Li-Po batteries (Lithium-Polymer) are one of the rechargeable types of batteries that have achieved to move Rc modelling a step forward. This is a basic reason for many novice and experienced modelers to switch to electrics. Li-Po batteries have such characteristics that make them unbeatable in comparison with the conventional rechargeable Ni-Cd & Ni-Mh such as:

         1. Low weight, produced in various shapes and sizes.

         2. High storage capacity.

         3. High discharging currents sufficient for the power starving electric motors.

      In brief, Li-Po batteries provide high specific power in almost infinite combinations of shapes and sizes. These advantages are vital for Rc cars, boats and airplanes.  Of course they have some disadvantages such as:

          1. They are more expensive than Ni-Cd & Ni-Mh.

          2. If certain rules are being followed then 300-400 cycles (charging/discharging) can be obtain.

          3. They are prone to ignition or explosion due to volatile electrolyte.

They require proper handling in order to last, otherwise they become useless even after a faulty charge-discharge cycle.

      Li-Po technology and differences from Li-Ion and Li-Po hybrid

      Nowadays most of the in use battery packs are Li-Po. Some high-end transmitters use instead Li-Ion batteries. While they are of the same consistence with Li-Po and need the same treatment, the differences focus on the battery’s elements construction and to the kind of the electrolyte.      

      Li-Ion batteries

      The electrolyte is of organic substance. Is highly flammable and prone to explosion if mistreated. Usually Li-Ion batteries are enclosed in metal case which enchases their weight. Buy rechargeable Li-ion Battery here

      Li-Po batteries

      Their electrolyte is solid, in contrast with the Li-Ion whose electrolyte is like a long solid film. This is located between positive and negative pole in many layers, sandwich like. In this way the construction of thin sizes and shape batteries is doable. A drawback with Li-Po is the relatively slow exchange of electrons through the electrolyte which reduce the battery’s charging and discharging speed. This problem can be overcome by heating the battery which practically is not able in various applies. If it can be achieved then the hazardous factor is reduced significantly.    

      Li–Po hybrids

      By introducing the gel electrolyte the ions exchange increases dramatically. The gel electrolyte reduces the possible leak of the battery although it remains highly flammable. Li-Po hybrids are not as dangerous as Li-Po batteries, but they may also ignite or explode if punctured, hit or short-circuited. When Li-Po hybrids were first introduced, they were more expensive in comparison to the Li-Ion due to the complexity of their structure. Nowadays with the cost been reduced significantly, Li-Po hybrids are more popular than Li-Ion. The Li-Po hybrids have the same elements structure as Li-Po which gives the ability to create shapes suitable for Rc models.

     Li–Po technology

     Each element in a Li-Po pack is wrapped in aluminum sheet which called pouch cell. These cells are ideal in order to create multiple elements connected in series or parallel with no empty space between them as in the usual cylindrical Ni-Mh batteries. Using a thin aluminum film instead of metallic ΄΄boxes΄΄ as in classic batteries, weight is reduced, something most wanted in Rc world. If a pack is unfolded inside is found a long film with thin layers of carbon with alternating wrap between anode and cathode. This is then wrapped and put into aluminum bags with the gel electrolyte. One distinctive feature of Li-Po batteries is that when they become warm the energetic performance increases as electrons exchange speed, gets higher. It’s advisable to keep the Li-Po batteries in a warm environment during winter, in order to achieve better performance.  

     RC Li-Po Specifications

     Voltage – Capacity – Discharge Rate (C)

     Voltage

      In contrary with Ni-Cd και Ni-MH batteries of which each element has a voltage of 1.2V, in Li-Po each element’s voltage is 3.7V.  Most helpful is the following board, where (S) indicates the number of elements of the specific battery.

BATTERY VOLTAGE	NUMBER OF ELLEMENTS
3.7 Volt	2S Buy 2S battery here
11.1 Volt	3SBuy 3S battery here
14.8 Volt	4SBuy 4S battery here
18.5 Volt	5S Buy 5S battery here
22.2 Volt	6S Buy 6S battery here
25.9 Volt	7S Buy 7S battery here
29.6 Volt	8SBuy 8S battery here
33.3 Volt	9S Buy 9S battery here

          It has to be mentioned that usually the battery manufacturers in order to increase the battery’s capacity, connect the elements in a parallel pattern. So at the end of the battery’s specs the letter ‘’P’’ is added stating the number of elements that are parallel connected. As an example, 2S2P indicates that 2 double elements in a row are connected in parallel. Keep in mind that each electric motor with his speed controller needs a specific voltage for a specific number of revolutions (RPM). This number is to be strictly followed in order to avoid unwanted change of RPM. 

     Capacity
     The term capacity, defines the amount of energy that can be stored in a battery and it is measured in Milliamps (mAh), indicating the current that the battery can provide within an hour of full discharging. As an example, a Li-Po battery of 1000 mAh under full load with 1 Ah, needs an hour to fully discharge.  Less than 2 Ah load needs half of the above time i.e 30 minutes.  If more flight time is required then increased Li-Po capacity battery has to be used, taking in consideration that increased capacity means increased weight.

      Discharge Rate (C)

      The third factor is the discharge rate, which is more or less misunderstood by many modelers.  Discharge rate indicates how fast a battery can be discharged safely. The faster the ions move from anode (+) to the cathode (-) the faster the rate ‘’C’’ .

      What is C?

      This letter indicates with how much current a battery can be safely discharged. As an example 10C means 10 (x) times more than the battery’s capacity. In the previous example a Li-Po battery 1000 mAh with 10C it could provide 10Ah current (10 x 1000mAh = 10.000mAh or 10Αh). Time wise, it would provide 166 mAh per minute, so the 1000 mAh battery would be discharged within 6 minutes.

      Here is how it’s calculated:

             1000mAh / 60 minutes = 16,6mAh/minute. Multiply by ‘’C’’ factor.

             16,6mAh/minute x 10 = 166mAh/minute and divide the battery’s capacity by 166mAh

             1000mAh / 166 mAh/minute = 6.02 min = 6min

      Most of the battery packs indicate except the ‘’C’’ factor and the instant ‘’C’’ (burst rating C). Generally the bigger the ‘’C’’, the most expensive the pack is. By many modelers battery packs with high C are being bought, but practically they are not in need of so high and intense currents. For sure the ‘’C’’ should not decrease too much to avoid the destruction of the battery or the speed controller. For a novice user it’s common to by battery with low ‘’C’’ and save money buying 3-4 battery packs. A rule of thumb states that battery packs with 25-30C are more suitable for electric planes/helicopters. For bigger in size and more demanding models 35-40C batteries are more proper. 

      The price of the Li-Po packs keeps reducing. So if a 35C pack has the same price with a 25C then the first one is more preferable because it will perform better in low temperature and will last longer. As everything, so Li-Po when misused they will lose part of their capacity and shorten their life time. If a double ‘’C’’ than originally needed battery pack is chosen, then it will most probably reach the 400 charge/discharge cycles, something that the manufacturers claim.

      The most reliable measurement is to register the Li-Po temperature under high load conditions, in order to justify the use of a high ‘’C’’ factor battery pack. A pack with C factor = 30 doesn’t provide such high rate of current. In real life ‘’C’’ factor is without significant use being impossible to be verified. On top of that keep in mind that as a Li-Po pack gets older, its internal resistance becomes higher and the pack gets warmer during discharging. A golden rule is as follows: Not able to hold the battery in our hand then it’s too hot. If your Li-Po pack becomes hot, then next time better choose a pack with higher ‘’C’’. Also prolonged stay in a hot car during summer time means lower battery performance.

      High discharging current

      One more cause that will increase Li-Po temperature is the drop of voltage /element below 3V. Even with a 40C pack which is being discharged with half of the available current (20 Amp), a voltage drop below 3V will cause temperature increase and drastic life expectance reduce. It’s wise to use the simple rule of 80%. This means that a battery should not be discharged more than 80% of its capacity to be on the safe side. As an example, in a 2000mAh battery the discharge current should not exceed 1,6
Αh having in mind that the battery’s capacity is at 100% (2000mAh). Here is where a digital charger becomes handy, because it’s easy to calculate the aging battery’s capacity.

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      If no digital charger is available then a digital voltmeter can be used to check each shell after a flight. One Li-Po shell would show 3,75V approx. A 3shell pack would show 11,25V, a 6shell would show 29,9V and so on. ATTENTION: Connecting individual shells in series or parallel though their exposed terminal is prone to shortcut that will ignite the shell.

Li-Po connection

      Charging

      The Li-Po leaving the assembly line are charged up to 50% of their full capacity. The Li-Po shells need special charging parameters so it is crucial to use chargers made especially for Li-Po. The maximum voltage per shell (in series) is 4,245V. Practically the charging voltage is regulated at 4,2V or 4,15V for enhanced safety. The use of charging current more than suggested, will not offset the expected reduce of battery’s life time or the ignition hazard. Charge can be accomplished in series or in parallel or in combination of both. In any case the shell’s characteristics must be the same.

      Handling

      Never leave the Li-Po batteries unattended when in charge. Always keep an eye on them, and feel them frequently for any possible rise of their temperature. Do not charge them whilst in the model. At home, place the battery being in charge, away from flammable liquids, or other ignite prone materials in a fire proof space or in a metallic box. If the programmed charging period has expired without the battery being fully charged, terminate the charging procedure and consider this battery as ‘’out of use’’.

      Discharge

      Different manufacturers suggest different values of lowest voltage for each shell. Some declare that the voltage of a discharged Li-Po shell, should not get lower than 3,0 V (free of load), or under 2,5 V (under load). If the shell’s voltage drops below the suggested volumes then the shell is going to be permanently damaged. First Li-Po generation restricted the discharging current to 2-3 C and second generation to 5-6 C. Nowadays there are shells that permit discharging up to 8-40C with reduce of their total capacity and life time as a ‘’by product’’.  When a new battery is purchased is wise to use thick permanent pen to right the maximum discharging current on it.

      The limitation of the discharging current per shell can be overwhelmed if two or more shells will be connected. As an example, if a shell can be discharged with 5A, by connecting two shells in parallel, then we note 10A, with three shells 15A and so on. When a Li-Po battery is discharged with a rate higher than suggested for a long period of time, then the shell’s temperature will rise to such an extent that due to an internal chemical reaction the internal shell’s resistance will rise. If it happens then the battery’s power is reduced significantly.   

      Calculations examples depending on Li-Po  “C” 

      Storage-Maintenance-Precautions

      The Li-Po’s have low discharging rate whilst in storage. Theoretically they can be charged today and be used after a month. Realistically it’s not so. Li-Po should not remain fully charged more than a month. If they put to storage fully charged, then their ability to regain their whole capacity will be reduced significantly. Also during their prolonged storage it has to be sure that the voltage/shell doesn’t drop below 3,0V. If it is time for this drop to occur, then is better to partially charge them up to 5-10%.    

      Don’t ever break the shell. Not even try to unfold the plastic cover. Do not throw them into fire. If your eyes come in contact with the liquid electrolyte then rinse them will plenty of water and pay a visit to an ophthalmologist. If they are punctured violently, leave them for 30 minutes in order to note if they react or not. Fire may arise from Lithium’s contact with the oxygen in the atmosphere.        

      Don’t try to charge a shell if any sign of damage is noticed. If a shell of a battery pack gets swell during charging, immediately interrupt charging and transfer it to a safe area because ignition hazard is imminent even after a long period of time. Do not try to charge them with reverse polarity. Storage and use temperature from 0⁰  to 50 ^o C. Their temperature should not exceed 70 ^o C. The flame that is being emitted from a burning Li-Po shell resembles a firecracker and reaches the height of 3-4 meters.

      They are dangerous.

     Believe it or not they are! The reason for this are the chemical substances that are made of, are being segregated by an insulate material which in case of breakage permits them to come violently in contact. The reaction is fierce, a substantial amount of gases is produced and the pack’s temperature rises to a very high degree. This dangerous combination makes them extremely dangerous because the violent detonation will lead to an explosion. It is advised to use a charging safe box or a safety bag. 

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