A little loss over the whole winter is typical. Also when the battery is cold it will be weak, but when it warms up it will go back to normal. This is also typical of any battery. You may have more of a charge than you think if you bring it inside and let it get to room temp. Don't charge it while it is cold. Riding in the cold will shorten the useable ride time, while it is cold.
Over the winter bring it inside and charge it a few times.