Not starting

Over the last couple of years, I've had trouble with the car starting.

If it was left for a week or more, it wouldn't start when I got back to it and because I travel a number of times a year and need to leave it at the airport, I bought a Buture jump starter which works perfectly.

Lately the problem has gotten worse and if I didn't use it over the weekend, by Monday it wouldn't start. It now needs to be jump started or charged every day!

When I go out to the car in the morning, it needs to be jump started so I can get to work and by the evening, it needs doing again.

I also have a battery charger and when I've fully charged the car, it starts fine but after a day or two, it won't start again.

I went and had the battery checked and it came back as good. I also took off the negative cable, fully charged the car, put the cable back on and it started. I then took the negative cable back off and left it overnight. The next day I put the cable back on and it started. I was told this means the battery is okay and it could be the alternator. I didn't drive the car that day and the next morning it wouldn't start again.

I had a new battery fitted about 3 years ago and a part used alternator fitted about 2 years ago.

Any ideas on what it could be, or my next step would be much appreciated!