We find ourselves taking our aircraft on domestic and international flights quite a lot. Getting the plane safely packed to endure the rigors of the baggage handling gorillas is hard enough, but what about the batteries? We've spent a ton of time combing the internet to understand the rules so hopefully this post can help you out when you fly. Here goes:
1. Do not put LiPos in your checked baggage. Should bad things happen the crew wants to be able to fight a battery fire.
2. Put the LiPos in your carryon baggage. If you've still got the original packaging, use it. If not, put each battery in a separate plastic bag. Tape over the connector and the balancing plug.
3. This isn't a rule, but I always take the batteries out at the TSA inspections and put them in a separate tub. There is not point in trying to hide them because they stick out like a sore thumb in the X-ray scan.
4. You can only carry 2 batteries greater than 100 Watt-hr (e.g. a 6S 5000 mAh) on the plane.
5. How many less than 100 Watt-hr? Here the rules are a little ambiguous with verbiage like "a reasonable number." I've carried 6 on international flights successfully.
6. This isn't a rule, but it's always a good idea to put the batteries in storage mode (about 50% SOC) to be safe.
7. I wish there was a single site that stated these rules but I haven't found one. The links below are a collection of them. I print out the relevant pages and have them with me in case of trouble. At several airports the inspectors were very thankful for this documentation and made copies of it for themselves.
8. Generally, getting through security in the US hasn't been too bad, although you should allow extra time because they almost always do the extra sniffing tests on them. Internationally is another story because finding an English speaker is often hard. My most difficult time was in Ulaan Baatar, Mongolia but we eventually prevailed.
Links and notes (the notes refer to batteries less than 100 Wh):
This document will be the most useful. In the chart on p. 1, LiPos are "Spare Lithium Batteries" (top chart), "Spare Lithium Ion (rechargeable) Batteries", less than 8 gram" lithium content. So you'll see that carry-on is allowed with no limit and checked is prohibited. I suggest you print this out and highlight the sections I mentioned above so the busy TSA guard can get to the important info easily. Page 2 of the document shows how to show that your batteries are less than 8 grams of lithium or less than 100 Watt-hours each (those two measurements are really the same thing).
This web site is directly from the DOT and has the statement "there is generally no restriction..." in the next-to-last paragraph. Print it out and highlight that passage.
This one talks about packaging the batteries for travel.
This one talks about the equivalent lithium content measurement. You probably won't need it, but print it out just in case.
This document is a bit redundant with the ones above but you may want to take a look at it.
This one is important because it is from the Int'l Air Transit Association, so it shows that the international rules are the same as the US. Print out p. 1, 12, and 13. Highlight the second ("...does not impose a limit...") and fourth paragraphs (US and ICAO/IATA rules are harmonized).