lithium polymer (and lithium ion) are so sensitive to overcharging, the individual cells that make up the battery pack are charged independently. With NiCad and NiMH packs, the standard charging method is to apply a voltage and current across the series-connected cells so that the same current passes through each cell. The voltage across the entire pack is the sum of the individual cells voltages, but the cell voltages are not necessarily all equal. In fact, it is almost certain that they are not. Since nickel-based cells are relatively tolerant of some overcharging, there is usually no danger. It is not uncommon for one cell in a NiCad or NiMH battery pack to be dead, and yet the rest of the cells charge normally. What that means is in the case of an 8-cell pack, with the nominal voltage of each cell being 1.2 V (9.6 V total), if a charging voltage of 10.4 V is present, each cell would ideally receive 10.4 / 8 V = 1.3 V. If one cell is dead, however, the voltage across each cell would be 10.4 / 7 V = 1.5 V. That 15% increase can be handled by most NiCad and NiMH cells, but a 15% overvoltage applied to a lithium cells would eventually result in a failure, and likely a fire. Li-Po batteries are also intolerant of over discharging, and tend to die if discharged below around 2.5 V. In operation, controller circuitry should prevent the cell voltage from dropping below 3.0 V. Cell temperature should never exceed 90 °C in order to prevent the internal separator polymers from melting and allowing plate shorting through physical contact.

To the left is my Mini Pulse XT aerobatic airplane. It uses the 450 motor and a 3 cell , 11.1 V, 2100 mAh, Li-Po battery. Futaba 4-channel radio.

Early lithium batteries had a rather high internal resistance, and had rather low discharge rates. As with all technology that is doggedly pursued, significant improvements have been made to the point that the contemporary Li-Po batteries may be substituted in most systems for the original NiCad or NiMH batteries. Discharge rates of 20C or 25C are commonly available on the commercial market. You can find 20C on google, there are Gens ace 20C 2000mah lipo, Gens 25C RC 2s lipo batteries in the market. In battery discharge terminology, each "C" is a discharge current equivalent to the value of the energy capacity of the cell (it is not the abbreviation for the Celsius degree unit). In the case of a 1,200 mAh rating, 1C is equal to a discharge current of 1,200 mA, or 1.2 amps. A 10C cell can deliver a continuous current to a load of 10 x 1.2 A = 12 A during its discharge cycle. The E-flight 450 motor shown to the right would require atleast a 12C battery to deliver full rated power. This motor, by the way, is the one I use in my electric-powered sailplane (2-meter wingspan) and in my 4-channel aerobatic airplane (an E-flight Mini Pulse XT).

In my opinion, Lipo batteries are grossly overpriced, given the huge production volumes in effect. When you consider that almost every mobile electronic device and cordless power tool uses these batteries, the cost should be a lot lower. Maybe it is partially product liability insurance that manufacturers have to purchase that keeps the prices high, but I just paid $50 each for two 3-cell (11.1 V), 2,100 mAh packs. That is at least 2x to 3x what is should be costing at this point in the lithium cell evolution.