There are three main battery types in use in R/C models today: Nickel Cadmium (NiCd), Nickel Metal Hydride (NiMh), and Lithium Ion (LiIon). There are some other types, but they are found much less often in R/C applications. For this article, I'm going to focus on the three major battery types.
Battery capacity is measured by milli-amp-hours (mAh), or amp-hours in larger batteries (Ah). The abbreviation 'C' denotes a battery's capacity when referring to charging or discharging characteristics of a battery. For example, a 720mAh battery pack might specify a maximum charging rate of 720 milli-amps, or 1 times C (abbreviated '1C') and a discharge rate of up to 5C (3600 milli-amps or 3.6 amps).
The amount of time it takes to discharge a fully charged battery pack is proportional to its capacity and the rate of discharge. A 720mAh pack would take almost an hour to discharge at a 1C (720mA) rate. An easy way to calculate the maximum discharge time is to divide 60 minutes by the discharge rate as referenced to its capacity, C. So a pack being discharged at 5C would be fully discharged in less than 60/5 = 12 minutes. Losses in the process reduce the discharge time to 8-10 minutes. Actual discharge time depends on the health of the battery pack and its temperature. Losses are higher at high discharge rates.
Charging time depends on the battery chemistry. In general, better battery life can be obtained by limiting the charge rate to 1C or less. Periodically charging a pack at C/10 (one tenth C) is a good practice because it allows each cell to equalize its charge, relative to the other cells in the pack.
Don't charge a battery that is hot. Artifically cooling batteries while charging does not cool the core of the battery. However, for those of us who fly when it is hot outdoors, it is a good idea to cool the batteries in a portable cooler prior to flight of e-powered planes (being careful to avoid severe condensation on the batteries or mixing the batteries with ice water). Ideally, batteries should be between 32 and 100 degrees F. They don't perform well outside this range and you certainly shouldn't charge them outside this range, particularly at the high temperatures.
A web site that has some good information on batteries of various types is Panasonic NiCd web site.
But this high current capability doesn't come without a cost. They contain cadmium, which is a hazardous material and should be not be discarded into landfills. Once a NiCd is depleted, it should be recycled at an approved hazardous material recycling center. Call (800)8-Batteries for the location of a recycling center near you.
NiCds are heavy, relaive to the other battery types. Therefore, their energy to weight ratio is lower than other batteries. Finally, if NiCds are not fully discharged and recharged on a periodic basis, they develop 'memory' which prevents them from fully discharging or accepting a full charge.
NiCd packs are often found in RC transmitters and internal combustion airplane receivers and in ground-based RC hobbies where weight isn't as much of a concern as in electric-powered airplanes.
NiMh have more energy per weight than NiCds. The tradeoff is that they cannot provide as much current as NiCd batteries. NiMh batteries can normally supply current at up to 5C. NiMh batteries should be charged at less than a 1C rate for good battery life. Peak chargers can also be used for NiMh packs, but the charger should be capabile of measuring the 5-10 millivolt per cell drop.
NiMh batteries have their own disadvantages. They are limited in discharge rates because they have higher internal resistance. Their charging is more critical. Trickle charging will degrade them. Even though their nominal voltage is the same as NiCd, 1.2 volts, their internal resistance causes them to deliver lower voltages under medium to heavy loads. Most people using NiMh cells for electric flight will add an extra cell to the pack to compensate.
More information on NiMh batteries can be found on the Panasonic NiMh web site. If you're interested in more detail about NiMh batteries, check out the Energizer NiMh info page.
Lithium Ion batteries are the newest of the three I'm covering. They are commonly found in cell phone battery packs and the plethera of outdated cell phones has created a healthy market in surplus packs. Li-Ion cells have the best weight to energy ratio, making them very attractive for lightweight park flyers.
Li-Ion batteries have several disadvantages for RC airplane use. First, they have a relatively low current delivery capacity of 2C. In addition, they must be charged at less than 1C, resulting in charging times of at least an hour.
Another factor is that Li-Ion batteries age over time, regardless of whether they are used or not. A typical Li-Ion battery will last for a year and will typically need to be replaced in two years. A number of places on the Internet sell surplus Li-Ion battery packs, typically from cell phones or laptop computers. Many of these cells may have already past the two-year point at which their internal resistance prevents them from providing much of their energy for flying airplanes. In my experience, buying cell phone packs and using the cells yields a number of good cells and a few bad cells. It is easy to determine a bad cell by measuring the voltage drop when a 1C load is applied. Better cells will have less of a voltage drop from a fully charged state.
The major disadvantage of Li-Ion is that they explode and catch fire when overcharged. Don't try to charge one on your NiCd/NiMh charger! A charger designed for Li-Ion cells will have the proper charge profile to prevent overcharging.
If you can obtain a charger for Li-Ion cells, and your application is for relatively low current, then you can save a significant amount of weight by using Li-Ion cells. Just be careful of the safety concerns!
More information about Lithium Ion batteries and safely charging them is available from the Panasonic Li-Ion or the Sony Li-Ion web site.
The Lithium Polymer battery has similar characteristics, with the advantage of lower weight and higher discharge current capabilities. A 2070mAh cell weighs 44gm and a 3270mAh cell weighs 64gm. These cells can be discharged at 4C. They are more expensive than other cell types, but their lower weight and high capacity allows one LiPoly pack to replace several packs of other cell types. The result is that they are economically equivalent to other battery types. More information on them is available from either the Sony LiIon site mentioned above or the Panasonic LiPoly PDF.
There seem to be a number of good chargers available, as well as many medium-to-low quality chargers. The best chargers handle a wide variety of battery types and have several charging and discharging cycles. A few also have computer interfaces which you can use to monitor battery charging and discharging cycles and graph the resulting voltage and current curves. Here are links to some very good chargers. Expect to pay about $170 - $230 for them.
Ginzel Orbit Microlader Great Planes Triton
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