I'm pretty sure you need to buy and find upgrades throughout the campaign just explore a little bit more and buy some stuff
^ There is a side quest... if I remember right, Dr Chakwas mentions if you do it she will be able to improve the efficiency of your medigel. Also, I don't know if anyone else noticed, but if you are taking damage at the time you use it, or if you have a partiallty
full health segment that hasn't started filling back up, then your medigel will usually not completely restore health.
I don't think it's half but it definitely doesn't fully heal you. Usually, if I was down to one bar it will heal me but leave the last bar empty. So yea I guess, now that I think about it, it is in the 50% to maybe 60% range.