So, I have to admit, something very strange happend and I would like to know if someone can give some explanation about it.
Since I have charged my iphone 3GS with the mintyboost (It's more like a mightyboost because it's still charging after 2 hours!!), the use of my battery on my iphone is better than ever (like in the first place, when I bought it!)
Today, I could use my iphone all the day and this evening I was surprised to discover that I didn't need to charge during the day.
The only explanation I found is the quality of the charging device but I'm not an expert so if anyone as experience to share I'm interested.