Winter 2007 Issue

Airborne Radio

Batteries: Care and Feeding

By Del Schier, K1UHF

Figure 1. A two-cell LiPo 3.2-Ahr pack tested at 1C, 10C, and 18C: 3.2 A, 32 A, and 57.6 A, respectively. Notice the severe drop in voltage and capacity at the higher rates. This test shows higher internal resistance than the latest technology.
 

This column is primarily about electric-powered model airplanes. Electric-powered model airplanes depend on batteries. With the availability of nickel-cadmium batteries, electric-powered models became a reality. NiCad batteries still perform well, but they are rapidly becoming unavailable due to the environmental hazards of cadmium and have pretty much been replaced with nickel metal hydride, NiMh. Either type of nickel battery is still a good source of power, but they now are quickly being replaced by lithium. We will discuss how batteries are used and how they should be cared for. I will confine this discussion to NiCad, NiMh, and lithium batteries. Most of the following information applies to rechargeable batteries that are not only used in airplanes, but also in amateur radio stations.

Standard lithium ion (Li Ion) batteries used to power a laptop computer are not normally used in a model airplane. The Lithium-polymer (LiPo) type is the battery of choice for a model airplane. You may be familiar with this type of cell, as they are commonly found in cell phones disguised inside a plastic case. LiPo cells have a soft poly bag over the interior and are relatively fragile when that interior is exposed. When made into packs for a model, they are usually wrapped with fiberglass tape and covered with heavy shrink tubing for lightweight protection. Li Ion cells are in metal tubes, which are more subject to explosion but otherwise are more durable. LiPo batteries are very similar in chemistry and characteristics to Li Ion and there are variations of this chemistry to include lithium manganese and lithium nano potassium.

Most of us are familiar with the average rating of dry cell batteries, that being 1.5 volts. This is not the case of the cells being discussed in this column. Because of their composition, each of the type of cells being discussed has its unique rating. For example, nickel cells are rated at 1.2 volts. However, this is the average voltage over a complete discharge cycle. Typically, they are 1.55 volts after resting from a full charge and .9 volts at full discharge. Lithium cells are rated at 3.7 volts average, and are exactly 4.2 volts at full charge and 3.0 volts at full discharge. Lithium nano potassium are 3.6 volts at full charge and 2.0 volts at full discharge.

The voltage of a battery is only one important characteristic. The most important rating is capacity—how much energy the battery will store. Capacity for batteries is rated in amps over time, Ahr or mAhr. Capacity may also be rated in watt hours or some other unit of energy storage. Another important rating is the battery’s internal impedance or resistance.

Model airplane motors generally draw high current and a battery must be of a size to do the job. The average current draw cannot cause the battery to overheat, and it must produce enough voltage to get the desired power from the motor. The capacity of the battery should also be chosen to provide a reasonable flight time. The problem is that you cannot just put a larger battery in an airplane if you expect it to get off the ground. Lead-acid batteries are a good choice in some applications, but not to power an airplane. Nickel batteries work well because you can draw extremely high currents from them and they are very durable. Lithium packs are far and away the lightest for a given capacity, about one third the weight of nickel chemistries.

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