OK time for drastic action
If possible backup and image your system - better safe than sorry
Use the powerconfig options/Power management in the control panel to switch off all
warnings and especially
actions related to battery level.
Charge the battery to full
Run the laptop on battery until it dies through lack of battery.
re-charge the battery
Repeat the run until die stage Hopefully
after charge you should see some improvement
Reset the power options to protect you in normal use
In future always run the system on battery at least once a week preferably until it runs down to the switch off stage. (this is not the drastic type as above but the controlled type at 5% remaining )
Also if you are buying a battery - As the laptop manufacturers buy in a battery rather than manufacture it, it is possible to obtain generic spares, often at a considerable saving - just check out any company before dealing with them.