Driving Motors in Battery

Many battery-powered systems and Internet of Things (IoT) applications—such as smart meters, smart sanitation products, video doorbells, robotic toys, personal hygiene products, and electronic locks—contain a motor, solenoid, or relay. The interaction between the battery and the motor physics produces some interesting design challenges, such as operating the system reliably as the battery voltage changes, minimizing standby power to increase system lifetime, and supplying large currents to the motor during startup and stall.

In this article, I’ll offer some tips to help overcome these design challenges.

The battery voltage range available to the motor driver depends on the battery chemistry, depth of discharge, temperature, load current, and the number of battery cells connected in series or parallel. Although battery modeling is a complex science, let's start with a simple battery model using the open-circuit voltage (VOCV), internal battery resistance (RBAT), and battery terminal voltage (VBAT), as shown in Figure 1.

Table 1 shows some examples of battery voltage ranges for various battery chemistries.

1.7 V/cell

3.4 V total

0.8 V/cell

1.6 V total

100-250 mΩ/cell

200-500 mΩ total

1.55 V/cell

4.65 V total

0.8 V/cell

2.4 V total

135 mΩ/cell (average)

405 mΩ total (average)

1.5 V/cell

6 V/cell

0.8 V/cell

3.2 V total

150-300 mΩ/cell (fresh)

600-1200 mΩ total (fresh)

4.2 V/cell

8.4 V total

2.75 V/cell

5.5 V total

160 mΩ/cell

320 mΩ total

RBAT and VOCV are key contributors to VBAT changing over the life of the battery. As the battery charge depletes, VOCV decreases, and RBAT increases. As the load draws current from the battery (IBAT), VBAT decreases because of the voltage drop across RBAT.

Figure 2 shows the relationship between VOCV, RBAT and IBAT across battery lifetime.

Depth of discharge (DoD) represents battery life as a percentage with respect to the full battery charge capacity given in milliampere hours (mAh). A 100% DoD represents a fully discharged battery.

Because VBAT changes with DoD and IBAT, the supply-rail rating for a motor driver must accommodate a range of possible battery voltages. For instance, many motor drivers designed for 24-V systems have a minimum supply rail of 4.5 V. With four alkaline batteries connected in series, a motor driver with a 4.5-V minimum supply rating may disable itself using undervoltage lockout before the batteries fully drain.

The DRV8210 and DRV8212 from Texas Instruments (TI) are examples of motor drivers designed for battery-powered applications with a supply rating of 1.65 V to 11 V. This accommodates the maximum voltage for a two-cell lithium battery stack (8.4 V) or an almost-discharged two-cell alkaline battery stack (1.65 V).

Battery-powered systems spend most of their operating lives in a standby state. For example, consumers may operate motorized blinds only twice per day, or lock and unlock an electronic lock as many as 20 times per day. The valve on a gas meter or water meter may only ever actuate once per year. The standby current of the overall system must be low in order to achieve long battery lifetimes in these systems.

Adding load switches on the supply rails of peripheral devices in the system is one way to keep standby current low. Another way is to use devices with low standby currents optimized for battery applications. The DRV8210 and DRV8212 have sleep currents <84.5 nA to help reduce system standby current consumption. Other ways to reduce system standby current are to eliminate resistor dividers and set device logic pins with pulldown resistors at 0 V when not operating.

Large currents from motors create two problems in battery systems: they use energy unproductively, and they can cause the system to prematurely go into a low-battery lockout condition because of the voltage drop across RBAT. There are two main causes of large motor currents: inrush current during motor startup and stall current. Figure 3 shows an example of these currents.

Implementing a soft-start routine for the motor by ramping the pulse-width-modulation duty cycle can mitigate the large inrush current during motor startup. Figure 4 shows examples of hard start and soft start implementations for a depleted stack of four AAA batteries.

In Figure 4(a), the motor inrush current causes the battery voltage to dip during a hard start because of the voltage drop across RBAT. If this system resets or goes into an under-voltage lockout state around 3.5 V, the motor will not be able to drive beyond the initial startup.

Figure 4(b) shows how using a soft start causes a lower voltage drop on the supply rail, which can help achieve a little more operating life from a system with a depleted battery.

To help control stall currents, adding a current-sense resistor can help the microcontroller detect a stall and disable the motor driver before drawing large stall currents for a long period of time. Stall conditions may occur from an unintended mechanical blockage or the mechanical load reaching an end stop (such as a deadbolt fully actuated in a smart lock).

Figure 5 shows an example system implementation using the DRV8212.

The microcontroller's analog-to-digital converter measures the sense resistor voltage and compares that voltage against a threshold stored in firmware. If the current measurement exceeds the threshold for a certain amount of time, the microcontroller disables the motor driver to save power. It's important to configure the duration of time to detect a stall so that the inrush current does not accidentally trigger the stall detection.

Figure 6 shows a motor current profile during a stall condition with stall detection implemented, whereas Figure 3 shows the motor current waveform without stall detection.

Battery manufacturers measure battery capacity in mAh, so limiting both the magnitude of inrush current and the duration of stall current helps lengthen battery life.

It can be challenging to design battery-powered systems that use motors due to the finite battery operating life, battery voltage variation, and large motor currents. Using a motor driver rated for the battery's voltage range eases design efforts by eliminating additional boost converters and accommodating the minimum battery operating voltage.

Minimizing the overall system standby current and using a motor driver with a low-power sleep mode reduces wasted energy draw from the battery. Both soft-start and stall-detection techniques can also help increase the operating life of battery applications by reducing the magnitude and duration of large motor currents in the system.

By using these techniques, system designers can lengthen the operating lifetimes in battery-powered systems such as smart sanitation products, motorized blinds, electronic smart locks, and others.

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