Introduction

 

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 offer a wide array of benefits. But each user must decide if the benefits outweigh the drawbacks. For more and more people, they do. In my personal opinion, there is nothing to fear from LiPo batteries, so long as you follow the rules and treat the batteries with the respect they deserve.

This guide was written after many hours of research. It is as accurate as I can make it without actually being a chemical engineer (though, in researching this article, I did talk to a few of them). That said, this guide isn't intended to be taken as definitive. It is a living document, and as common knowledge regarding LiPo batteries changes, so too will this guide.

 

Let's first talk about the differences between LiPo batteries and their Nickel-Cadmium and Nickel-Metal Hydride counterparts.

 

LiPo Packs versus NiMH Batteries

LiPo batteries offer three main advantages over the common Nickel-Metal Hydride (NiMH) or Nickel Cadmium (NiCd) batteries:

LiPo batteries are much lighter weight, and can be made in almost any size or shape.

LiPo batteries offer much higher capacities, allowing them to hold much more power.

LiPo batteries offer much higher discharge rates, meaning they pack more punch.

But, just as a coin has two sides, there are some drawbacks to LiPo batteries as well.

LiPo batteries have a shorter lifespan than NiMH/NiCd batteries. LiPos average only 300–400 cycles.

The sensitive chemistry of the batteries can lead to fire if the battery gets punctured and vents into the air.

LiPo batteries need special care in the way they are charged, discharged, and stored. The required equipment can be expensive.
 

Voltage / Cell Count

A LiPo cell has a nominal voltage of 3.7V. For the 7.4V 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 cells in Series. So a two-cell (2S) pack is 7.4V, a three-cell (3S) pack is 11.1V, and so on.

 

Capacity

The capacity of a battery is basically a measure of how much power the battery can hold. Think of it as the size of your fuel tank. The unit of measure here is milliamp hours (mAh). This is saying how much drain can be put on the battery to discharge it in one hour. Since we usually discuss the drain of a motor system in amps (A), here is the conversion:

1000mAh = 1 Amp Hour (1Ah)

I said that the capacity of the battery is like the fuel tank - which means the capacity determines how long you can run before you have to recharge. The higher the number, the longer the run time. Airplanes and helicopters don't really have a standard capacity, because they come in many different sizes, but for R/C cars and trucks, the average is 5000mAh - that is our most popular battery here in the store. But there are companies that make batteries with larger capacities. Traxxas even has one that is over 12000mAh! That's huge, but there is a downside to large capacities as well. The bigger the capacity, the bigger the physical size and weight of the battery. Another consideration is heat build up in the motor and speed control over such a long run. Unless periodically checked, you can easily burn up a motor if it isn't given enough time to cool down, and most people don't stop during a run to check their motor temps. Keep that in mind when picking up a battery with a large capacity.
 

Internal Resistance and C-Rating

There are many people out there that believe a higher C-Rating will make their vehicle perform better. We know from our previous discussion on C-Ratings that you need to account for the power draw your motor has when picking out the right C-Rating for your battery, but does more equal better? Many people say yes.

But there isn't anything intrinsic to the C-Rating that substantiates their claims. It's simply not true that a higher C-Rating makes your car or airplane faster.

However, there is a correlation between the C-Rating of a battery and the internal resistance of that battery. In general, batteries with a higher C-Rating also have a low internal resistance. This isn't always the case, as there are always variances in manufacturing, but the general idea seems to hold true, and a lower IR will make a car or airplane faster.

This is a case of correlation not equalling causation. It's really the internal resistance making a battery faster, not the C-Rating.