Bob Henderson, Contributor
Atmospheric monitors use sensors to measure gas. Some types of sensors need more power, while other types need less power. Very low-power sensors may use so little power that a set of disposable or rechargeable batteries can last for months or years of operation. But, no matter what kind of sensors are installed, portable atmospheric monitors need power—and that means the instrument depends on batteries.
There is no perfect type of battery. Each type of battery has benefits and liabilities. How well the instrument performs is a combination of the type and capacity of the batteries; the type of sensors installed; the environmental conditions in which the instrument is used; and the power requirements of the instrument electronics.
Types of Batteries
There are three major types of batteries that are commonly used in portable instruments: disposable alkaline, rechargeable lithium ion (Li-ion) and rechargeable nickel metal hydride (NiMH) batteries.
Portable instruments can be powered by disposable alkaline batteries, rechargeable batteries—or they might be able to use both types of batteries. A primary advantage of rechargeable batteries is overall cost-effectiveness. Frequent (or daily) replacement of disposable batteries can be expensive and is increasingly viewed as environmentally objectionable. Some instrument designs offer interchangeable rechargeable and alkaline battery packs. Other designs allow the optional use of either alkaline or “off-the-shelf” rechargeable batteries.
Contractors who only use their instruments occasionally often find disposable batteries are an easier solution than charging and maintaining rechargeable batteries. For other instrument users, simply having the ability to use disposable batteries “in a pinch” is a strong design advantage.
Make sure any disposable or rechargeable, off-the-shelf batteries you use are approved by the manufacturer. The owner’s manual will list the batteries which are approved for use. Using a non-approved battery, even if it fits the instrument and seems to work, can void intrinsic safety and other certifications carried by the instrument.
Alkaline batteries have the benefit of convenience, but they suffer from poor performance in low temperatures. Generally, when the temperature is below freezing, instrument users should avoid alkaline batteries. The batteries may work for a while, but once the internal temperature in the battery drops below freezing, the amount of available power drops as well.
The most common types of rechargeable batteries are lithium ion (Li-ion) and nickel metal hydride (NiMH) batteries. Each type of rechargeable battery has specific advantages and limitations. The weight of the instrument, run time, time to recharge the battery and the number of charging cycles that the battery can survive without loss of capacity are all affected by the type of battery included in the design. Less obviously, the temperature code and operating ambient temperature range over which the instrument’s certification for intrinsic safety applies are also affected (or limited) by the type of batteries used in the design.
Design Improvements, “Smart” Battery Chargers
Battery and battery charger manufacturers have made major improvements in their designs over the last few years. Today’s “smart” battery chargers contain electronics for assessing the condition of the battery pack during charging and are able to drop from a “fast” charge rate to a “trickle” the moment charging is complete. The trickle charging rate is too low to produce damage or loss of capacity due to heating. As a result, instruments containing rechargeable batteries can be recharged in a very short period, while still being left on the charger for long periods of time without damage.
Li-ion Battery Concerns
Li-ion batteries do not suffer from charging “memory” issues, and they do not lose capacity as a function of over-charging. Li-ion batteries have low internal self-discharge rates and lose power only very slowly in storage. They also do not require periodic cycling to prolong life. The materials used in Li-ion batteries are environmentally friendly, and Li-ion batteries are better in cold temperatures than alkaline batteries.
Lithium ion batteries share a major concern, however, which is the possibility that mechanical damage can lead to an internal short leading to a “thermal runaway” condition. This can, in turn, lead to a fire.
If you slice a Li-Ion battery in half, it looks like a jelly roll with many extremely thin layers. A non-conductive separator layer is used to keep the cathode and anode layers apart. The electrolyte consists of salts and other additives in a solution that contains flammable solvents. It serves as the conduit of lithium ions between the cathode and anode layers. Mechanical damage that allows the anode and cathode material to directly come into contact can cause a short, causing an increase in temperature of the electrolyte and battery components. As the temperature becomes hotter, other components in the battery begin to break down, worsening the short-circuiting. If the heat is not dissipated quickly enough, the reaction becomes self-sustaining. The higher the temperature, the greater the current flow, increasing the temperature still further—until the electrolyte and other components in the battery reach the auto-ignition temperature and burst into flame.
Li-ion battery fires are extremely difficult to put out. This is the reason airlines prohibit electronic devices equipped with rechargeable, Li-ion batteries being checked in baggage. While you are allowed to take Li-ion-battery equipped devices with you into the cabin, the safety briefing warns you to be careful you do not do anything that could mechanically damage the device (i.e., getting it caught in a reclining seat mechanism). If you have ever seen video footage of a burning Li-ion battery pack, you will know why the airlines are so concerned.
Fortunately, certification laboratories like UL® and CSA® take this issue very seriously. Unlike the Li-Ion batteries in many consumer products, the rechargeable battery packs in portable gas detectors are evaluated as part of the Intrinsic Safety certification process for the complete instrument. Testing includes deliberately creating a short and measuring the temperature rise within the battery.
Gas detector battery packs have fuses to interrupt uncontrolled flow of power and are encapsulated with fire-resistant potting that helps to spread and dissipate heat. They might include other components, such as metallic “heat pipes” or vanes to further spread heat quickly and evenly through the battery pack and reduce the chances of “hot spots” in the battery, which reach the auto-ignition temperature of the other components in the instrument.
Nickel metal hydride (NiMH) batteries have several safety advantages over Li-ion batteries. The electrolyte is not flammable, and they are not prone to run-away short circuiting. NiMH batteries are also a particularly good choice for low-temperature operation. While all types of rechargeable batteries are affected by cold temperatures, NiMH batteries are typically usable down to -20°F (-29°C) with only a modest loss of operation time. They can be used for shorter periods of time, even in colder temperatures.
NiMH batteries are durable and able to survive up to 500 complete charging cycles without a significant loss of capacity. To avoid harming the battery, compared to Li-Ion battery chargers, NiMH chargers can take a little longer to fully recharge depleted batteries. While rechargeable NiMH batteries can be left on the charger for prolonged periods of time without damage, they still benefit from periodically being deep-discharged. Most instruments that include this type of battery also include an automatic deep discharge cycle.
For maximum flexibility, being able to use disposable batteries is a strong design advantage. But, when battery safety, cold temperature operation and / or the certifications carried by the instrument are the major concerns, NiMH batteries should be strongly considered. IHW
[Bob Henderson is President, GfG Instrumentation, Inc., Ann Arbor, Mich.]