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Sport Aviator Editor, Frank Granelli led off this technical information series on “Batteries” to teach the basics as they apply to our RC systems (both receiver/servos and transmitters). Frank explained both Ni-Cd (nickel cadmium) and Ni-MH (nickel metal-hydride) type batteries as are supplied with most modern day RC systems.

If you haven’t as yet, I urge you to go back in this same Sport Aviator category (Flight-Tech ---- How to Articles) and read the Part-One and Part-Two articles. In a casual and easy to read format, Frank, describes such things as battery types, their characteristics, how to charge them, how to test them, how long you can fly on a charge, how to store them and what kind of battery life you should expect.

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My job in this Part-Three will be two –fold:

            1) To introduce you to Li-Poly (lithium polymer) and Li-Ion (lithium ion) batteries and let you know of possible applications to power (operate) your RC systems and what might be gained (or not gained!) using these type batteries.

            2) Then I intend to go on to describe the truly wonderful advantages of using these lithium batteries to provide electric power for your model aircraft. I think you will be surprised what these new type batteries can do for you.

Although Ni-Cd, Ni-MH, Li-Poly and Li-Ion are all rechargeable batteries, their characteristics (and internal chemistry) vary considerably. For the most part, Ni-Cd and Ni-MH batteries can be charged and used the same way, although the Ni-MH cells require a slightly lower fast charge current.

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The popular AstroFlight 110 Deluxe peak detect charger can fast or slow charge both Ni-Cd and Ni-MH battery cells)

The lithium batteries are a totally different breed, with different characteristics, different charging techniques, different operational considerations and different storage requirements.

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Three typical, dedicated Li-Poly chargers. The Peak Electronics Sirius lithium charge at the left, the AstroFlight 109 lithium charge in the middle and the FMA Direct CellPro 4S balanced charger at the right

If improperly charged or mishandled, lithium batteries can be quickly ruined and in some cases can even cause hazardous situations, like a meltdown or outright fire!

You might have had some concerns in the past identifying the difference between Ni-Cd and Ni-MH cells. They are usually cylindrical in shape and sometimes not very prominently marked. Li-Poly batteries are usually contained in flat, rectangular shaped aluminum pouches making them easy to distinguish from other type batteries. Li-Ion cells tend to look more like Ni-Cd and Ni-MH cells. So right away, that’s a flag raiser! Make sure you know what battery you have so that you can select the proper charger, charge current and the proper operating conditions.

Let’s proceed with some details on the new and very popular Li-Poly batteries. Later I’ll get back to Li-Ion batteries. Unlike the 1.2 volts nominal (per cell) for Ni-Cd and Ni-MH cells, the Li-Poly cell has a much higher nominal voltage of 3.7 volts per cell. It is said to be fully charged when the cell reaches 4.2 volts.

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On the other end of the scale, Li-Poly batteries shouldn’t be discharged below 2.5 volts per cell, but as a conservative recommendation, don’t let it go below 3.0 volts per cell. If you let your Li-Poly battery goes below 2.5 volts per cell it will suffer irreversible damage. Keep this in mind when using Li-Poly batteries, never let them be charged above 4.2 volts and never discharged below 2.5 volts (3.0 volts better!).

Since Ni-Cd and Ni-MH cells have much lower voltage you can’t substitute battery types using the same cell count. For example a six (6) cell Ni-Cd battery pack will have a nominal voltage of  7.2 volts (6 X 1.2V), while a two (2) cell Li-Poly battery pack will have 7.4 volts (2 X 3.7V).

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One of the biggest advantages of going to Li-Poly batteries is the fact that they can offer more capacity, in a lower weight battery, that is physically much smaller in size. As an example, a 4 cell 600 mAh standard receiver size Ni-Cd pack might weigh 3.3 ounces, while a 2 cell 800 mAh Li-Poly battery might weight only 1.5 ounces. At first glance this appears to be half the weight with 25% more capacity. But then you must consider the voltage going to the receiver and servos. The 4-cell Ni-Cd pack will send about 5 volts to the receiver. The 2 cell Li-Poly would send 7.4 volts which is much too high for your receiver and servos. To cope with this higher voltage problem, you will need a regulator to reduce the 7.4 volts down to a useable 5 to 5.3 volts. These regulators, usually included as part of the on-off receiver switch system, are sold by MPI, Perfect Switch, FMA Direct and Batteries America to name a few.

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What you have to decide is whether the weight savings (only 1.6 ounces!) justifies a different charging technique and an add-on voltage regulator? Also keep in mind that when charging Li-Poly batteries they must be removed from the aircraft and then attached to a dedicated Li-Poly charger. Li-Ion batteries, although based on the same lithium chemistry, are safer to charge since they have internal protection circuitry to protect against dangerous overcharging. Li-Ion batteries can be charged inside the aircraft but must otherwise still be treated the same as Li--Poly batteries when charging and discharging.

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With Ni-Cd and Ni-MH cells you can charge the pack while it remains inside the aircraft’s fuselage. Another offset to discuss is cost! A standard four cell Ni-Cd battery pack can be obtained for as little as $11.00. The 2 cell 800 mAh Li-Poly pack may cost $30.00 and the regulator another $20, bringing you up to $50.00 total.

I think you can see where this discussion is going. Using the newer Li-Poly batteries only buys you an advantage when you are talking about a very small aircraft, like maybe a 6 ounce parking lot flyer or even smaller. But once you get to about 10 ounce aircraft weights and more, the Ni-Cd and Ni-MH cells are a better deal for powering RC systems. The possible exception to this is that some very large model aircraft might benefit from Li-Poly power especially when using a large number of high-powered, high-resolution servos. Keep in mind that at this point we are still talking about fueled powered aircraft.

(Ed. Note: Most competition pilots, Precision or Scale Aerobatics, use Lithium Ion receiver battery packs regulated to 5.3 volts. This saves about 4 ounces in weight while providing more energy to the servos. An 88 in. oz. output servo produces about 105 in. oz. of torque operating at 5.3 volts. Servos also center better at the higher voltage. The extra cost is not usually a factor since these aircraft already cost upwards of $4,000!)

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Taking this one step further, you decide to substitute Li-Poly batteries in your RC transmitter. Special Li-Poly battery packs are available that will fit directly into the same space used for an 8 cell Ni-Cd or Ni-MH pack in your RC transmitter. But this kind of battery substitution is even less likely than it is for the airborne (receiver/servo) power. To operate your transmitter you will need a three (3) cell Li-Poly pack that will supply 11.1 volts (3 X 3.7V). One of the typical packs I have in mind is rated at 1800 mAh and cost approx. $75.00. Yet, I can still go to Batteries America (http://www.batteriesamerica.com/) and buy an 8-cell Ni-MH battery pack, with a cable and connector, rated at 1650 mAh. That pack would cost me $34.95 or half the price of the Li-Poly equivalent. The Ni-MH pack could be charged while it remained inside the RC transmitter (using the external charging jack!). But the Li-Poly battery pack would have to be unplugged and removed from the transmitter every time to be recharged.

So in the case of the transmitter, and considering that weight is not a factor in this application, the Li-Poly battery offers you no real advantage.