Key Considerations for Battery Powered Motors

This article was featured in Design World Magazine and Designworldonline.com

New commercial equipment designs continue to drive smaller, lighter, and more mobile solutions. This has resulted in a rapidly accelerating transition to battery-powered equipment designs from traditional wired products. Proper motor selection for any automated equipment application is critical to optimizing system performance, however, battery-powered applications require additional considerations for both the motor and the battery. This article reviews the process for selecting the correct motor-battery combination for commercial equipment applications.

Power Requirements

In any electric motor application, the desired equipment performance dictates the power requirements of the motor. The rated power of the motor is calculated from the combination of speed, torque, and duty cycle of the application that in turn establishes the critical voltage, current, and capacity requirements of the battery.

The supply voltage and current directly correlate to the speed and torque that a motor will produce. In addition to voltage and current, capacity is the third critical factor in selecting the proper battery for a specific application. Battery capacity is based on the desired operational run time of the equipment between recharging cycles and is simply the amount of charge a battery can hold. The standard measure for battery capacity is milliampere-hours (mAh) or amp-hours (Ah), which indicates how long the battery will last based on the current it outputs. The calculation of the battery life at a certain current draw is the battery capacity (Ah) / output current (A) = battery life (hours). For example, an AA battery with a rating of 2500 mAh outputting 100 mA would last approximately 25 hours.

Performance Considerations

One key motor performance parameter to consider in a battery-powered application is efficiency.  Maximizing motor efficiency helps minimize the required power capacity and hence the size and cost of the battery solution. For this reason, brushless DC (BLDC) motors are preferred over brushed DC motors but are typically higher in price. Brushed DC motors are lower in initial costs and simpler to integrate, but their reduced efficiency rating may require a higher cost battery solution or reduce the operating time between battery recharging cycles.

The typical battery discharging process requires addressing several performance considerations, primarily motor speed. With most battery types, the terminal voltage decreases as the battery discharges.  Since motor speed is directly proportional to the battery voltage, as the terminal voltage decreases, so will the motor’s speed.

Batteries also see a decrease in terminal voltage as the output current (load) increases, which also negatively impacts motor speeds at higher torque loads. These factors do not consider the characteristics of the motor winding itself, where output speed decreases as the motor load increases, even with constant battery voltage (see Graph 1, below).

Graph 1: Typical DC Motor Performance Curve

One method to address this potential speed fluctuation issue is to incorporate speed control electronics into the system, ensuring the motor maintains the desired speed as the battery discharges over time or as the load on the motor changes.

Motor output torque must also be considered, as the discharge current of a battery is limited. Since motor output torque is directly proportional to the current supplied, the maximum output torque of the motor could be limited by the battery discharge current rating. A properly sized battery should have a discharge current rating that meets or exceeds the current requirements of the motor.

Battery Considerations

An essential criteria in battery selection is making sure that the battery will not only supply the motor’s voltage and current requirements when fully charged, but also continue to meet those requirements as it approaches full discharge. Weighing the trade-offs between battery type, size, and cost while still meeting the critical performance requirements is the next step in the process.

All rechargeable and non-rechargeable battery designs are based on the conversion of chemical energy into electrical energy. Today, most small, rechargeable batteries are made using Lithium-Ion (Li-ion) technology, due to its high energy density properties. These rechargeable and low maintenance batteries are more expensive than traditional technologies such as Nickel-Cadmium based cells. Larger batteries have traditionally been lead acid designs, although Li-ion has become increasingly popular due to longer life, smaller size and weight, and overall ease of maintenance and recharge ability compared to lead acid batteries.

Battery size directly correlates to the energy storage capacity of a given battery. For product designs with minimal space available for batteries, like medical hand tools, utilization of a high energy density battery is beneficial despite the potential cost and capacity disadvantages.

The way different types of batteries discharge over time is another important factor in proper selection.  While many of today’s high energy density battery technologies maintain a relatively stable output voltage until they approach full discharge, traditional battery technologies offer a gradually decreasing voltage versus time curve until they reach full discharge (see Graph 2, below).

Graph 2: Example Discharge Curves for Different Battery Types

As explained previously, the battery discharge curve is important because output voltage directly affects the output speed of the motor and understanding how close a battery is to full discharge can prevent damaging the battery and potentially the system. Allowing a battery to fully discharge during operation may ultimately damage the battery, rendering it no longer rechargeable. To prevent this, a preset cut-off voltage will typically disengage the motor once the battery voltage dips below the preset threshold. Determining and implementing the proper cut-off voltage is typically easier with a battery that has a gradual discharge curve than a battery with a flat curve.

Additional battery selection considerations include the number of recharge cycles the battery can endure, availability, and environmental factors.