Can anyone recommend a battery replacement for this poor excuse of a cycling computer?

Ideally one that doesn't drain almost as fast off (properly powered off) as when on? When fully charged it's doing its job and survives quite happily for my typical 3-4h rides but for some reason it has this constant battery drain meaning I've grabbed what I think would be a fully charged unit to find it has virtually no power left. I don't understand this as it's a lithium ion battery which should only lose 1-2% of life per month. My device goes fully flat from full in under two weeks. The device is two years old and I ride about 3000 km per year.