See this thread.
https://phantompilots.com/threads/faa-rules-flying-with-lipo-batteries.107866/

Thank you all for your input.
In the meantime I have learned that the final verdict rests with the airlines and the security personnel.

These people are trained to deal with the most common issues concerning the majority of passengers. They have procedures clearly stating what is and what is not allowed in the checked and in the carry-on baggage.

Carrying the Lithium Polymer batteries for the electric model airplane is not addressed explicitly in the procedures the airlines and the security personnel use to verify their decision (below).

In the document titled: "Batteries Carried by Airline Passengers - Frequently Asked Questions;
Question1: What kinds of batteries does the FAA allow in carry-on baggage (in the aircraft cabin)?

Answer to Question1:
"Lithium ion batteries (a.k.a.: rechargeable lithium, lithium polymer, LIPO, secondary lithium). Passengers may carry all consumer-sized lithium ion batteries (up to 100 watt hours per battery). This size covers AA, AAA, cell phone, PDA, camera, camcorder, handheld game, tablet, portable drill, and standard laptop computer batteries. The watt hours (Wh) rating is marked on newer lithium ion batteries and is explained in #3 below. External chargers are also considered to be a battery."

"Q3. How do I determine the watt hours (Wh) rating of a battery?
A3. To determine watt hours (Wh), multiply the volts (V) by the ampere hours (Ah). Example: A 12-volt battery rated to 8 Amp hours is rated at 96 watt hours (12 x 8 = 96). For milliamp hours (mAh), divide by 1000 (to get to Ah) and then multiply by the volts."

My comment: one 5S 2800 mAh Thunder Power is 53.2 Watt hour (3.8V per cell storage voltage assumed).

In Canada the problem is addressed on http://www.catsa.gc.ca/guidelines-batteries: the spare battery (singular! Is this typo or intentional limitation) is permitted only with air carrier approval. Transport Canada guidelines and the FAA guidelines have similar (but not the same) content that is presented in a slightly different way.

My comment: why is that? I feel nobody knows for sure.

Again, because carrying the Lithium Polymer batteries for the electric model airplanes is not addressed explicitly in the procedures, the personnel must use their own discretion to interpret the rules and, for many, this can be too difficult and too risky.

Human nature I suppose...

Regards,
M

If the captain says "No!" they don't go......

Even though the regs say it's ok to take lipos as carry-on, at Dubai last year the lipos were confiscated at the scanning area.
The Worlds competitors had to buy new batteries when they arrived in Australia.

What some people are doing now is buy cheap Zippy Compacts at the destination they are flying at. Or you could get someone to buy them for you there.
Of course you'll have to give them away when you go home.

It's a bit "pot-luck" really.