You are allowed to bring lithium batteries in your hand luggage if they are under 100Wh per unit.
If you want to bring batteries over 100Wh, you must obtain permission from the airline. In that case, you may bring a maximum of two units, and they must not exceed 160Wh each. Additionally, the equipment that the batteries are intended for must be brought along with the batteries.
If you want to bring lithium-ion batteries over 160Wh, these must be transported as fully regulated dangerous goods. You can read more about this on the Danish Civil Aviation and Railway Authority's website.
Batteries that are part of an electronic device (e.g., mobile phone, computer, etc.) may only be carried in hand luggage. Most electronic devices, such as tablets, smartphones, and laptops, have batteries that are less than 100Wh and can be carried in hand luggage.
If you are unsure about the size of the battery, you can check this on the manufacturer's website. Since some airlines have specific regulations in this area, it is always a good idea to contact the airline before departure.
It is not allowed to carry single batteries in checked luggage.
Relevante links
You can read more about the rules on dangerous goods here, where the full regulations can also be found (trafikstyrelsen.dk).
Read IATA's guidelines on battery regulations (new window).