There must be some service already in sailfish monitoring battery levels and shutting down the phone at 4%. I wonder if it is possible to tweak it to play an alert at, say, 5%. Also i'd like to have an alert for full battery.

Can this be done, anybody knows how?

It would be a very useful addition, I agree.
Anyone?

Once I get my Jolla booting, I might have a look :P

I don't know about the Sailfish phones, but I used to be annoyed to no end with my N9's battery being empty, without any warning sign whatsoever. Also, I have my devices laying around in other rooms of the house, so I ended up with a "remote" solution. Provided you have ssh running, you could try a oneliner like this:

It would send me a notification message on my laptop when my battery would drop below 15%.

veeall, try this Battery notifer
https://openrepos.net/content/schturman/battery-notifer

My Jolla notifing me every percent drop <= 10%. Why i need to install any battery notifier for that?

I'm sure you don't need
My Jolla do the same, but probably some people like to see big notification

How to do this?

Just open this file and see how I did it:
/usr/share/openrepos-batterynotifer/notifer3.py

What should i do?