Choosing Batteries for Robots

Robots are named after a Czech word “Robota” which means slave or any mechanical item that can help its master. Robots are mechanical devices that help perform specific tasks for humans. The term 'Robot' can be used for any mechanical device equipped with software-based intelligence to perform specific physical tasks. The design of a robot basically involves three functionalities –

A robot must be able to detect the environment with the help of external sensors. For detection, various types of sensors that can read physical quantities such as temperature, light intensity, infrared radiation or others depending on the robot application or sensors for robotic vision, touch sensitivity, voice recognition, proximity detection must be connected to the control of electronic circuits. the robot.

Robots are useless without software-based intelligence and decision-making capabilities. They must be able to sense the environment and make decisions based on it so that the tasks assigned to them can be carried out. For decision making, a robot's control circuit may have a programmable controller, a microprocessor or a minicomputer.

Robots are designed to perform physical tasks. To do this, they have actuators, motors and drive systems coupled to their mechanical assembly. These actuators and motors are programmatically controlled by control circuits.

Robotic cars or robotic vehicles are one of the many robotic projects developed to move to perform physical tasks. The movement of this robot itself involves the application of energy and the use of motors. When making such a robot, there needs to be a balance between the load (weight of the robot), battery and motor used. These three elements are connected to each other and choosing the right one requires a lot of math and experimentation. Just as in the previous tutorial it was discussed how to choose the right motor for a robotic application, in this tutorial it is determined how to choose a suitable battery for the selected motor according to the load.

Before learning about battery types, let's first take a look at the basic specifications of batteries.

In the previous tutorial, the basic specifications of the DC motor were discussed. To run a motor in a robot, it is necessary to choose a battery with suitable specifications. There are many types of batteries available to power motors. The main specifications that must be taken into consideration when selecting a battery are as follows –

Voltage Rating – This is the maximum terminal voltage that a new battery can be expected to deliver to electronic components.

Current Rating or Capacity Rating – The amount of current a battery can supply in one hour is called current rating or capacity rating. This rating is associated with the power that the battery can provide. For example, a battery with a capacity of 1 amp-hour will be able to continuously supply a current of 1 amp to a load for exactly 1 hour, or 2 amps for half an hour, or 1/3 amp for 3 hours, etc. ., before being completely discharged.

For example, an average automotive battery might have a capacity of about 70 amp-hours, specified for a current of 3.5 amps. This means that the amount of time this battery could continuously supply a current of 3.5 amps to a charge would be 20 hours (70 amp hours / 3.5 amps).

C and E Rates – Discharge current is often expressed as C rate to normalize battery capacity. 1C means the discharge current will discharge the battery in one hour. A 5C rating for this battery would be 500 Amps, and a C/2 rating would be 50 Amps. Likewise, an E rate describes the discharge power. A 1E rate is the discharge power to discharge the entire battery in 1 hour.

Let's now move on to the types of batteries available for use in robotic projects. There are following types of batteries available for use in robots –

Alkaline Battery – Alkaline batteries are non-rechargeable batteries that come in ratings of 1.5V and 9V. 1.5V battery comes in different varieties such as AA (about 1,000 mAh), AAA (less than 1,000 mAh), C (2,000 mAh to 2,500 mAh) and D (about 2,000 mAh). The 9V battery comes in the range of 50 to 500 mAh. These batteries are low cost and readily available, but they do not last long and discharge quickly.

Nickel-metal hydride (Ni-MH) batteries – Ni-MH batteries are rechargeable, where the nominal voltage of each cell is 1.2 V ranging from 600 mAh to 3300 mAh. These batteries are a little more expensive, but they last longer and have a high current capacity.

SLA batteries are still the cheapest option for high capacity. They require almost no maintenance for several years and can go through thousands of charge and discharge cycles until the discharge does not exceed 30% of its capacity. Additionally, SLA batteries can produce tons of current and are easy to charge. These batteries are widely available and inexpensive, but they are like heavy rocks for mobile robots and are not preferred for hobby robots.

Lithium-Ion (LI-Ion) Batteries – Lithium-ion batteries are rechargeable batteries, the same as those used in cell phones and cameras. Lithium-ion batteries come with a rating of 3.7V. These batteries are easily available and have a high capacity. They are also lightweight.

Lithium Polymer (Li-Po) Batteries – These are becoming the most popular type of batteries for use in robotics due to their light weight, high discharge rates and relatively good capacity, except that voltage ratings are available in increments 3.7V.

To match a battery with a motor, the specifications of the motors must also be known. The DC motors used in robotics are mainly gear motors. Geared DC motors have a gearbox mechanism that decreases the motor speed and increases torque. The following specifications of DC geared motors must be taken into consideration –

Voltage Rating – Voltage rating is the maximum sustained voltage that can be safely applied to the DC motor without risk of electrical failure. Suppose the voltage range of the motor is 5V – 12V, this means the motor can be powered between 5V and 12V.

Free Current or No-Load Current – ​​As the name suggests, no-load current is the current at which the motor runs freely without any load attached to its shaft. As an unloaded DC motor rotates, it generates a backward-flowing electromotive force that resists the current applied to the motor. The current through the motor drops as the rotational speed increases, and a free-spinning motor has a very small current.

Stalling current – ​​Stalling torque is the torque produced by a device when the output rotational speed is zero. It can also mean the torque load that causes the output rotational speed of a device to become zero, causing it to seize up. Stalling is a condition when the engine stops rotating. This condition occurs when the load torque is greater than the motor shaft torque, that is, this is a breaking torque condition. In this condition, the motor consumes maximum current, but does not rotate. This current is called Stalling current.

Power – The power of a motor is usually expressed in watts. The power rating is derived from the simple relationship, power = voltage x current. For example, a motor driven at 5 V with 100 mA of current consumed is consuming 0.5 Watts of power.

Torque – The force directed in the circle is known as torque. It is said to be as simple as how much weight the DC motor can lift a given distance.

Voltage Rating – When making a hobby robot, there are chances that the robot may have a faulty power supply because it may be using a 9V battery in the circuit. 9V batteries are intended to be used primarily for electronic components and not to power motors as they have low output current and low capacity. To power motors, batteries such as AA, AAA, D or C or even batteries are more suitable as they are manufactured for a specific voltage rating and configuration.

A 9V battery is actually made up of flat packs in series. These batteries have very small capacitances, maximum found at 500 mA. These batteries can be used to power certain electronic components in the robots, such as the microcontroller, and other low-power electronic components in the robot, such as sensors. Motors, compared to typical microcontrollers, consume many times more power, usually in the hundreds of milliamps, and when they stop (keep the motor shaft still) they can draw several amps, which can drain a 9V battery in a few minutes.

Many robots utilize a Motor Driver IC that has supplies for both the logic side and the motor side. By connecting 4 AA batteries on the motor side of the controller, the 9V battery or even the same AA pack can be connected to the logic power as well as the motor power. However, the motor supply must not be connected to a 9V logic supply. Because in this case the logic side may work, but the motor power may not work.

Battery Type – NiMH is the preferred choice for powering motors because they have decent capacitance, are low cost compared to their size, and have decent current output. Furthermore, NIMH batteries are rechargeable and safe to use compared to NICAD batteries. NiMH batteries do not have a memory effect and do not contain cadmium, which is a toxic chemical.

Another popular choice these days are lithium polymer batteries. They are designed to produce power many times greater than their capacitances. Some batteries can produce hundreds of amps for a short period of time. However, lithium polymer batteries are dangerous to use and need to be handled with care. Therefore, it is best to start with NIMH batteries first.

Another preferable choice may be lead-acid batteries, which are made of spongy lead grids with special electrolytes on each plate. Each cell has a potential of 2V. The most common cell configuration is 6 cells or 12V. For a large robotic project that requires many amplifiers, the lead-acid battery is the best choice. Lead-acid batteries have relatively low power density for their weight, but they can easily produce hundreds of amps for long periods of time and are easily chargeable.

There are also lithium-ion batteries that can be used in hobby robots. Lithium-ion batteries are generally used as camera and laptop batteries. However, they cannot produce large currents at once. This is why Li-Po batteries are more preferred. Li-Ion and Li-Po batteries are actually very similar, but they have different cell constructions. However, lithium-ion batteries are best for low-current electronic components, such as for a robotic controller or sensor circuit. But that excludes engines. Lithium-ion batteries are a bit tricky to charge compared to other batteries and that is why it is still better to use a NiMH battery instead of a lithium-ion battery.

Torque and RPM – The torque and RPM of an engine are already known from its specifications. Using Torque, RPM and average efficiency, the total power the engine needs to move can be calculated.

Let us represent the total power required by the engine by P.

The total energy required by the motor can be found by the following equation –

E=P*T

Where,

T is the time in seconds

E is the total energy required

P is power rating

A factor of 1.2 must be multiplied in the above equation to include the power required by other electronic components such as motor controllers, etc., and to account for efficiency losses. This is the energy required to be supplied by the battery. The energy equated from this equation is in Joules, but battery specifications are typically in AH (ampere-hours). Therefore, if a 12V motor with 100 AH rating is powered, the total power required can be equalized as follows –

12 * 100 *3600 (seconds in an hour) = 4320000 Joules of energy.

This power must be greater than the power requirements for all engines combined. Depending on the weight of the battery, the required specifications can be increased or decreased.