Select the Right Battery for your Robot DC Motors – Part 1 of 2

This blog has two parts. The first part provides basics of DC motors and batteries, the second part outlines how to select the motors and batteries.

This is the first part. To jump straight to the second part where I show how to calculate battery power requirements for your evil robot, click on Part-2

Every robot enthusiast wants to build something that is moving…more or less…

In a large number of cases, while we get everything right in terms of the electronics, circuits, interfaces, control software, we generally end up with one problem that we did not think before….how to power my motors that make the robot move?

Select Battery for Robot DC Motor - Various Types of Batteries

Select Battery for Robot DC Motor - Various Types of Batteries

I may be wrong in generalizing the situation, but it has happened to me when I started building robots, and have seen a lot of other folks getting into this situation. You build everything right, tested everything with your power adapter and when the batteries were connected, the robot either refuses to move or the batteries die too soon.

I’m not yet an expert and am still experimenting with the batteries, but, I guess, based on little successes that I have, I decided to write this down for others who might benefit and I might also learn a thing or two from other experts who can point me to the right direction after reading this blog.

Before we begin, let me make myself clear, this blog talks about DC motors (geared or non-geared). We are not talking about servos, steppers etc. The principles are still the same though.

This blog is divided into the following sections:

  1. Basic DC motor specifications that you need to consider [Covered in part 1]
  2. Basic battery specification that you need to consider. [Covered in part 1]
  3. Different types of batteries. [Covered in part 1]
  4. Matching motor with batteries. The mathematics. [Covered in part 2]

Basic DC Motor Specifications

DC motors for robotics come in many variants. Two most common are geared and non-geared motors. In simple terms, geared motors have a gear mechanism that slows down the motor speed and increases the torque. Thus, some standard RPM values that I get inIndiaare 60 RPM, 100 RPM, 150 RPM, 200 RPM etc.

The other motor is plain and simple non-geared one. It’s just the motor running at its full speed at rated voltage and current.

Whatever is the variant, when you have a DC motor, from the perspective of powering the motor, you need to consider the following specifications:

Parameter

Meaning

Example

Voltage Rating The voltage range that the motor is designed to operate. Within the range you can select a voltage value that suits your needs best. 3V – 12V Meaning, motor can be powered by a source with a voltage value between 3V and 12V. Thus, 4.8V, 7.4V, 9.6V are all valid values
No Load Current The current drawn by the motor when its running free with nothing connected to the shaft. (For small motors, when you hold them in the hand and the shaft is not yet connect to the wheels, propellers etc).This indicates the minimum current your power supply must be able to provide in order for the motor to turn. 50mA This is a single value indicating the minimum required current
Stall Current When you increase the load on the motor shaft, at some point of time, the motor shaft will stop turning. This is called motor stall. The current drawn by the motor in this state is called the stall current. Please note, this is also the maximum current that will be needed by the motor 300mA This is a single value indicating the maximum required current from the source
Power This is generally given in “Watts”. In a large number of cases, I have seen that the watt rating is not specified. That should not be a problem as long as we have the voltage and current ratings. The power rating can itself be derived in simple terms as:Power = Voltage x CurrentThus, theoretically, a motor driven at 5V with 100mA of current draw, is giving 0.5Watts of power (or for a resistance, this would be the power dissipated as heat)Alternatively, the power delivered is 0.5 Joules/second. 1 Watt This is a single value at specified voltage and current rating, or a value indicating the maximum power.
Torque This is an indicator of how much weight the motor can lift at a given distance. If the rating is 1Kgcm, it means, that 1 cm from the shaft, you can attach a maximum of 1Kg load and the motor can lift it. 1Kgcm A single number. More on this in the calculation/mathematics section

Basic Battery Specifications

In the section above, we learned the very basics of dc motor specifications. Now let’s look at what battery do I need to run them. Please note, one of the variants is BLDC (Brushless DC Motor). We will not cover that here. It’s a different subject all together and someday, when I have experimented enough, I’ll write about it.

The table below outlines the battery specifications that we need to keep in mind:

Parameter

Meaning

Example

Voltage Rating The voltage a new battery is expected to deliver 1.5V Standard alkaline batteries (e.g. Duracell AA)
Current/Capacity Rating The maximum amount of current a battery can deliver in one hour. This is a very confusing specification. Suppose a battery is rated at 1500mAh. That means, it can deliver 1500mA in one hour. However, that is not entirely true. The rating is defined by the manufacturer after testing it at a different rate. For example, during testing, manufacturer can discharge a battery at a rate of 150mA per hour and if the battery lasts for 10 hours, you get a rating of 1500mAh 1500mAH Meaning, 1500mA current can be delivered over a period of one hour
Discharge Rate This indicates what is the maximum current a battery can deliver (remember, current/capacity rating is how much capacity is stored in the battery)A 1C rating means, the battery can discharge at Capacity x 1. 1C, 2C 20C Consider the battery capacity as 1000mAh. A 1C rated battery will give a maximum current of 1000mAA 20C rated battery can discharge upto 20000mA current.
Weight This is not available for all battery types. Mostly, I have seen LiPo batteries have weight specified, others do not specify that. 120gms(For a typical 3S1P LiPo cell)

Different Type of Batteries

When I started reading, I was inundated with the types of batteries available. What became the driving force behind listing the batteries below is ease of availability, most-common scenarios and cost.

The table below outlines some batteries that are common in hobby robotics. The specifications are “typical” and you should look at the manufacturer datasheet to get the exact numbers:

Battery

Type

Voltage Rating of Single Cell

Current/Capacity Rating

Discharge Rating

Typical usage/Comments

Alkaline(Non-Rechargeable) 1.5V AA – Around 1000mAH 0.1C – 1C - Very small robots (1 or 2, 3V motors)- Very cheap

- Readily available

- Don’t last long

AAA – Less than 1000mAH 0.1C – 1C
C – Around 2000-2500mAh 0.1C – 1C
D – Over 2000mAH 0.1C – 1C
9V Varies, 50-500mAh 0.1C – 1C Not good for any robotics motor driving application
Ni-MH(re-chargeable) 1.2V Varies. The ones available in local shops range from 750mAh to over 3300mAh Around 1C -Readily available-Costly

-Lasts long

-High capacity

-Easier to charge than LiPo

Once you have tested with alkaline batteries, try these

SLA(Sealed Lead Acid) I do not use it for hobby robotics. Too heavy for home built robots.
Li-Ion Cellphone batteries 3.7V Varies. The ones available in local shops range from 750mAh to over 3000mAh(Look inside your cellphone) 0.1C – 1C -Readily available in cellphone shops-Very costly

-Lasts long

-High capacity

-Easier to charge than LiPo

-Very low weight (Lower than NiMH)

Go for these when there is a special need (e.g. weight)

LiPo 3.7V Varies. The ones available in Indian online shops range from 1500mAh to over 10000mAh 1C – 30C -Only available in specialty shops (dedicated to RC, robotics)-Costliest

-Need another costly charger

-High maintenance

-A bit dangerous. Not for even intermediates

-Very high capacity

-Need special controllers called “ESC” to control BLDC motors.

-Always used almost exclusively with BLDC(You can though use it with other motors)

To summarize, you should start with alkaline batteries (a large majority of websites say not to use them, but I disagree. They have a place in robotics history!) for small to medium size motors.

Once you have a hang of it, see if you can work with NiMh. Go for LiPo batteries only in extreme cases. If you can build your mobile platform with alkaline or NiMH, you have gained expertise :-)

Disclaimer : You bear the risk of anything unexpected…don’t blame me! You cannot build a NASA robot with these. The values are only rough approximations

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5 thoughts on “Select the Right Battery for your Robot DC Motors – Part 1 of 2

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