In new standards for shipping lithium batteries that took effect on August 7, 2015, US DOT updated the accepted method for expressing the power of a lithium-ion battery. Before August 7, 2015, shippers in the US measured the power of lithium batteries by “equivalent lithium content.” Under the current rules, battery power must be expressed in watt-hours.
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When applied to a lithium-ion battery, watt-hour rating is a measurement of how much energy (in watts) the battery will expend over one hour. Knowing the watt-hour rating of a lithium-ion battery is crucial to ship it safely by ground, air, or vessel. Shippers use the watt-hour rating to determine how the battery must be packaged, marked, and labeled
, as well as what kind of quantity limitations apply to the shipment or whether the batteries are forbidden from certain modes of transport
—namely passenger aircraft.
Finding the Lithium Battery Watt-hour Rating
For newer lithium-ion batteries, determining the watt-hour rating should be as easy as checking the battery itself. Since December 31, 2011, all lithium-ion batteries, large (“Section I”) and small (“Section II”), must be marked with their watt-hour rating. As we move forward, it will become less and less likely that your batteries will not be marked with a watt-hour rating.
However, if you do not see the watt-hour rating marked on the battery, the next place to check is technical documentation, like a product specification sheet. If the watt-hour rating doesn’t appear in either place—on the battery or on the spec sheet—you can calculate it using other information that may be available.
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Formula for Calculating Lithium Battery Watt-hour Rating
If your lithium batteries do not have the watt-hour rating clearly marked on them, and the information is not included on a specification sheet, you can do some quick math to determine the watt-hour rating yourself, using some other information commonly found on the battery.
You will need to know:
- The battery’s nominal voltage (V); and
- its capacity in ampere-hours (Ah).
Multiply these two numbers to get the watt-hour rating (Ah × V = Wh).
If the capacity of your battery is expressed in milliampere hours (mAh)—like in the image above— you will need to divide by 1,000 to calculate the ampere-hours (Ah) before multiplying.
If All Else Fails, Contact the Manufacturer
If the watt-hour rating of your lithium batteries is not readily available, or if you cannot calculate it or are unsure about the results, the best course of action is to contact the manufacturer directly to get accurate information. The stakes are too high to make guesses about how to prepare batteries for transport—US DOT fines are as high as $75,000 per day, per violation. Ground and air carriers are well aware of the hazards posed by lithium batteries, and will reject shipments that do not meet the latest regulatory requirements.
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