Frequently asked questions

How can I use USB to charge a battery?

The Universal Serial Bus (USB) standard has evolved dramatically from its introduction in 1995. Designed originally as a replacement for the antiquated RS-232 interface, USB not only provided enhanced serial communications, it also allowed the ability to provide power to remote peripherals devices. Originally, this was limited to a fixed 5V supply with a maximum current of 1000mA, but subsequent revisions have allowed for significant increases in both data speeds and power capabilities.
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The present power standard allows for currents of up to 5 amps at a number of common voltage (ie. 5V, 9V, 12V, 15V, 20V, 24V, 36V and 48V) as well as several programmable modes known as PPS. These enhancements necessitated that the number of wires in a fully implemented configuration expand from 4 to 12, with several of the additional lines being used to negotiate the voltage and current capabilities of the overall system. 

While the full details are far too involved to be discussed here, these enhanced power configurations were included precisely with the idea of battery charging in mind. In systems that have an internal power conversion stage, the USB supply is simple set to an appropriate fixed voltage level and the host’s internal charge controller manages the pack. In systems that lack internal power conversion, this function can now be managed in the USB supply, provided the supply supports the appropriate PPS mode. The host still needs to monitor its battery and then adjust the supply accordingly, however.

What is “intrinsic safety”?

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.

What is the difference between a cell and a battery?

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.

What “standard” rechargeable batteries are available?

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.

How do I ship a battery?

Generally speaking, any battery is supposed to be at a 30% state of charge or less when it is shipped in order to minimize potential risk.

If the pack is shipped via air, it needs to be in a special container that both protects it from crushing and shock and will contain a fire if the pack should experience a thermal event. Air shipments require special labeling that call out the potential hazard. Batteries should never be shipped on commercial passenger planes.

If the pack is shipped by land or ocean the carrier must be made aware they are carrying hazardous material.

Passengers may carry a fully charged laptop battery on a commercial plane as long as it is inside the computer and does not exceed 100 watt-hours in capacity.

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