Can I fit a 39Wh battery in my MacBook Air 2011 which states 35Wh?
Solution 1:
The battery you want to buy should be fine (in terms of the specs and compatibility). The 39 watt hours basically means that on a full charge it should last longer than a 35 watt hour battery (about 11.4% longer). Many after market MacBook batteries do usually offer a little more capacity in that regard.
As for the battery itself, I've never used that particular brand, so cannot vouch for its quality. However, since it seems to have a 5 star rating, it looks promising. As long as it offers a decent warranty (I can't read French, so can't tell), I'd buy it.