Hi there,
I'm from a country where it's about 28-35 degrees year around, and high humidity.
This means that also inside there is usually quite warm, as the aircon is not powered on while away from apartment.

What's the best alternative?
Is it to store batteries in a closet?
Or refrigerator the best option? ( about 3 degrees I guess)

Thanks.

I keep mine in the refrigerator if I'm not going to use it for a week or more.

You need to keep it in a sealed air tight box with a packet of silica gel to absorb any condensation, recharge the silica gel in the microwave before each use, and when you take it out of the refrigerator leave it sealed in the box for an hour to warm up properly before recharging (if you take it out of the box immediately it will get condensation all over the electronics).  Always recharge to full before use.

The only advantage is that the battery will have a longer lifespan, you could just buy replacement batteries every year instead.

DJI's pdf says not to store them in an environment higher than 60C and that the ideal storage temp is 22-28C.  

http://dl.djicdn.com/downloads/p ... _Guidelines__en.pdf

Please follow the guideline PDF that Mark provided. You may want to also invest in a LiPo storage container that might help with this.

It's very sad that you can't use the conditioner. I live in 30 degree temperatures all year round, and I can't live without air conditioning. There were times when it broke down, then I used a refrigerator. I know this is very unhealthy because it makes the refrigerator deteriorate. But what can you do, while I was waiting for airconservicing.or I had to escape the heat somehow. Perhaps only people who live in this temperature will understand this. I even find it hard to work, it makes me want to move to another country where it's much colder.