I normally carry an extra battery for my electronics. I am travelling in Europe.
Is it allowed to have an extra battery in a hand luggage?
Probably, but with care
Although the EU security regulations don't prohibit spare batteries, there are separate safety restrictions. IATA regulations state that:
In all cases, each battery must either be in a separate plastic bag, or have its terminals covered to prevent a short circuit.
The British CAA guidelines for example are a little more restrictive, requiring airline approval for anything between 100-160Wh. Not sure whether any EU member states have similarly strict guidelines.
For comparison, a laptop battery is typically less than 100Wh
Yes, you can. There is the list of prohibited objects in hand luggage according to EU regulations. As batteries are not listed you can carry them without further trouble.