LiPo batteries (abbreviation for Lithium Polymer) are a type of rechargeable battery that has swept the electric RC world. They are the main reason electric vehicles are gaining in popularity and now in many cases overpowering fuel powered vehicles. When going electric the initial cost of the batteries and charger are expensive but then eliminates constant purchasing of expensive nitro.
LiPo batteries have characteristics that make them a great choice for RC vehicles over conventional rechargeable battery types such as NiCad, or NiMH.
LiPo batteries are light weight and can be made in almost any shape and size.
LiPo batteries have large capacities, meaning they hold lots of power in a small package.
LiPo batteries have high discharge rates to power the most demanding electric motors.
Basically LiPo batteries provide high energy storage to weight ratios in an endless variety of shapes and sizes.
There are a few down sides with RC LiPo batteries but there is no perfect solution.
LiPo batteries can be expensive but coming down in price all the time.
Safety issues - because of the volatile electrolyte used in LiPo’s, they can catch fire or explode.
LiPo batteries require unique and proper care if they are going to last for any length of time more so than any other battery technology. Charging, discharging, and storage all affect the lifespan – wrong procedures and a LiPo is no good in as little as one mistake.
LiPo Battery Ratings
Unlike conventional 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. The benefit here is fewer cells can be used to make up a battery pack.
Vehicles will use 1S LiPo’s and up. The voltage of the LiPo battery that is chosen depends on how much speed/power the person is seeking and what the motor and ESC is capable of handling. Most 1/10 scale cars will use 2S batteries and some 3S. For example the Traxxas 1/10 scale vehicles with the Velineon motor and ESC recommend 2S batteries, or 3S (max) to get the high speeds that the vehicles are advertised as capable of doing. The higher the voltage, the more power the motor/ESC is capable of producing.
This is a list of LiPo RC battery pack voltages with cell counts most beginners will be using. If you are wondering what the 2-6S in parenthesis means; it is a way the battery manufactures indicate how my cells hooked in series (S) the battery pack contains.
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)
I should point out you may run across packs or cells hooked up in parallel to increase the capacity. This is indicated by a number followed by a "P". Example: 2S2P would indicate two, two celled series packs hooked up in parallel to double the capacity.
Capacity / mAh
Capacity indicates how much power the battery pack can hold and is indicated in milliamp 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. This is a measure of run time, not power/speed.
For example a 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.
In conclusion if you are seeking more run time, increase the capacity/mAh of your battery pack. Unlike voltage, capacity can be changed around to give you more or less run time. Because of size & weight restrictions, you have to stay within a certain battery capacity range seeing that the more capacity a battery pack has, the larger and heavier it will be.
Discharge rate is simply how fast a battery can be discharged safely. The faster the ions can flow from anode to cathode in a battery will indicate the discharge rate. In the RC LiPo battery world it is called the “C” rating.
What does this mean?
Capacity begins with “C” so that should give you a pretty good idea. A battery with a discharge rating of 10C would mean you could safely discharge it at a rate 10 times more than the capacity of the pack, a 15C pack = 15 times more, a 20C pack = 20 times more, and so on.
Let's use our 1000 mAh battery as an example; if it was rated at 10C that would mean you could pull a maximum sustained load up to 10,000 milliamps or 10 amps off that battery (10 x 1000 milliamps = 10,000 milliamps or 10 amps). From a time stand point (divide by 60 minutes), this equals 166 mAh of draw a minute so the 1000 mAh pack would be exhausted in about 6 minutes.
Most RC LiPo Battery packs will show the continuous C rating and a burst rating as well. A burst rating indicates the battery discharge rate for short bursts of extended power. An example might be something like “Discharge rate = 20C Continuous/40C Bursts”
The higher the C rating, usually the more expensive the battery. You can not go with too low a discharge C rating or you will damage your battery and possibly your ESC (electronic speed control). Higher power systems of today require higher discharge ratings in order to not discharge a battery too quickly and keep constant flow of power to other electronics (ex. servos/receivers). One example is the Mamba Monster system from Castle recommends a 35C or greater discharge rate.
All this said, RC LiPo packs are coming down in price all the time. If you find a 35C pack for the same price as a 25C when that is all you need, go for the 35C pack - it will run cooler and have a longer life span. Like most things, pushing a Lipo pack hard close to its limits will wear it out and reduce it's useful capacity in very short order. If however you get a pack with a C discharge rating at least double of the maximum you intend to pull out of it, with proper care, there's no reason you shouldn't be able to get at least 400 charge and discharge cycles out of it with little degradation of the battery.
The general rule is if you can't comfortably hold a LiPo pack tightly in your hand after using it, it's too hot. If you find your packs are getting warmer than this, it's a good bet you should consider moving up to a higher discharge rating for your next LiPo pack.
Leaving your packs in the car on a hot sunny day can be harmful on the battery. Internal or external heat - both have the same negative effect, hot LiPo's are miserable and they won't last long.
OVER DISCHARGING - THE NUMBER ONE KILLER OF LIPO'S!!!
The other thing that will heat a pack up fast and can ruin the battery is if you push it right down to or lower than 3.0 volts per cell under load. When using LiPo batteries the ESC on the vehicle should have a low voltage Cut-off, if not there are low voltage cut-offs that can be added to the vehicle. This is the same concept that cell phones and laptops use, when they reach a certain battery output they automatically shut themselves off. This is because they also use lithium batteries and this prevents battery damage.
Charging RC LiPo Batteries
Charging RC LiPo Batteries is a topic in itself. LiPo, LiIon, and LiFe batteries obviously have some very different characteristics from conventional RC rechargeable battery types. Therefore, charging them correctly with a charger specifically designed for lithium chemistry batteries is critical to both the life span of the battery pack, and your safety.
Maximum Charge Voltage and Current
A 3.7 volt RC LiPo battery cell is 100% charged when it reaches 4.2 volts. Charging it past that will ruin the battery cell and possibly cause it to catch fire.
It is critical that you use a charger specified for LiPo batteries and select the correct voltage or cell count when charging your RC LiPo batteries if you are using a computerized charger. If you have a 2 cell (2S) pack you must select 7.4 volts or 2 cells on your charger. If you selected 11.1V (a 3S pack) by mistake and tried to charge your 2S pack, the pack will be destroyed and most likely catch fire. Luckily, all the better computerized chargers out there these days will warn you if you selected the wrong cell count. Going past 4.2 volts per cell, even to 4.21 volts will shorten battery life.
RC LiPo Battery Charging Current