Selecting a Battery for Wi-Fi DAQ Devices

Overview

The modular form factor of NI Wi-Fi data acquisition (DAQ) devices is designed to provide wireless connectivity for a broad range of applications, from examining the quality of soil to testing the structural integrity of bridges. The NI WLS-9163 Wi-Fi C Series carrier supports several C Series measurement and control modules for streaming data over an IEEE 802.11b/g connection for such remote applications. However, some applications require both a wireless data connection and a wireless power connection.

The following is a guide for selecting a battery for your Wi-Fi DAQ device when power is not readily available.

Assessing Your Application Needs

Every wireless data acquisition application is different, and so, too, are its power requirements. Before choosing a battery, it is important to ask yourself a few key questions:

Are there alternative sources of 9-30 VDC power available?

Sometimes battery power is the only option for an application; at other times, however, batteries may supplement an alternative power source or serve as a backup. Solar panels, for example, are a common power solution for remote, outdoor applications. The panels will never provide a constant source of energy, and a battery can provide power when sunlight is low or absent. Most off-the-shelf solar panel solutions are prepackaged with a battery and charging circuit already. For other applications, for example a factory floor, DC power may be more readily available than a data cable drop. In such an application, a large capacity battery may not be necessary.

Is recharging an option?

There are many different battery chemistries available (discussed below); however, there are only two fundamental classes of batteries: single-use and rechargeable. Single-use batteries, such as the ubiquitous alkaline AA, have less upfront cost if you plan to use battery power infrequently as a backup power source. However, rechargeable batteries can be more economical in the long term if you intend to use batteries as your primary source of power.

How long does the wireless DAQ device have to run on battery power?

The single most important factor to consider is how long your application must run on batteries. This will affect the type and size of battery you should use. Wi-Fi DAQ devices can run on small- to moderately-sized batteries for many hours to a full day. Multiple-day or week(s)-long applications can require much larger batteries if recharging (such as with a solar panel) or periodic servicing is not an option.

Choosing a Battery Chemistry

There are multiple means of storing energy in a battery, and not all batteries are made the same. The most popular chemistries today are alkaline-, lithium-, or nickel metal hydride (NiMH)-based. (Lead acid and nickel cadmium (NiCd) are older technologies still in use today, but their weight and longevity, respectively, may not be attractive for a wireless data acquisition application.)

Alkaline: The most common type of single-use or disposable battery is alkaline. Most commercial off-the-shelf AAA, AA, C, D, and 9-V batteries are alkaline-based. These batteries are inexpensive, store well, and can have high energy densities. However, most alkaline batteries also have relatively low voltages, typically around 1.2 to 1.5 V. You would need at least eight such batteries to power a 9 VDC Wi-Fi DAQ device. The 9-V battery is of course the exception, but these have much lower capacity.

NiMH: Perhaps the most common rechargeable chemistry for consumer products is NiMH (nickel metal hydride). These batteries are available in the same standard sizes and voltages as alkalines (AAA, AA, C, D, and so on) and provide similar to slightly lower capacities. The advantage of NiMH over alkaline (in addition to reuse) is a more constant voltage level over the life of the battery. They are also inexpensive relative to other rechargeable chemistries. A potential drawback to NiMH batteries is a short shelf life. The self-discharge rate for a NiMH battery at room temperature can be on the order of 1 to 2 percent per day, compared with alkalines, which self-discharge about 2 percent per year. You can reduce this by storing the batteries at colder (even freezing) temperatures.

Li-Ion: Many hand-held electronics, such as laptops and mobile phones, use Lithium Ion (Li-Ion) or Lithium Polymer (Li-Poly) batteries. These chemistries are popular because of their low self-discharge rates (5 percent per month) and high energy density; Li-Ion batteries are much lighter than alkaline or NiMH batteries with equivalent capacities. For wireless data acquisition applications, they also have the added benefit of higher voltage levels (typically 3.6 or 3.7 VDC per cell). Most Li-Ion batteries are available in the industry standard 18650 size (similar to an AA) or flat rectangular cells (as in a mobile phone or camera). However, because Li-Ion can become unstable if punctured or damaged, most Li-Ion batteries come prepackaged at common voltage levels, such as 11.1 VDC or 18.5 VDC, with added current regulation circuitry. Li-Ion batteries are also more expensive and typically have longer lead (shipping) times than other chemistries.

 

 

Table 1. Different battery chemistries have relative pros and cons for Wi-Fi data acquisition applications.

Choosing a Size

The size and weight of a battery is determined by capacity, energy density, and voltage level. The length of time your wireless data acquisition application has to run continuously is directly proportional to the capacity of your battery. Battery capacity is measured in milliamp hours (mAh). For example, a 1000 mAh battery can provide 1 A of current for one hour, 500 mA for two hours, 250 mA for four hours, and so on. Energy density is a related metric that specifies the capacity per unit of mass in watt hours per kilogram (Wh/kg). A battery with a high energy density, such as Li-Ion, will weigh less than a battery with a lower energy density, such as NiMH, for the same battery capacity. Battery cell voltage also has an impact on total weight and size, because eight NiMH or alkaline batteries are (roughly) equivalent in voltage to three Li-Ion batteries.

Battery Capacity Requirements

The total size of your battery solution depends greatly on the needs of your Wi-Fi data acquisition application: which C Series module you are using, how long you need it to run, and which voltage level you choose. As an example, consider the NI WLS-9234 acquiring at the full rate on all channels. When specifying a battery, it is important to know the peak current usage and continuous (or normal) current usage. The WLS-9163 carrier has a peak in-rush current of 1.67 A. Figure 1 below shows the startup sequence for the WLS-9234, with a steady-state current of about 220 mA after 20 seconds with a 12 VDC power supply. (These results were obtained using an NI PXI-4110 programmable power supply and configuring the WLS-9234 to authenticate with a WPA2 access point without Ethernet connected).

Figure 1. The WLS-9234 boot sequence shows a power profile with a steady-state power current usage around 220 mA with a 12 VDC power supply. (Download additional power curves below.)

When acquiring at the full 51.4 kS/s rate on all four channels, the NI WLS-9234 consumes an average 290 mA continuously. If you need to run this device on batteries for eight hours, you would need at least a 2320 mAh capacity battery:

The following table summarizes typical power consumption for all Wi-Fi DAQ devices. You may also view complete test results in the .zip file attached to this document.

 

Table 2. Typical continuous power consumption for Wi-Fi DAQ devices.

Calculating the approximate battery life of different cell chemistries is straightforward from here. First, because not all battery configurations are exactly 12 VDC, it is convenient to work with watts (W) and watt hours (Wh). Simply multiply the results in Table 2 above by 12 V to determine the continuous power usage for your Wi-Fi DAQ device. Then divide into the Wh capacity (mAh X nominal voltage) for your battery to determine how many hours your Wi-Fi DAQ device will operate.

Example Battery Configurations

Consider the following battery configurations for the example WLS-9234 application:

8x AA Alkaline Pack: Eight AA size alkaline battery cells with 2000 mAh capacity and 1.2 V average voltage connected in series.

8x AA NiMH Pack: Eight AA size NiMH battery cells with 2500 mAh capacity and 1.2 V nominal voltage connected in series.

10x 4/3 A NiMH Pack: Ten 4/3 A size NiMH battery cells with 4000 mAh capacity and 1.2 V nominal voltage connected in series.

6x 18650 Li-Ion Pack: Six 18650 size Li-Ion battery cells with 2400 mAh capacity and 3.7 V nominal voltage in a 3/2 configuration (two parallel sets of three cells in series).

Table 3 below summarizes these results for all Wi-Fi DAQ devices.

Device	8 AA Alkaline	8 AA NiMH	NiMH 4/3 A Pack	Li-Ion 18650 Pack
NI WLS-9234	5.5 h	6.9 h	13.8 h	15.3 h
NI WLS-9219	6.9 h	8.6 h	17.2 h	19.1 h
NI WLS-9237	5.6 h	7.0 h	14.0 h	15.6 h
NI WLS-9215	5.8 h	7.3 h	14.6 h	16.2 h
NI WLS-9211	7.7 h	9.7 h	19.3 h	21.4 h

 

Table 3. Example battery configurations for Wi-Fi DAQ devices.

Connectivity

After deciding on a battery pack chemistry and configuration, wiring the batteries to the WLS-9163 carrier is easy. Use the NI 9976 (part number 196739-01) screw terminal accessory kit to connect power. You may need to splice the leads off to a different connector for recharging as well.

Summary

No one battery pack can solve every remote or portable power application. With some planning, however, you can choose a custom battery solution for all your wireless data acquisition applications.

Additional Resources

NI Wi-Fi Data Acquisition

Energizer Battery Detailed Specifications

Downloads

power_curve.zip

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