Most of us have suffered the loss of power of a battery after some months or if we are lucky (or select hardware wisely) after some years. There are a lot of misconceptions and a lot of myths around this problem. But there are experts that can tell us what’s going on and what we can do about it.
One of the worst things you can do to a Li-ion battery is to run it out completely all the time. Full discharges put a lot of strain on the battery, and it’s much better practice to do shallow discharges to no lower than 20 percent.
And for us leaving in Greece especially during Summer temperatures:
Another thing that Li-ion batteries hate is heat. This somewhat less of a problem for cell phones, but a big problem for notebooks. Even using a battery at room temperature for a year can bring its capacity down by as much as 20 percent, and the interior of most computers is a mite cozier than than that. So in a unfortunate twist of fate, laptop batteries usually spend the most time in the worst possible state: plugged in at 100 percent charge, running at an elevated temperature.
Read what you can do to take care of your Li-ion batteries here: Ask Ars: What is the best way to use a Li-ion battery?.