How To Choose the Batteries for an All-Terrain Robot When the DC Motors Require 24V to Run

This is one of the questions that comes up when I was planning a 4WD mobile robot able to deal with difficult terrains. And because without electrical energy my robot is just a simple piece of furniture, in this article I have to find the best report between technology and price for the batteries that will keep my robot up and running for 1.5 hours.

Furthermore, to simplify the power system of the robot I will use two 12V batteries. I will not use multiple batteries with different nominal voltage for each type of components just because the voltage range may differ. I’ll keep everything as simple as possible.

I know that there are advantages and disadvantages in using only one nominal voltage. One of the advantages is that I can recharge the batteries in the same time with only one battery charger. The second advantage is the low weight of the platform. A light weight would likely be an advantage to traction given that the robot will work outside. A lighter robot requires less power so I need smaller batteries to get more run time.

As a disadvantage, I have to use a lot of voltage regulators since some of the electronics usually operate at different voltages than the actuators and sensors.

Define the needs

These are the theoretical values of the DC motors that I’ll use to build the robot. Here is how I choose the 24V DC motors for the all-terrain robot that I have to build.

  • RPM: 148
  • Torque: 2 X 6.12Nm = 12.24Nm

Moreover, I’ll take as the example a DC motor closer to the above specifications. This is a 350W at 24V with 10.7Nm that pulls no more than 18.7A at the stall.

I consider that an average of 1.5 working hours on a charge is more than enough for this robot.

I did some calculations here:

  • The DC motors: 4 X 24V nominal, 18.7A at stall
  • The time between recharges: 1.5 hours

The formula to calculate the capacity of the batteries is:

  • 4 motors x 18.7 A/motor x 1.5 hours = 112.2 Ah capacity of the battery

Well, let’s take a deep breath to try cutting some numbers. The stall current is the maximum current drawn when the motor is applying its maximum torque. But the motors will generally run at the maximum torque for a limited time of period. Of course, I have sensors and electronics that will eat my batteries, but these components consume a negligible amount of current compared to the DC motors. So, the capacity of the batteries needed to run for 1.5 hours is less than 112.2 Ah.

Therefore, I’ll consider an average current drawn by each motor of 9A.

Recalculate the battery capacity:

  • 4 motors x 9 A/motor x 1.5 hours = 54 Ah capacity of the battery

Therefore, two 12V 54 Ah batteries will last around 1.5 hours.

The best report between technology and price

Below I explore all the chemistry used in batteries that come into my mind.

NiMH

These batteries are in use in hybrid vehicles like the Toyota Prius and do a great job. The NiMH batteries are not affected by memory, meaning every charge should bring the battery up to full capacity. In general, these batteries have a good price and weights less than other batteries which can save me some torque for slopes and uneven terrain.

But I have a problem with these batteries. I didn’t find a battery that fits my needs. So, I have to go on to the next batteries.

NiCd

If you’re using cordless power tools such as a cordless drill, you’ve probably used a NiCd battery. One great thing about these batteries is that they can be charged rapidly without damaging them. But there are also disadvantages in using NiCd batteries. These batteries lose part of the capacity each time after the charge/discharge cycle.

In addition, since my robot will work in really cold weather (in Romania, the winter can reach temperatures below -20 degrees) I can damage the batteries at recharge. Yes, I can damage the batteries because the gas absorption reaction is not adequate when the temperature is below 0 degrees. Next.

LiFePO4

These batteries are extremely lightweight than standard batteries and have a faster charging rate. But the price is so… $690 for a 12V 50Ah, and $1,246 for a 12V 100Ah Lithium Ion battery. Since I’m not a millionaire, I have to jump quickly to a new category.

Sealed Lead Acid

The SLA batteries are still the cheapest option for high capacity and, like any old rusty trucks, these are widely available. They require almost no maintenance for several years and a thousand of charge/discharge cycles if the discharge is no more than 30% of capacity. In addition, the SLA batteries can output tons of current and are easy to charge.

But these batteries are like heavy boulders for mobile robots. For example, a 12V car battery store a few dozen amp hours at a weighing of around 15Kg. A considerable weight even for a heavy-duty robot. Moreover, I need two of these.

During the mid-1970s, researchers developed what is called a maintenance-free lead-acid battery. These batteries can operate in any position since the liquid electrolyte is gelled into moistened separators and the enclosure is sealed. So, I can mount the batteries without taking too much into account their position.

Using powerful DC motors, I need batteries with deeper discharges at higher current and for a higher number of cycles. These batteries are only good for power supply high-torque DC motors.

I know that the electric vehicles use lithium batteries, but they’re more concerned with the weight for a high travel range. Also, the drones use lithium as well, for their low weight and high capacity.

For me, keeping the robot light-weight is not the primary concern, but money is.

As a conclusion, I have to find two 12V SLA batteries with a capacity higher than 50Ah to run my robot.

Here are four of them:
Ritar 12V 60Ah 12V 75Ah Sealed Lead Acid Battery
24v-battery-001

UB12550 12V 55Ah Scooter Wheelchair Mobility Deep Cycle SLA AGM Battery
12v-battery-002

12V 55Ah SLA Sealed Lead Acid AGM Rechargeable Replacement Battery
12v-battery-003

UB12500 12V 50Ah Toy Car Play Mobile Scooter Rechargeable SLA
12v-battery-004

Disclaimer:
This article is written to help the robot hobbyist gain some understanding for the batteries used to power up the robots. The author assumes that you have sufficient knowledge about electricity and the basic operation of batteries. Please consult manufacturer’s data sheets in order to use batteries. The author will not be held responsible for accidents or injuries resulting from use of the information herein.

References:

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