A frequently asked question by air travelers, especially tourists is if they can carry back-up batteries with them in an aircraft. Each airline can have its own set of guidelines about what can be carried in your check in baggage, what can be taken in check in baggage and what is completely prohibited from taking into the aircraft.
There are various kinds of batteries and the aircraft regulatory bodies have guidelines for them which fit into the below points.
Spillable Wet Cell Batteries- These are the kind of batteries used in Cars and motorcycles, and are strictly not allowed on the aircraft either in carry-in baggage or the checked-in baggage. The only exception is that if they are a part of the customer’s wheelchair. In this case, the airline takes caution to remove the battery from the wheelchair and place it in a special container. Non-spillable wheelchair batteries are usually allowed. No backup is allowed.
Spare Lithium ion batteries- Spare lithium batteries were forbidden from being carried but recently there have been certain guidelines that airlines follow to allow these batteries to be taken provided they fulfil certain conditions. Spare lithium batteries must be carried in the carry-on baggage only as per the FAA guidelines and should be safely packaged. For the devices containing these batteries, If the lithium ion battery does not exceed 100 WH, these can be carried in as checked in or carry in baggage without any approval If the lithium ion battery exceeds 100 WH but does not exceed 160WH, you need approval from airline operator to carry it. Also, for this range, there is a limit of two spare batteries per person. Batteries being carried for further sale are not allowed. Any batteries above that wattage are strictly not allowed by airlines.
Dry Cell Alkaline batteries- These are the AA, AAA, C, D batteries used quite commonly, these can be carried in the check in as well as carry in baggage. You must take care that they are safely packed and there are no chances of short circuit with these.
Dry Cell Rechargeable batteries- These are the Nickel Metal Hydroxide batteries and Nickel Cadmium batteries. You should have these in their original safe packaging or should ensure that these are safely packed so that they never short circuit by taking care of safely packing the terminals.
Batteries that are installed in your devices such as laptops, chargers, cell phones, etc. are completely allowed and there is no restriction on the number. You can carry these in your carry on or checked in baggage. However the airline rules do change with time, so you should keep a close eye on the regulations that are released by FAA from time to time. Also please note that it is possible for each airline operator to have its own hazmat guidelines over and above the ones released by FAA so you can always check with the operator with whom you are travelling.