Tuesday, 1 September 2015

Electronics and Propulsion System in RC Flying - Part 4 - Battery

4. Battery.

  • Lithium polymer (LiPo) batteries are now standard for the RC hobby mainly because of their light weight and high power output.
  • LiPo batteries are very dangerous when used incorrectly, so be sure to read all the safety information before using one!
  • LiPo batteries are composed of multiple cells with each cell measure 3.7 volts.
  • A Lithium polymer is technically a lithium-ion polymer battery.
  • It is very similar to the lithium ion battery but without some of the shortcomings.
  • LiPo Batteries are available in different type, sizes, weights, voltages and capacities.
  • Comparing to metal case used by other small rechargeable cells, LiPo Batteries uses a thin (110 μm), polymer-based packaging material to contain the electrochemical materials. This allows the system to have a flat thin (2 to 5 mm) form factor. 
  • Since the case of the cell starts as a sheet of polymer-laminate, changing the footprint of the cell is cost effective.
  • Also, Lithium Polymer cell uses a 'stacked' construction, adjusting the electrode/electrolyte structure is also easy. 
  • A strict charging procedure is necessary to properly and safely charge Lithium Polymer batteries.
  • Almost all LiPo chargers contain a protective circuit to prevent overcharge and over discharge.
  • This circuit limits the charged cell voltage to a maximum 4.2 Volts.
  • Some circuit also contains a thermal sensor, which disconnects charge if the temperature reaches 90 °C (194 °F).
  • If a cell is inadvertently overcharged, the cell may heat up and end up in flame. 
  • While discharging each cell should cut off at 3.0 Volts.
  • Operating temperatures for discharging should be within 10 deg C to 60 deg C .
  • For charging LiPo batteries you must use a charger designed for Lithium Polymer Batteries only.
  • Do not use a charger intended for NiCd / NiMH batteries - you will permanently damage the cells . 
  • Lithium Polymer cells cannot be "fast charged".
  • Never charge at a current rate higher than the capacity of the battery.
  • For best charging, low charge rates should be used when possible.
  • You must not over discharge your Lithium Polymer cells or they may become permanently damaged.
  • Charge at a constant current until the battery voltage reached the 4.2 voltage limit at which the current is reduced to maintain 4.2 volts. 
  • Operating temperatures for charging are 0 to 45 C 
Lithium Polymer Storage Guidelines 
  • Store the cell in a dry location between –20 C and 30 C.
  • Keep out of direct sunlight. 
  • When storing for an extended period, store between 10 C to 30 C 
  • Store at 40% of capacity. 
  • When charging the first time after long-term storage it may take several cycles to achieve original performance. 
Lithium Polymer Precautions. 
  • Lithium Polymer Batteries and packs if subjected to abuse may cause damage to the pack or cause explosions resulting in personal injury. 
  • Do not expose the battery to extreme heat 
  • Do not short circuit battery 
  • Do not puncture or modify the battery or pack 
  • Do not immerse the battery pack in water 
  • Never reverse charge the battery 
  • Charge only with charger specified by equipment manufacturer 
LiPo Battery Charging Current 
  • Selecting the correct charge current is also critical when charging RC LiPo battery packs.
  • The golden rule here used is - "never charge a LiPo or LiIon pack greater than 1 times its capacity (1C)." 
  • For example a 2000 mAh pack, would be charged at a maximum charge current of 2000 mA or 2.0 Amps. Never charge higher than this or the life of the pack would be reduced.
  • These are the two main numbers you should look for when going to purchase.
  • Unlike NiCad or NiMH battery cells that have a voltage of 1.2 volts per cell, LiPo battery cells are rated at 3.7 volts per cell.
3.7 volt battery = 1 cell x 3.7 volts (1S)
7.4 volt battery = 2 cells x 3.7 volts (2S)
11.1 volt battery = 3 cells x 3.7 volts (3S)
14.8 volt battery = 4 cells x 3.7 volts (4S)
18.5 volt battery = 5 cells x 3.7 volts (5S)
22.2 volt battery = 6 cells x 3.7 volts (6S)
29.6 volt battery = 8 cells x 3.7 volts (8S)
37.0 volt battery = 10 cells x 3.7 volts (10S)
44.4 volt battery = 12 cells x 3.7 volts (12S) 
C rating on a lipo 
  • So what does the C rating on a LiPo mean?
  • The C in C Rating stands for capacity or in simplest terms, the C rating is the maximum safe continuous discharge rate of a pack. 
  • If you see 10C on your battery, it means it can be discharged at 10 times that pack's capacity.
  • An easy way to find your battery's discharge rate is , just multiply the number from the C rating by the pack's capacity. 
  • Please remember that 1000 milliamps equals one amp.
  • Here's an example, using an 11.1V 2000mAh 10C
11.1 volt 2000mAh -10C
2000 milliamps = 2 amps 
2 Amps x 10 = 20 amps continuous discharge

  • This means that, you can safely draw up to 20 amps continuously from that 11.1V 2000 mAh 10C LiPo without doing damage to your battery.
  • The higher the C rating, usually the more costly and heavier the battery gets
Capacity of Lippo
  • Their stored energy expressed either in amps-hour Ah or milliamps-hour mAh.
  • For example, a battery with a capacity of 500mAh should deliver 500mA during one hour before it gets totally discharged (flat). 
Lipo Voltage Choice
  • You now need to work out what voltage you are going to use.
  • Try and keep max amps to around 50~60% of the capacity/C rating of the Lipo  Pack, 
  • For example, if you purchase a 2200mAh 20c pack, then it is rated for 44A constant discharge, so keep the max amps at around 20A~25A IF possible.
  • For low powered models, choose 20c packs
  • For general flying choose 20c~25c packs
  • For high performance models 30c + packs
  • Up to 50w: 1s~2s 
  • up to 100w: 2s~3s 
  • 100w Up to 500w: 3s (This is the practical upper limit for 3s Lipo's, so basically, models of 5lb AUW/2.27kg) 
  • 500w up to 800w: 4s (This is the 0.40~0.46 glow equivalent range favoured by many club flyers) 
  • 800w up to 1000w: 5s 
  • 900w up to 1500w: 6s (this is the 0.60~0.90 ic equivalent range) 
  • 8s~10s packs are for very large and generally specialised models. 
  • A very good rule you should follow for long life of your Lipo battery ( Lipo batteries are costly so be careful). 
  • This simply means that you should never discharge a LiPo pack more than 80% of it's capacity. 
  • This again is where good chargers pay for themselves many times over, so you can see how much capacity the battery takes allowing you to adjust your flight times accordingly to stay within that 80% rule to get the most life out of your pack.
  • If you don't have a computerized charger to confirm the amount of capacity, another good indicator is to measure the open circuit voltage (no load voltage) of the pack or individual cells right after a flight/drive with a digital volt meter or other similar digital voltage measuring device.
  • An 80% discharged LiPo cell, will give an approximate open circuit voltage of about 3.72 to 3.74 volts. 
Lippo Battery- Manufactures Data

Example 1 - Turnigy 4000mAh 4S 40C Lipo Pack 
Spec.- Minimum Capacity: 4000mAh
Configuration: 4S1P / 14.8v / 4Cell
Constant Discharge: 40C  Peak Discharge (10sec): 50C
Pack Weight: 476g   Pack Size: 154 x 50 x 29mm
Charge Plug: JST-XH 
Discharge plug: 5.5mm Bullet-connector

Example 2 - Turnigy 2200mAh 3S 30C Lipo Pack 
Spec. - Minimum Capacity: 2200mAh
Configuration: 3S1P / 11.1v / 3Cell
Constant Discharge: 30C   Peak Discharge (10sec): 40C
Pack Weight: 197g   Pack Size: 108 x 34 x 27mm
Charge Plug: JST-XH   Discharge Plug: XT60 

How to select LiPo Battery for your model.
  1. First you have to decide the power requirement of your model and select the motor.
  2. Once you select the motor you can decide the ESC required. 
  3. After selecting the motor and ESC next step is the Battery.
  4. For selecting most appropriate battery you have to decide upon the following things.
  5. Capacity - flight duration depends on your battery capacity.
  6. C ratting - remember the 80% rule and select the C rating - this also depend upon your motor.
  7. Voltage - no of cells or voltage as per selected motor.
  8. Try and keep max amps to around 50~60% of the C rating.
  9. For example, if you purchase a 2200mAh 20c pack, then it is rated for 44A constant discharge, so keep the max amps of your motor at around 20A~25A if possible.
  10. Battery size and weight -  you should be able to accommodate the battery in a convenient way in your model -  to correctly set the CG of your aircraft you may have to adjust the battery position - so be careful about the dimensions and weight of the battery.
  11. So you have to make a balance between all the above factors and rules for proper selection of Lipo battery, so that you can achieve a safe and long operational life for your Lipo.
  12. There are different grade LiPo batteries avialiable, such as, grade A, B, C. This indicates the quality of battery.
  13. Then as for other parts conduct a proper comparison study and survey to get good LiPo battery for a reasonable price.

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