LiPo (Lithium Polymer) batteries have become the industry standard in remote control over the last 5 or so years. Many Cars, Boats and Planes now use a LiPo battery so understanding basic LiPo care and practices are important for every RC enthusiast. Understanding LiPo battery specs are something that many people find confusing, What is "C" rating, what does 3S mean, what happens if I increase my capacity or voltage are all very common questions. Well have no fear, we are here to explain what this all means and how it affects YOU when purchasing your next LiPo.
Capacity is the first thing people talk about when upgrading a LiPo, I'm sure everyone has heard or thought about how to make their RC car, plane or boat last longer per run, and increasing the capacity of the battery is what will get you there! Capacity is expressed on the battery in the form of milliamp hours (mAh), when you see 5000mAh written on the front of your battery, this is your capacity. Increasing this number will increase runtime, decreasing this number will decrease runtime. I like to think if it like litres, the bigger the number the more you have, 2 Litres of water will take longer to drink than 500ml, think of your battery capacity like this, the bigger capacity the more stored energy (Water) you have and the longer it will take to use! The main thing to consider when increasing the battery capacity for your model are the physical dimensions of the new battery. Normally the bigger the capacity, the bigger the battery and the heavier the battery is (Weight is more important for Aircraft CoG). Confirming that your new prospective battery fits in your model is step one, but you can rest assured, if you increase your capacity, your runtime will increase too! In an aircraft, the extra weight that the bigger battery will likely have should be thought about. A heavier battery may affect the Centre of Gravity (CG) of the model and make it harder to fly. A heavier plane will fly faster, have a higher stall speed and land faster. Will that be a good thing?
The voltage of the battery is commonly confused with capacity in the sense that many people think a higher voltage battery will last longer. This is not the case, a higher voltage ultimately will have your Brushed or Brushless motor spinning at a higher RPM and in turn have the vehicle moving much faster. If I have a 1000KV brushless motor this means the motor will perform 1000 RPM/Per Volt applied to it. If I am powering this motor with a 2S LiPo this will give me a nominal voltage of 7.4v, this will give me 7,400rpm out of my motor. If I power that same 1000kv motor with a 4S 14.8V LiPo I would have 14,800rpm output from that same motor. The main consideration when increasing voltage is damaging your ESC and overheating the motor, so be sure to read your manual and understand the maximum voltage your equipment can handle.
Sticking with the Voltage theme, another battery "Spec" is the way a LiPo voltage and Cell setup is described. You would have heard the term 2S or 3S LiPo before. This refers to 2 cells in Series (Meaning we add the voltage together) to give you the batteries total voltage. When talking voltage a nominal voltage (3.7v) is given for each cell, for example a 2S LiPo is a 7.4v (3.7v x 2) pack , A 3S 11.1v (3.7v x 3) and a 4S 14.8v (3.7v x 4). This can be where people get confused, as each cell's Nominal (Or in this case "Mid" point) is 3.7v, fully charged, however, each cell is 4.2v in a standard LiPo and flat is around the 3.0v per cell. The Nominal Voltage is used when talking about LiPo's, this is an industry standard and something that is worth knowing.
Possibly the most misunderstood and misinformed spec relating to LiPo batteries is the "C" rating. The discharge rating or C rating refers to how fast the battery can be discharged without being damaged. I think an easier way to think about C rating is the batteries ability to do work. Imagine you had a water bottle full of water, if you then went and put a pin sized hole in the bottom, water would leak out of the bottle until it was empty at a reasonably slow rate, think of this as a battery with a low C rating. Now if we went and placed a hole that was 2 cm in diameter, water would be able to leak out much faster, think of this as a high C rating battery. This is especially important in a vehicle that requires a lot of power very quickly, you need high C rating to meet the requirement of the car if the battery is not capable of this, then you risk damaging the battery and at a worst case causing a catastrophic and terminal failure of the battery.
When discussing C rating there are two numbers that are spoken about, these are the Burst C rating and this refers to the maximum discharge rate of the battery for a 10-second window. Then there is the Continuous C rating, this is more important and Is imperative to know for the health of the battery. The way to calculate the Continuous C rating is pretty simple, using the formula as follows: 60C = 60 X Capacity of battery in Amps (1000mAh = 1Amp). So the continuous C rating of the battery I used as an example would be 60C X 5AH (5000mAh) = 300Amps. This is simple to now find the battery that will suit your requirements. If you had a Car with an 80 Amp ESC then a battery that was able to give 100amp continuous would be sufficient and is above the capability of the ESC to give you some wiggle room! It is also worth noting that generally for any given cell count and capacity, a higher C rated battery will be heavier than a lower C rated battery.
Understanding the battery specifications and ultimately getting the right battery for the right application is very important, this can not only save you money and time but it will also mean that you're safely operating the battery inside its window of performance and not causing any unnecessary damage or risk by overexerting it. LiPo batteries must be treated with respect and understanding the C rating is a great start to having your LiPo performing at it's best for as long as possible!