A number of people have started using LiFePO4 (Lithium Iron Phosphate, sometimes known as LiFe) batteries in their boats due to their great power density, their light weight, and because they are so much safer than the other lithium batteries. Unfortunately, the UN and TSA (Transportation Security Administration) have not differentiated the rules between the other types of lithium batteries and Lithium Iron Phosphate batteries. As a result, LiFePO4 batteries and cells are governed by the more restrictive rules.
Under current rules (as of December 2011), small lithium ion batteries and cells (which includes LiFePO4 cells) of up to 100 watt-hours may be carried without restriction as carry on baggage only (these are considered small batteries). Batteries and cells between 100 and 300 watt-hours can have one installed in a device and two spares, again carry on only. Cells and batteries above 300 watt-hours are forbidden. Most of our batteries fall into the small battery category, as most boats I’ve seen use the larger individual cells in the 10 to 20 amp hour range which corresponds to 33 to 66 watt hours per cell. So, you can pack as many of these for personal use as carry on so long as they are made safe to prevent shorts and damage.
Part of the problem is that the TSA agents are often unaware of the regulations concerning traveling with lithium chemistry batteries. I’ve been forced to check the batteries because the TSA agents felt that the batteries would be safer there. For that reason, I would highly recommend carrying a copy of this TSA page in your luggage right along with the batteries: https://apps.tsa.dhs.gov/mytsa/cib_results.aspx?search=lithium%20battery . The page that has the specs on what constitutes a small versus large battery is on the page http://phmsa.dot.gov/safetravel/batteries. This is what TSA used to make their rules, but the TSA agents only accept rules handed down directly from TSA and their website. I’ve had them flat out tell me they don’t care what the DOT rules are. It might also be a good idea to have a printed page of http://phmsa.dot.gov/safetravel/definitions#larger_lithium_metal handy to show that the bricks we haul around are actually classified as small batteries and cells.
I hope this guide is helpful when flying with your batteries to your next model warship combat adventure.