With appealing attributes such as low weight, high energy density, and ever greater discharge rates, Lithium-Polymer (LiPo) batteries have transformed all facets of RC. The emergence and continual improvement of these batteries has provided a significant performance boost for RC cars, boats, airplanes, and helicopters, while also paving the way for new vehicles such as multi-rotors. Lipo batteries can quickly become damaged or even catch on fire. Today, I will be sharing the basics knowledge of lithium polymer batteries so you can use them safely and effectively in your future projects.
What is a Lipo Battery?
Lithium Polymer batteries (henceforth referred to as “LiPo” batteries), are a newer type of battery now used in many consumer electronics devices. They have been gaining in popularity in the radio control industry over the last few years, and are now the most popular choice for anyone looking for long run times and high power.
Lipo batteries are a type of rechargeable battery that has taken the electric RC world by storm. When compared to their old NiCd/NiHm batteries, there are 3 main points that make them a better option:
- Have nearly double the capacity with half the weight of NiCd or NiMH battery packs
- Retain charge significantly longer than NiCd or NiMH batteries when not being used
- Do not develop a "memory" as NiCd batteries do
- With LiPo cells in combination of series and parallel, LiPo packs can be built to power most all RC aircraft
But, just as a coin has two sides, there are some disadvantages to LiPo batteries as well.
- Have very short shelf life of two to three years
- Very sensitive to high temperatures
- Very sensitive to high and low voltages. Exceeding either will damage the battery
- Prone to explode if exposed to heat, or charged/discharged improperly
Read more about LiPo vs NiMH: Comparison of Two Most Popular Battery Type on The Market
What Are Lipo Battery Ratings?
There are three things that you should pay attention to when buying your battery. The Voltage, the Capacity (mAh), and the C Rating (C). Here is a quick overview of how each is indicated.
Voltage / Cell Count
A LiPo cell has a standard voltage of 3.7V. For the 7.4V Lipo battery above, that means that there are two cells in series (which means the voltage gets added together). This is sometimes why you will hear people talk about a "2S" battery pack - it means that there are 2 cell lipo batteries in Series. So a two-cell (2S) pack is 7.4V, a three-cell (3S) pack is 11.1V, and so on.
The voltage of a battery pack is essentially going to determine how fast your vehicle is going to go. Voltage directly influences the RPM of the electric motor (brushless motors are rated by kV, which means 'RPM per Volt'). So if you have a brushless motor with a rating of 3,500kV, that motor will spin 3,500 RPM for every volt you apply to it. On a 2S LiPo battery, that motor will spin around 25,900 RPM. On a 3S, it will spin a whopping 38,850 RPM. So the more voltage you have, the faster you're going to go.
Here is a list of Lipo RC battery pack voltages with cell counts most beginners will be using;
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)
If you are wondering what the number in parenthesis means, it is a way the battery manufacturers indicate how many cells hooked in series “S” the battery pack contains.
Battery packs can also be wired in parallel to increase the capacity. This is indicated by a number followed by a "P". Example: 3S2P would indicate 2 x three celled series packs hooked up in parallel to double the capacity.
Capacity indicates how much power the battery pack can hold and is indicated in miliamp hours (mAh). This is just a fancy way of saying how much load or drain (measured in milliamps) can be put on the battery for 1 hour at which time the battery will be fully discharged.
As an example, an RC Lipo battery that is rated at 1000 mAh would be completely discharged in one hour with a 1000 milliamp load placed on it. If this same battery had a 500 milliamp load placed on it, it would take 2 hours to drain down. If the load was increased to around 15,000 milliamps (15 amps), a very common current drain in a 400 sized RC helicopter while hovering, the time to drain the battery would be only about 4 minutes.
Almost all Lithium Polymer batteries you buy today will come with a ‘C’ rating. The ‘C’ in C rating stands for Capacity. The C rating is the maximum, safe, continuous discharge rate of a pack as specified by the manufacturer, so when you see 20C printed on your label, it means it can be discharged at 20 times the pack’s capacity,"continuously". Capacity is usually measured in mAh and a popular cell size is 2200mAh. Therefore, so by way of example:
2200mAh 20C Cell = 2.2A x 20 = 44A continuous discharge
Along with the continuous discharge rate, some batteries display ‘burst’ discharge rate. This is typically twice the continuous discharge rate. It usually means the battery is capable of allowing twice the current, but only for a few seconds at a time.
Lithium Polymer batteries come partially charged and are significantly more volatile than the alkaline, Ni-Cd or Ni-MH batteries used in RC applications. Here are some guidelines for using Lipo Batteries.
Before the First Charge
- Make a visual and physical inspection of the battery. Look for any damaged or loose leads or connectors, broken shrink tubing or other irregularities which may cause a short circuit and eventual fire.
- Check for proper polarity of the factory installed leads and connectors. Use a digital voltmeter to ensure that the polarities are correct.
- Check the voltage of the battery using a digital voltmeter or the E-flite Power Meter (EFLA110). The voltage of the cells within the battery should be between 3.5V and 3.9V each.
The voltage of a new 2S (7.4V nominal) battery should be between 7.0V and 7.8V The voltage of a new 3S (11.1V nominal) battery should be between 10.5V and 11.7V The voltage of a new 4S (14.8V nominal) battery should be between 14.0V and 15.6V The voltage of a new 5S (18.5V nominal) battery should be between 17.5V and 19.5V The voltage of a new 6S (22.2V nominal) battery should be between 21.0V and 23.4V
Charging Guidelines and Warnings
1. You MUST use a charger specifically designed to charge Li-Po batteries ONLY. The charger must be capable of “constant current / constant voltage” charging. Failure to use the proper charger may result in a fire causing personal injury and/or property damage. DO NOT use an Ni-Cd or Ni-MH charger at any time.
2. The battery should always be charged in an open area away from, and not placed on, any flammable materials. The battery should never be charged inside of the model or a full-size vehicle. It is also suggested that an “ABC” type fire extinguisher be readily accessible in the charging area in case any issues resulting in fire are experienced.
3. Never charge the battery unattended.When charging the battery, you should always remain in constant observation to monitor the charging process and react to potential problems that may occur.
Now you know most of what you need to know about LiPo batteries. I make no claims that this article teaches you everything there is to know about LiPo batteries, but hopefully it helps give you some insight into how they work.LiPo batteries have the potential to overtake NiMH batteries in general use in the next few years, quicker than any battery in history. It is certainly an exciting time for the hobby, and things are changing on a frequent basis. Just remember to have fun, and ask questions! The only dumb question is the one you don't ask!