Batteries

General Battery information

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This is not necessarily specific to any one battery type. This page is to help explain the different types of batteries used for E-Flight as well as give some general information. This was information that was posted in a thread on the Ikarus BBS and some information was gathered and put here. Thanks to everyone to added to the discussion.

Terminology

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C = Capacity or charge current
1C = Charge the pack at 1x it's capacity (3000mAh at 3.0 amps, 2400mAh at 2.4amps)
C/10 = Charge at 1/10th the capacity (3000mAh at 300mAh, 2400 mAh at 240mAhs)
Lio = Lithium based battery cell. Lio-Polymers, Lio-Ion are examples
Out of balance battery - pack is when some of the cells in the pack still had some charge left in them while others have no charge remaining.
IR = Internal Resistance
Trickle Charge = A trickle charge is less than C/10 and is meant for continuous charging for standby devices such as cordless phones
Slow Charge = C/10
Peak charge = Charge at 1C and Slow or Trick after pack peaks.
Fast Charge = Charging above 1C

Out of balance battery

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Pack is out of balance is when some of the cells in the pack still had some charge left in them while others have no charge remaining. So when you charged the pack three of the cells peaked before the others did. That would leave the other cells a little under charged. So when you ran the pack out the undercharged cells dumped before the fully charged ones causing the under charged cells to feel warm. Cells begin to really heat up when they reach their "dumped" point.

Fixing an Out of Balance pack - The C/10 charge theory

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Just do a C/10 (c = capacity i.e. 2400mAh pack is charge 1c at 1.2A) charge for 14-16 hours, the magic of "slow charge" is that it equalizes the cells. As the cells fill up some will get full before others, but at the C/10 rate there is no danger of over charging because the cells that are full can safely dissipate the "over charge" as heat, without damage, giving the other cells time to fill up. This is also why it's a good idea to occasionally C/10 charge all your packs, in normal use over a number of cycles they will start to become unbalanced and the C/10 charge will rebalance them. Setting a C/10 trickle rate after a pack peaks will also do this, the pack will peak when the first cell or two does, but the others may not be quite full yet, if left at the C/10 trickle for some time after peak the cells will rebalance. Some people say that they see a reduction in power if the trickle charge after the pack has peaked, stating they perform best hot off the charger. This is not as noticed with NiCds but it does seem to be true for NiMHs, but Suspicions are it is temperature related as NiMHs always seem to perform better when warm and the time spent trickle charging will let them cool.

Charging at higher rates

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This is to drive up the initial voltage of the pack. "Car jockeys" charge at high amp rates to get that extra edge at the start of a race. But the higher voltage only lasts for about 20-30 seconds and then the pack drops off to its normal 1.2 volts per cell. But in flying you don't need or want that higher short-term voltage. You want the pack to charge at a lower amp rate so it fully deep charges for long run out times.

Zapping Cells-

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NiCad cells build up crystalline formations, as they are manufactured and later as they are cycled. These crystals cause tiny bridges to form between the plates inside the cell. The more crystal formations the harder it becomes for the cell to absorb and release energy effectively. This is refereed to as IR or Internal Resistance. A "Zapper" pretty much explains it's self. It's basically a transformer and huge capacitors that store up a very large charge and then releases it through the cell in a couple microseconds. This tends to shatter the crystals breaking down the IR in the cell and making it more efficient. Commercial zappers are very expensive and can pass as much as 30,000kva through a cell. There are less expensive "maintenance" zappers that put up to 90volts at up to 1kva through a cell to help keep crystals from building up.

There is still some debate on whether or not zapping actually does anything for NiMH though. There is also still discussion on whether or not "Zapping" cells will shorten the life. People have shown as much as a 14% improvement in NiCad cell performance with high current draw (above 40amps) applications. But again it may shorten the cell life and number of times the cells can be cycled.

The Biggest Problem With Battery packs

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With our batteries is that they eventually go out of balance and depending on your charger's peak sensitivity, some cells will be overcharged while the others catch up, or the slow cells will not get fully charged and will be over discharged or even reverse charged as the pack runs down.

Only Charge batteries Once a day.

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Batteries don't keep track of time, they don't know if it's today, tomorrow or yesterday. As long as you allow them to cool down before putting them back on charge, you can charge them as many times as you like. Of-course the more you use them the less time they'll last in terms of years, but not in terms of cycles.

Memory Effect

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Nicads in every application suffer "memory effect" to varying degrees, the newer sintered cells are affected less but NiMh do not suffer this phenomenon at all. The problem with NiMh is that for a given current the output voltage is less than NiCd, because of the voltage drop across the higher internal resistance of NiMh cells. That's why one or two extra cells are added to increase the voltage when using NiMh. Note that some NiMh cells are becoming available whose internal resistance is approaching that of NiCd cells.

Cell types

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Nickel Cadmium (NiCd)

NiCd cells can be charged at a pretty high rate. Many time NiCad packs are charged with a rating of up to 2.5 times their capacity. A 2400 mAh cell is thus charged at 6 Amps in 24 minutes. Chargers exist of up to 12A per hour (Robbe's latest). With non sorted packs you shouldn't go above 6A though. The differences between the cells will make the charger false peak and the pack will never be full.

Get a good charger that can handle both NiCd and NiMh cells. Make sure you can charge at least 16 cell packs and set your budget decide on the maximum charge rate. You are bound to charge larger packs sooner or later and a high charge rate will get the time to charge the pack down.

If you want to go above 16 cells, get a second 16 cell charger and split the pack up. Then charge the two packs simultaneously. Two 16 cell chargers are cheaper than one 32 cell charger. This is due to the fact that the power needed to charge big pack is pretty high. This power translates in special electronics and heat precautions.
Another advantage of having two chargers is that when you fly packs smaller than 16 cells, and you have 3 packs, you can fly indefinitely. There are always two packs charging while you fly with one. A good charger doesn't take more than 20 minutes to charge a pack. With a 10 minute flight, you can simply rotate the packs after every flight.

Nickel Metal Hydride (NiMh)

This type of cell has a much larger capacity than NiCd cells. A NiMh cell of 3300 mAh capacity is the same size as a NiCd cell of 2400 mAh. The disadvantage of these cells is that they can't handle bigger currents. For our applications of up to 25 Amps they are very usable. Nevertheless they tend to drop their voltage a bit more than NiCd cells. The latest generation of High Voltage NiMh cells have this a lot less.

Charging NiMh cells must be done a lot more careful than NiCd cells. Most manufacturers say that you shouldn't charge over 1C (1 time the capacity per hour), although many people report charging NiMh cells at 2C without a problem. Whether this over charging will shorten the life of the cells remains to be seen. To be safe it is recommended to stick with the 1C rule.

A few examples:
Panasonic 3300 mAh packs can be charged at up to 3.3 A
Sanyo 3000 mAh packs can be charged at up to 3 A
Sanyo TwiCell 1800 mAh AA cells can be charged at up to 1.8 A

Lithium Ion (LiIon)

These cells have lower weight and higher energy storage capability than NiCd or NiMh, but have a relatively low current delivery capacity of 2-4C. In addition, they must be charged at less than 1C, resulting in charging times of at least an hour. The result is that for high current applications like the Eco8, a number of packs must be paralleled to achieve the necessary total current capacity. Each LiIon cell has a nominal voltage of 3.6 volts vs the 1.2 volts of NiCd and NiMh, so 1/3 the number of cells is needed for the same voltage. Paralleling six packs of three 18650 cells (10.2v at 1400mAh/pack) would create a battery bank capable of delivering 25Amps at a 3C rate and would weigh 27oz (792gm) and provide 8400mAh. Total weight is about the same as a NiCd or NiMh pack, but the capacity is 2-4 times as much, translating into longer flight times.

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.

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 models.

Lithium Polymer (LiPoly)

If you're interested in longer flight times and a safer battery pack, check out the LiPoly cells that have recently become available. They are 3.6 volt cells and can deliver current at 4C rates. They are the lightest cells you can buy today, but are expensive. However, the additional flying time tends to offset the price, making them comparable to buying several packs of NiCd or NiMh cells. Paralleling four packs of three 2070mAh cells produces a battery bank that can deliver 33Amps, weighs 18.5oz (528gm), and provides 8200mAh. Creating a battery bank by paralleling two packs of three of the 3270mAh cells that could deliver 26Amps, weigh 13.5oz (384gm), and have a capacity of 6540mAh.

Charging LiPoly cells is similar to charging LiIon - you need a charger that can handle their special charging characteristics. The nice thing about these cells is that they are not prone to explosion and fire when overcharged - they swell and do get warm or hot. They are still not candidates for charging on your NiCd/NiMh charger. Only use the correct type of charger!