How to find the right battery? Crashbotics S01E01

duracell-aa-aaa-battery

Finding the right battery for your project can be a real puzzle if you don’t know what to look for. But all you really need is to understand a few basic concepts and all the pieces will fit together. In this two-part video series, we will cover everything you need in order to choose the right battery for your next project.

Primary and Secondary Batteries

Batteries are the result of what is known as “electrochemistry”, which is a way to convert a chemical reaction into an electric current. They are two main types of batteries: primary and secondary.

Primary batteries are non-rechargeable. The AA, AAA and 9V batteries that you will find in most stores are mostly of the primary type.

Secondary batteries are rechargeable. The most commonly used secondary batteries in robotics today are Lead Acid, NiMh, Lithium Ion, and Lithium Polymer.

Some example of Primary batteries

What are Battery Cells?

A battery is made up of one or multiple cells. A cell is an electrochemical device with the ability to generate current. There are three main components of a cell, the anode, the cathode, and the electrolyte. When a load is connected between the anode and the cathode of a cell, an electric current will start to flow through the circuit.

When choosing a battery for your project, there are three specifications that you have to look for. The nominal voltage, the capacity and the discharge rate of the batter, also known as the “C-Rating”.

Voltmeter

What is the Nominal Voltage of a Battery?

The nominal voltage of a cell depends on its chemistry, and it is based on the average voltage that its chemical reaction can produce. A brand new or fully charged cell will have a voltage that is higher than its nominal voltage, and while being used it will gradually decrease until it reaches what is known as the cutoff voltage where the battery where the battery is considered dead or completely discharged.

What’s the Single Cell Capacity of a Battery?

The capacity of a battery represents the amount of energy stored inside of it. It is almost always given in milli-amp hour (“mAh”) or amp-hour (“Ah”), but what is an Amp-hour exactly?

As an example, if you have a battery with a capacity of 2000 mAh or 2 Ah, this battery will be able to provide 2 Amps continuously for an hour before being considered completely discharged

6v-2800mah-nimh-battery

A 6V, 2800mAh, NiMH Battery – https://www.robotshop.com/en/6v-2800mah-nimh-battery.html

Even if a battery can give 2 Amps, it does not mean that it WILL give 2 Amps. If you use this battery to run a DC motor that needs 500 mA to run at, this battery will only give the amount of current the motor needs. Since this battery has a capacity of 2000 mAh, it should be able to make this motor spin for 4 hours.

If your motor needed 4 Amps to run, this battery could, in theory, make it spin for half an hour. Your motor needs 20 Amps, it will run for 6 minutes… but wait, there’s a catch.

Discharge and C-rating of a Battery

The discharge rate tells you how much current you can draw safely from a battery. If you try to draw more current, the battery may get physically damaged, reducing its capacity permanently. Always respect the battery’s discharge specifications!

37v-2000mah-5c-lipo-battery

This 3.7V, 2000mAh LiPo Battery has a discharge rate of 5C

What do the C-rating means exactly?

Let’s say that this 2000 mAh battery has a continuous discharge rating of 5C, all you need to do to find the continuous discharge current is to multiply the capacity by the C rating. In this case, the battery will be able to safely provide 2000 mAh times 5C = 10 000 mA for… 12 minutes.

Watt-Hour

One last unit that is commonly used when talking about batteries energy is the watt-hour. Watt-hour means that the battery can deliver a certain amount of energy for one hour.

But, how can you find the watt-hour value of a battery if it’s not in the battery’s datasheet? You have to multiply the capacity of the battery pack by the nominal voltage of the battery. For example, if our 2000 mAh or 2 Ah battery has a nominal voltage of 3.7 volts, you can simply multiply the capacity by the nominal voltage to get 7.4 Watt-Hour of energy with this battery.

crashbotics-tutorials-batteries

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That’s it for the first part of this crash course on batteries. In the next video, we will see how battery packs are made by connecting different cells in series and in parallel. Tips and tricks on how to charge your battery packs and, last but not least, we will compare every battery chemistries and see how to choose the right one for our next project.

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