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There are lithium ion (Li-ion), lithium iron phoshate (LiFePO4), lithium polymer (LiPo) and lithium titanate batteries.
All lithium batteries are in fact a type of lithium ion battery, not just the ones we call lithium ion.
Lithium ions move from a negative electrode through a semipermeable membrane to a positive electrode to supply power.
The process is reversed when the lithium battery is being recharged.
There are a variety of lithium chemistries and each has its good and bad points.
LiCoO2 Lithium Cobalt Oxide has a high energy density but it has a low discharge rate and is a flammable safety hazard.
LiCoO2 are the most prevalent Lithium Ion batteries but lithium manganese oxide (LMO) are also common.
Generally lithium ion batteries are in a cylindrical configuration in a metal can.
The Panasonic NCR-18650 and it's clones are the most commonly available Lithium Ion batteries
The Panasonic and most other Lithium Ion batteries now include internal short circuit and overload protection.
LiFePO4 Lithium Iron Phosphate has a lower energy density, but is safer, has a higher discharge rate and a longer lifetime.
LiFePO4 have become popular for RC transmitters, more capacity than NiMH batteries and faster charging.
They are occasionally used in planes but are heavier than other lithium batteries with equivalent capacity.
LiPo Lithium Polymer batteries are flat packs with a polymer separator with higher discharge rates and lower energy density.
LiPo batteries use normal lithium ion chemistries, the polymer separators reduce capacity but permit higher discharge rates.
Changing the physical and chemical nature of the separators, anode and cathode allow specific features to be optimized.
LiPo batteries are currently most commonly used for UAV but Lithium Ion has capabilites we need to consider.
Lithium Ion and Lithium Polymer Battery Energy Densities
Battery & Link to Information Used    Volts (VDC)    Capacity (mAh)    Mass (g)    Energy Capacity (WH)    Energy Density (Wh/kg)
Panasonic NCR 18650B Lithium Ion    3.6    3.2    0.047    11.52    245.11
Zippy Flight Max Lithium Polymer    22.2    5    0.703    111    157.89
MaxAmps MA6S11000 Lithium Polymer    22.2    11    1.235    244.2    197.73
MaxAmps MV5450 Lithium Polymer    3.7    5.4    0.131    19.98    152.52
The Above table shows the comparative energy densities of various popular batteries.
The column on the far right shows energy density per unit weight of the battery and is what you should use for comparison.
The following comparisons are based on the weight of the various batteries being the same - the capacity will vary.
The Panasonic Lithium Ion battery has almost twice the energy density of an average (Zippy) Lithium Polymer battery
The Panasonic supplies at least 25 percent more than a top of the line MaxxAmp Lithium Polymer battery.
The Zippy Compact Pro LiPo battery performance is representative of the majority of batteries in common use in our UAVs.
Substituting a premium battery like the MaxxAmp can directly result in 25 percent longer flight times for equal battery weight.
And if you can accomodate the Panasonic Lithium Ion batteries slow discharge rate you can nearly double your flight times.
Lithium Polymer
The Lithium Polymer battery is popular because it has a high discharge rate, and the square package fits in our UAVs.
The above cut away shows how the individual cells are stacked and wired and how the balance plug is connected to each cell.
To acheive equal energy capacity, each cell needs to be voltage matched to the other using the balance plug during charging.
LiPos batteries are readily available in a wide variety of sizes: Milliamp Hours per cell and Number of cells
For our UAV use two cells to 6 cells in series and 100 to 10,000 milliamp hours are commonly available.
Claimed life span is 600 nominal discharge cycles although reality for most is probably more like 300.
Failure mode can be simple exhaustion, but "puffed" cells that have failed internally are not uncommon.
LiPo batteries can also fail catastrophically from external or internal short circuit or from physical damage and burn vigorously.
They can even burn up while just sitting there not doing anything at all, although fortunately this is less common now.
The energy density available in LiPos can permit flights from 10 to 30 minutes in copters and perhaps up to an hour on airplanes.
Lithium Ion
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Lithium Ion batteries can have one major advantage, up to twice the energy density of a LiPo.
They are always cylindrical and generally have a metal casing giving up some space and weight, but not much.
Even with those constraints Lithium Ion batteries normally have at least 70 percent more energy density by weight than LiPos.
This directly translates to 70 percent longer flight time (if you can use them).
The Achilles heel of the Lithium battery for our use is it's low discharge rate.
Current Lithium Ion batteries have a 2C Discharge rate and if you drain them above that rate it will damage the battery.
In fact many of the popular Lithium Ion batteries now include built in protection against charging or discharging too fast.
But for our use this only means that if you need more power than the battery is rated for it simply won't provide it.
If you are flying a multicopter and a motor needs to speed up for stability and the power is not available you have a problem.
Or if your airplane needs more power than is available, it can pull down the electronics and servos to where they won't function.
Nonetheless, the high energy density definitely justifies designing UAVs that can work within the Li Ion discharge envelope.
A properly designed copter could get 40 to 60 minute flight times and an airplane up to two hours with lithium ion batteries.
A secondary advantage of lithium ion batteries is that they can operate to over a thousand charge cycles if properly cared for.
The NCR-18650 Lithium Ion Battery pioneered by Panasonic is available in a variety of single cell energy capacities.
Since lithium ion batteries come as single cells you will need to either "spot weld" several together with tabs or use a holder.
It is recently becoming possible to buy stock and custom battery packs of 18650 type lithium ion batteries.
The picture above on the right is two 3400mAh Panasonic NCR 18650B 3.7 volt batteries..
GEB 8043125
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There is at least one flat pack style battery claiming equal capability with the Lithium Ion the GEB 8043125
The GEB 8043125 seems to have demonstrated an energy density rivaling a standard cylindrical lithium ion battery.
You can get them from Team Black Sheep and Range Video.
So far there have been results that seem to corroborate the performance claims but there is a down side.
Capacity begins to drop around 50 cycles so cylindrical li-Ions like the 18650 or regular LiPos are better for normal use.
The Battery below left weighs 765 grams and has 18,300mAh 3S3P with a 2C discharge rate of 36 amps at 11 volts.
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Lithium Battery Safety
Using Lithium Batteries Safely is Extremely Important as they can easily catch fire or even explode.
Lithium batteries need special handling because physical damage or short circuiting them is likely to cause them to catch fire.
Store them in a special bag (above right) when not in use or while charging and keep the bag where it could contain a fire.
If a LiPo pack starts to expand (puff up) or if it won't take a full charge on all cells, dispose of it.
Before disposal a LiPo battery should be discharged by connecting it to a resistive load (light bulb or charger-discharge function).
It is a good idea to keep a metal bucket filled with sand handy in case a LiPo fire needs to be quenched.
DO NOT put water on a burning LiPo Battery, lithium takes oxygen directly out of water and keeps on burning.
Lithium batteries are especially likely to misbehave while charging or discharging or when dropped.
Although Lithium Ion batteries have more energy, 18650 batteries are safer due to internal overload protection and steel case.
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All Lithium batteries require tightly controlled charging methods and a quality charger is very important.
Here is a link to an Excellent RC Helicopter Fun article on Battery Chargers (I prefer chargers with built in power supplies).
It is extremely important when charging lithium batteries to charge at the correct current and to provide for cell balancing.