I have a Macbook Air Early-2015 model since 2017.
Like 4 or 5 months ago, I updated from High Sierra to Mojave. Initially there was no problem. But after a few weeks my Macbook started to shutdown suddenly at about %15 (in changes time to time, sometimes it can be 5%). And when it is closed, I couldn't open it without connecting charge cable.
Because of the quarantine I stuck at home and using it while charger is connected, even it has 100% charge, but not for too long like 1 month or so. But I don't think that's the reason.
Anyway. I controlled battery health with coconutBattery and it was about 80-85%, I can't remember clearly, maybe even more, but I'm sure it was not less. Then I did a research and found SMC & NVRAM reset. I did those resets and calibrate the battery by charging to 100% and draining until it shuts down itself.
The sudden shutdown problem seemed to be solved, but after a few days the Service Recommended warning started to appear. And when I checked with coconutBattery, it said 55% battery health. Still same value with small changes. I mean, how this can be possible? The cycle count is not even close to its limit, it just have 350 cycle. I have friends who has same computer for more than 5 years and more than 800 cycle without changing the battery and their batteries' health are much better than mine.
It seemed like a software problem to me. Before try formatting or downgrading to High Sierra I just wanted to ask you. Thus I won't waste my time by backing up and formatting if it won't make a difference.
Now I have to charge it like every 4-5 hours. And I realized that sometimes the battery percentage is going up. Like, I am looking the percentage and it shows 65% and I am going back to my work, write something in Word etc. 5 minutes later when I check that again, it shows 66%. That's odd.
I don't want to buy a new battery, I can't spend that much money right now.
I apologize for the long text, I just wanted to explain everything :)
Thanks in advance.