How old is your computer? Just last year I had a similar problem turn out to be my battery charger had an older one I plugged in and shazam it charged right up.
If it always said 90% that sounds like it is plugged in and charging all the time or most of the time. That's bad for the battery and will have shortened it's life. Modern devices usually have an option to limit charging to 80% to optimise the life of the battery under normal charge and discharge cycles but if the intended pattern of use is to leave it plugged in then the optimum setting setting is 60%. Is it possible your device has been set to the optimum for continuous plugged in use ? It's not uncommon for the actual value to be 1% below what has been set.
On a Windows device to find out what's happened to the battery open a command prompt and type powercfg /batteryreport and press enter. This will save a battery report file and it will tell you where it has been saved. Opening the file and scrolling down to Battery Capacity History will provide information as to how much capacity is left in the battery compared to it's design capacity.
My ancel tester will give a "battery health" in percentage but its a combination of volts and amps. Charge the battery with a low amperage charger / maintainer - allow it to rest a few hours to get the surface voltage off and test again.
I would charge all the way up and then turn off all of the stand by modes and run it till it turns off. Then charge all the way up and see if the battery or software recalibrates itself.