Military & Defense Power Mission-critical reliability in extreme conditions

Devices that are operated in potentially dangerous environments are often required to be certified for intrinsic safety. A typical example would be the use of two-way radios in a mining operation. Since mines can accumulate odorless, explosive gasses like methane, it essential that the battery powered radios not be able to provide a source of ignition. Equipment that is certified as intrinsically safe is required to undergo extensive scrutiny and testing to verify this fact.
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In battery-powered systems, the certification requires that charge and discharge currents are (redundantly) limited to prevent over-voltage and over-current situations. Attention must be given to all creepage and clearance dimensions. Temperature rise of all parts is also considered. The shorting of any exposed terminals is not allowed to produce sparks. Various elements of the system may require potting or coatings to limit the effects of contaminants or possible mechanical damage.

In popular usage, the terms “cell” and “battery” are often used interchangeably, though this is not technically correct.  In reality, a cell is the smallest discrete unit of electrochemical energy storage while a battery is a grouping of these cells.  (By way of analogy, an atom is the smallest discrete amount of an element while a molecule is some combination of atoms.)

When a consumer purchases a package of AA-cells at a retail store, each cell is a discrete electrochemical unit that produces about 1.5V.  Most devices that use these cells require more than 1.5V to operate, however.  To accommodate these higher voltage requirements, multiple cells are connected in series inside the device.  This combination forms a battery.

Most people are unaware that the little 9V batteries (commonly used in smoke detectors and portable radios) are true batteries in the technical sense.  They internally contain six small cells wired in series.  Likewise, most car batteries contain six cells packaged together as one unit.

By far, the most common standard rechargeable batteries are the lead-acid variety used in automobiles. These come in a variety of sizes from multiple vendors, but are generally large, heavy and have a liquid electrolyte that may require service.

With the advent of laptop computers, a standard series of small rechargeable lithium ion batteries was developed that have since found a home in many kinds of handheld equipment and portable devices. These are available from multiple vendors in many sizes and voltages.

Typically these batteries use blade-style connectors for easy replacement and have internal intelligence that tracks the state of health of the pack. As batteries have become subject to increased regulation in the last decade, the use of (pre-certified) standard packs can significantly reduce development costs for new products.

Though there is no specific naming convention for these packs, the model numbers often contain the digit combinations 201, 202, 203 and 204 as a legacy of the original names.

A number of vendors also sell their own pre-certified “standard” batteries. While no additional certification may be required for these packs, they are only available from a single source.

A “smart” battery is one that contains an internal fuel gauging circuit. By continually monitoring the current going into and out of the pack, the gauge is able to keep an accurate measure of how much energy is presently available.

Since this feature was part of the effort to standardize battery packs in the 1990s, an industry specification was developed that lays out how this, and other pack information, is stored and communicated. Collectively, this is known as the Smart Battery System (SBS) specification. The communications link is called the System Management Bus (SMBus).

The SBS specification allows for 36 different pieces of information about the pack. These include dynamic parameters such as voltage, current, temperature, and state of charge, as well as static parameters like the original design capacity and date of manufacture. See www.sbs-forum.org for more details.

SBS-compliant packs use a 2-wire communications scheme based on the I²C standard originally developed by Phillips. Larger packs, like those typically found in motive applications, often opt for some variant of the CAN communications used in automobiles. Lastly, there are also some proprietary, single-wire schemes, though these are used mostly in legacy applications.

The name “GaN” comes from the chemical symbol for gallium-nitride, a semiconductor material used for making power transistors.

The earliest transistors were made from the semiconductor element germanium (chemical abbreviation Ge). As transistor technology took off, however, there was a strong push to transition over to silicon (Si) as it is much better behaved and much more plentiful. And for the next 70 years, silicon received the vast bulk of the research and development effort.

Silicon and germanium are not the only viable semiconducting materials, however. There are also combinations like silicon carbide (SiC) and gallium phosphide (GaP) which have found niche applications. Unfortunately, they are often difficult to process using the standard silicon tools and techniques.

In today’s highly digital world, most switching applications are managed by silicon MOSFETs (Metal-Oxide-Semiconductor Field-Effect Transistor). These have improved markedly since their introduction in the 1970s, and have now reached maturity with little room for further improvement. One of their biggest limitations, however, is power loss during switching transitions, and their switching speed is limited by their (high) gate capacitance.

MOSFETs made from gallium nitride (GaN) can have characteristics similar to the best ones made from silicon but can switch considerably faster. For this reason, they are rapidly being adopted in the manufacture of switching power supplies, motor drives, etc. Since the switching speed also governs the size of the other necessary power parts, GaN-based supplies can be much smaller than their silicon counterparts. It is not uncommon to get a space saving of 50%.

Because they are simultaneously electrical devices and chemical devices, batteries merit special scrutiny from regulatory agencies. This is exacerbated by the conflicting requirements of the “ideal” pack. Users expect them to be small, light, robust and have high energy density. Unfortunately, compacting a lot of (potential) energy into a small package is inherently risky.

To begin with, UN DOT 38.3 regulates the safety of transporting batteries. It mandates testing for short-circuiting, crushing, thermal cycling, vibration, shock and altitude change among other factors.

IEC 66281 is an expanded version of UN DOT 38.3 that also includes drop testing. This certification is required for international shipping.

UL 1642 again covers many of the same tests, but focuses more on chemical and electrical hazards at the cell level as well as the safety of user serviceability.

IEC 62133 is the expanded international version of UL 1642.

UL 2271 pertains to batteries used in e-bikes and scooters.

In medical applications, batteries must meet the safety requirements of IEC 62133, UL 2054, ISO 13485, and IEC 60601-1.

Operation in hazardous environments may require additional certifications. Likewise with batteries that exceed 60V.