[upv5]

If you only do low mileage especially at this time of year with lights on, heated screens, heated seats etc your battery will get low.
I suggest buying a simple charger from a reputable maker and give it a full charge which may take 24 hours or more. Good ones have polarity protection and an indicator when fully charged. Leave the bonnet ajar for ventilation.
You can safely do this with the battery attached and you are correct it is safer to connect the negative to an earth on the other side of the engine bay, I use the engine lifting eye. Disconnect this first when you remove the charger so if there is a spark it is not next to the battery.
You will probably find you only have to do this once a year as during the summer the electrical loads are much lower.
Batteries do have a limited life but Volvo original and Bosch silver often exceed 10 years if looked after.

John