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To many R/C enthusiasts, batteries are the single most important component of an R/C car or truck in terms of acceleration, top speed, and run time. Good quality batteries that are part of a properly constructed pack can be the difference between maximum enjoyment of your R/C car or truck or mediocre performance that leaves you wanting for more.

The purpose of this article is to provide you with some basic information regarding batteries and pack construction so you will be able to make an informed decision when you purchase your next battery pack.

Battery Types

There are many types of batteries on the market however three chemistries in particular are used by R/C enthusiasts: Nickel Cadmium, Nickel Metal Hydride, and Lithium Polymer. Each of these are reviewed below.

NiCd: Alkaline nickel battery technology originated in 1899 when Waldmar Jungner invented the nickel cadmium battery. The materials were expensive compared to other battery types then available and its use was limited to special applications. In 1932, the active materials were deposited inside a porous nickel-plated electrode and in 1947 research began on a sealed NiCd battery that recombined the internal gases generated during charge rather than venting them. These advances led to the modern sealed NiCd battery which is in use today.

Among the rechargeable batteries, the NiCd remains a popular choice for applications such as portable radios, emergency medical equipment, professional video cameras, power tools, and R/C vehicles. Over 50% of all rechargeable batteries for portable equipment are NiCds. The introduction of newer battery chemistries caused the use of the NiCd to drop slightly. However, recognition of the limitations on the alternative chemistries has led to renewed interest in the NiCd chemistry in some sectors.

Some of the distinct advantages of the NiCd over other battery chemistries are:

1) Fast and simple charge
2) High number of charge/discharge cycles (if properly maintained, the NiCd  provides over one thousand c/d cycles.).
3) Excellent load performance, even at cold temperatures (the NiCd can be recharged at low temperatures).
4) Simple storage and transportation (the NiCd is accepted by most air freight companies).
5) Easy to recharge after prolonged storage.
6) Forgiving if abused.
7) Economically priced.
8) Available in a wide range of sizes and performance options.

    The NiCd is a strong and silent worker; hard labor poses no problem. It prefers fast charge over slow charge and pulse charge over DC charge. Improved performance is achieved by interspersing discharge pulses between charge pulses. Commonly referred to as burp or reverseload charge, this charge method promotes high surface area on the electrodes, resulting in enhanced performance and increased service life. Reverse load also improves fast-charging because it helps to recombine the gases generated during charge. The result is a cooler and more effective charge than with conventional DC chargers.

    Another important purpose of reverse load is to minimize the crystalline formation for improved battery performance and prolonged service life. Research conducted in Germany has shown that reverse load adds 15% to the life of the NiCd battery.

    The NiCd does not like to be pampered by sitting in chargers for days and being used only occasionally for brief periods. In fact, the NiCd is the only battery type that performs best if fully discharged periodically. All other chemistries prefer a shallow discharge. So important is a periodic full discharge that, if omitted, the NiCd gradually loses performance due to the formation of large crystals on the cell plates, also referred to as memory.

    The word memory was originally derived from cyclic memory, meaning that a NiCd battery can remember how much discharge was required on previous discharges. Improvements in battery technology have virtually eliminated this phenomenon. Tests performed at a Black & Decker lab, for example, showed that the effects of cyclic memory were so small that they could only be detected with sensitive instruments. After the same battery was discharged for different lengths of time, the cyclic memory phenomenon could no longer be detected.

    The problem with the modern NiCd battery is not so much the cyclic memory but the effects of crystalline formation. In most cases, however, there is a combination of the two phenomenon (from now on when memory is mentioned, crystalline formation is being referred to). The active materials of a NiCd battery (nickel and cadmium) are present in finely divided crystals. In a good cell, these crystals remain small, obtaining maximum surface area. When the memory phenomenon occurs, the crystals grow and drastically reduce the surface area. The result is a voltage depression which leads to a loss of performance. Some of the capacity may still be present but cannot be retrieved because of the battery’s low voltage table. In advanced stages, the sharp edges of the crystals grow through the separator, causing high self-discharge or an electrical short.

    Another form of memory that occurs on some cells is the formation of an inter-metallic compound of nickel and cadmium which ties up some of the needed cadmium and creates extra resistance in the cell. Reconditioning by deep discharge helps to break up this compound and reverses the capacity loss.

    The battery’s capacity will gradually deteriorate if repeatedly charged and not fully discharged (never discharge a battery to 0 volts, 0.9 volts is considered discharged). Another problem with NiCds is a lower cell voltage (1.2 volts) when compared to single use batteries (1.5 volts). Despite having a lower initial voltage, NiCds provide better performance than single use batteries due to a much flatter discharge curve. Lastly, the cadmium contained in NiCds is highly toxic and very bad for the environment. Always recycle old NiCd batteries, never throw them away!

    NiCd cells should be primed before use.  Priming refers to charging then discharging the cell.  Generally, a NiCd cell must by charged/discharged (c/d) 5 to 7 times. In some cases, up to 50 c/d cycles are required before the cell reaches it’s full potential.

    Battery manufacturers recommend to slow charge a new NiCd battery for 24 hours before use. This initial trickle charge helps to redistribute the electrolyte to remedy dry spots on the separator that may appear when the electrolyte gravitates to the bottom of the cell during long storage. A slow charge also helps to bring all the individual cells within a battery pack up to an equal charge level because each cell may have self-discharged to different capacity levels during storage.

    Although most NiCd batteries are suited for fast charging in an hour or so, fast charge should only be applied between 41F and 113F. When charging a NiCd below 41F, the efficiency of oxygen recombination is greatly reduced and pressure build up occurs. Sometimes, hydrogen can be generated as well.

    To compensate for the slower metabolism at cold temperatures, a low charge must be applied, especially at the beginning and end of the charge cycle. Special methods are available for charging at cold temperatures. The NiCd is the only commercial battery that can accept charge at extremely low temperatures.

    The charge acceptance of a NiCd at higher temperatures is drastically reduced. A battery that provides a capacity of 100% if charged at a moderate room temperature can only accept 70% if charged at 113 F and 45% if charged at 140 F. This is demonstrated by the typically poor summer performance.

    All batteries should be kept in cool and dry storage. Refrigerators as a storage media are recommended, but freezers should be avoided because not all chemistries are suited for storage in freezing temperatures. When refrigerated, the battery should be placed in a plastic bag to protect against condensation. Placing a desiccant pouch in the plastic bag is a good idea.

    The NiCd battery can be stored unattended for up to five years. For best results, a NiCd should be fully charged, then discharged to 0.9 volts per cell and storage in a cool, dry place. A NiCd that is allowed to self-discharge is subject to formation of large crystals (memory).

    Prolonged storage of NiCd (and NiMH) requires priming the batteries before use by applying a slow charge followed by one or several discharge/charge cycles. Depending on the length and temperature of storage, two to five cycles may be required to regain full performance. The warmer the storage temperature, the more cycles are needed. Some cycling may be required after as little as two months of storage.

    NiCd Cutaway

    nicdcut.gif

    NiMH: NiMH (Nickel Metal Hydride) provides incremental improvements in capacity over the NiCd at the expense of reduced cycle life and lower load current.

    Research of the NiMH system started in the seventies as a means for hydrogen storage for a Nickel Hydrogen battery. The metal hydride alloys were unstable in the cell environment and the desired performance characteristics could not be achieved. As a result, the development of the NiMH slowed down. New hydride alloys were developed in the 1980’s that were stable enough for use in a cell. Since the late eighties, the NiMH has steadily improved, mainly in terms of energy density. Design engineers have indicated that the NiMH has a potential of yet higher energy densities.

    Some of the distinct advantages of today’s NiMH are:

           30% more capacity over a standard NiCd.
      Less prone to memory than the NiCd. Periodic exercise cycles need to be done less often.
      Fewer toxic metals. The NiMH is currently labeled "environmentally friendly".

    Unfortunately, the NiMH also exhibits some negative attributes and in some aspects lags behind the NiCd. For example:

    Number of cycles: The NiMH is rated for only 500 charge/discharge cycles. Shallow rather than deep discharge cycles are preferred. The battery’s longevity is directly related to the depth of discharge. Therefore, stop running your R/C vehicle at first sign of slow down. DO NOT run it until it no longer moves!
    Fast charge: The NiMH generates considerably more heat during charge and requires a more complex algorithm for full-charge detection than the NiCd if temperature sensing is not available. (Most NiMH batteries are equipped with internal temperature sensing to assist full-charge detection). In addition, the NiMH cannot accept as fast a charge as the NiCd; its charge time is typically double that of the NiCd. The trickle charge must be controlled more carefully than on the NiCd.
    Discharge current: The recommended discharge current of the NiMH is considerably less than that of the NiCd. For best results, manufacturers recommend a load current of 0.2C to 0.5C (one-fifth to one-half of the rated capacity). This shortcoming may not be critical if the required load current is low. For applications requiring high power or a pulsed load, such as a high performance R/C vehicle, the more rugged NiCd is preferred by some people.
    Self-discharge: Both NiMH and NiCd are affected by reasonably high self-discharge. The NiCd loses about 10% of its capacity within the first 24 hours, after which the self-discharge settles to about 10% per month. The self-discharge of the NiMH is one-and-a-half to two times higher than that of the NiCd. Selecting hydride materials that improve hydrogen bonding to reduce self-discharge typically also decrease the battery capacity.
    Capacity: The NiMH delivers about 30% more capacity than a NiCd of the same size. The comparison is made with the standard, rather than new ultra-high capacity NiCd. Some ultra-high capacity NiCd cells provide a capacity level approaching that of the NiMH. (Ultra-high-capacity NiCd batteries cannot provide as high a load current as standard NiCd batteries. They are also less durable in terms of cycle-count but are longer lasting than NiMH batteries). As of late, NiMH cells have seen a dramatic jump in capacity with some sub-C cells having capacity ratings of 3600 MAH.
    Price: The price of the NiMH is about 30% higher than that of the NiCd. Price may not be a major issue if the user requires high capacity and small size. In comparison, ultra-high capacity NiCd cells are only slightly higher priced than standard NiCd cells. Capacity for cost, the ultra-high capacity NiCd is more economical than the NiMH.

    Lastly, be aware that a charger specifically labeled for NiMH must be used.  Never use a charger labeled only for NiCd!

    Below is a graphic that shows the relative discharge curves of the three most popular battery chemistries.

    batcurves.jpg

    Lithium Polymer: LiPoly cells are a dream come true R/C fans and are on the cutting edge of battery technology. Compare the voltage, capacity and weight of LiPoly cells to NiCd or NiMH cells and you will see that LiPoly cells provide more energy per gram than any other battery - up to 4 times the specific energy (Watt hour/Kg) of NiCd or NiMH batteries!

    In addition, the use of a polymer electrolyte allows for very flexible design, including construction of prismatic cells that measure as little as one millimeter (0.039") in thickness. Batteries that resemble flexible rubber mats that can be rolled or formed to fit tight spaces will also be feasible.  LiPoly manufacturers are not bound by the cylindrical design necessitated by NiCd and NiMH chemistries. LiPoly cells are also very light weight.

    The original concept of the Li-polymer battery is based on the use of solid electrolyte. This design offers great potential with respect to fabrication, ruggedness, safety and low cost. It also avoids the high flammability of the liquid electrolyte used in the Li-ion and metallic lithium batteries, should hazardous leakage occur through cell container rupture. However, limitations in conductivity of the solid polymer have resulted in adding some liquid to the solid electrolyte meaning the risk of fire has not been eliminated.

    With all these positive points it’s no wonder R/C plane and helicopter enthusiasts have been quick to adopt this technology. So with all this good stuff, what’s the downside to LiPoly? 

    lipoly.jpg

    Fire risk, cost, and poor performance when cold. When charging, LiPoly cells represent a real risk of fire.  Therefore, it is often recommended that they are only charged out doors and always under supervision.  Being a new technology, LiPoly cells are more expensive than either NiCd or NiMH. In time, the cost should come down. When taking the negatives into consideration, LiPoly cells loose much of their attractiveness. 

    Nonetheless, LiPoly (or some derivative of) is the battery chemistry of the future.  Over time, look for it to take away much of the rechargeable battery market from the NiCd and NiMH chemistries.

    The Ultimate Battery Technology – UltraCapacitors: What if I told you a type of battery exists that can be fully recharged in under 1 second, has a perfectly flat discharge curve, can be fully discharged in under 1 second, requires no maintenance (cycling, discharging, etc.), can go through an infinite number of charge/discharge cycles, and doesn’t loose capacity over time; would you be interested? Of course you would.  Well, this type of battery has existed for many years and it’s called a capacitor.

    So why haven’t capacitors replaced traditional batteries yet?  Ironically enough, it’s due to lack of capacity.  While capacitors have all the features listed above, they don’t have enough capacity for a given physical size to be useful, until now (almost!).

    A new type of capacitor is making its way into the market, a capacitor with a much greater capacity than capacitors of old – the ultra capacitor.

    Like batteries, capacitors provide electrical energy using chemicals stored within a container. But one of the traditional drawbacks of capacitors is that they don't provide nearly enough juice as a common battery of a similar size.

    Honeywell Specialty Materials in Seelze,Germany, has developed a chemical solution that will help make so-called supercapacitors.  Like batteries, these devices could hold enough energy to power R/C vehicles, computers, or even (real) cars.  The heart of Honeywell's solution is a fluorborate salt, a chemical whose physical structure allows it to store a tremendous number of electrons. The chemical is dissolved in a special solution at high concentrations — about 300 grams of salt per liter of solvent — and then embedded onto carbon plates.

    This chemical process is important because it improves how capacitors store and deliver energy.

    The main difference, compared to batteries, is that the battery uses a chemical reaction to store electricity.  In a capacitor, it's a pure physical process. The chemicals in the plates are where you are storing the electrons.

    But by using special chemical concoctions such as Honeywell's salt, ultracapacitors can now hold about a million times more energy than of capacitors of similar size — and without losing the important advantages of ordinary capacitors.

    For example, storing electrons in a capacitor doesn't require any chemical reaction. That's why capacitors can be charged in seconds and release huge amounts of power quickly, too. (A typical use of capacitors is in electronics such as camera flashbulbs.)

    Another advantage of ultracapacitors is their extremely long life cycle. Batteries can produce electricity only as long as their chemicals hold out. Even rechargeable batteries have a limited life since their chemicals wear out from repeated charging and discharging.

    For a capacitor, the charge/discharge cycle is typical. There is no overcharging, no effect on the lifetime of the chemicals, and 100 percent capacity all the time.

    Such characteristics now make ultracapacitors an ideal candidate for uses traditionally filled by conventional rechargeable batteries. One such new use, for example, would be in new hybrid electric cars.

    Currently, hybrid cars use heavy, rechargeable nickel-metal hydride batteries to store energy recaptured when drivers step on the car's brakes. That energy can then power a small electric motor to move the car at slow speeds, saving the car's engine — and gasoline — for higher speeds.

    Ultracapacitors could easily do the job of the hybrid's battery without the weight or cost. 

    Automobile makers have reportedly been working on supercapacitor-equipped cars. A Toyota spokeswoman says the Japanese company had a few years ago shown a Prius concept car using what it called an ultracapacitor. But the company says it's unlikely such a car will make it to showroom floors anytime soon.

    Many say that even with chemicals such as Honeywell's fluroborate salt, supercapacitors won't catch up with the capabilities of traditional batteries. And there are several disadvantages that still need to be worked out.

    Zapping NiCd and NiMH Cells

    Zapping  is said to increase the cell voltage by 20 to 40mV when measured under  a 30A load. This would increase the cell voltage from 1.25V to about 1.28V.  (Note that industry tends to rate nickel-cadmium at 1.25V whereas the  consumer market has adapted 1.20V. It is simply a preference of rating).  According to experts, the voltage gain is stable; only a small drop is  observed with usage and age.

    Zapping is done by putting a very high current through a cell. Usually a bank of capacitors, about 70,000uF, is charged to about  70V. The energy is discharged through the cell resulting in a few thousand Amps for a very short time. This spot welds the cell's  internal connections.

    The welded connections reduce the cell's internal resistance.  This results in a higher voltage under high loads. For this reason, many serious R/C competitors use zapped cells.

    Zapping only works on some cells. It appears that only high current cells such as Sanyo SCR series, Panasonic SCP, and GP cells series respond to zapping. This is probably because these cells use metallic electrodes. Many high capacity cells use a foam electrode and  foam can't be welded. Both NiCd and NiMH cells can respond to zapping.

    Cells are zapped with something called a zapper (great name!). Hardcore R/C racers sometimes buy a zapper of their own but most people rely on buying cells that have been zapped by a cell retailer. Either way, if you want to squeeze every last drop of performance from your batteries, make sure they’re zapped.

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