I have an electric shaver with a built in battery. Can I put it in my checked luggage on EU flights?
According to the IATA guidelines, you can have up to 15 personal electronic devices with a built-in lithium-ion battery that has a capacity of less than 100 Wh, which should be the case of an electric shaver battery, either in your carry-on or as checked luggage, without operator approval.
Spare batteries and batteries of higher capacities are subject to different rules.
If carried in checked baggage, "measures must be taken to protect the device from damage and to prevent unintentional activation".
You may still want to check your airlines rules, though as far as I know, most if not all follow IATA guidelines.