Spare lithium ion batteries can be brought onboard an aircraft in carry-on baggage, so long as:
- each battery has no more than 8 grams of lithium content or;
- in the case of extended-life batteries like those NCharge laptop batteries from Valence, no more than 25 total grams of lithium are carried at any one time.
The reason for the ban: lithium ion batteries are extremely flammable. If one catches fire in the cabin, people will notice. In the cargo hold, it's much harder to keep an eye on them. Makes sense, and it's not likely to affect most people anyway, so don't let it bother you.