First you may need a new MagSafe power adapter. Check this Apple T/N to make sure you have the correct one and the things you need to check: [https://support.apple.com/en-us/HT203207|Apple Portables: Troubleshooting MagSafe adapters] Make sure the cable & connectors are in good shape. Any damage you should replace the adapter.
Next did you calibrate your battery? Here's a good write up: [https://www.newertech.com/batteries/power-calibration-guide/|How To Calibrate a Mac Laptop's Power System]
Lastly, you should download this gem of an app! [https://www.coconut-flavour.com/coconutbattery/|CoconutBattery] If you are still having issues lets get a screenshot of the main window and post it here for us to see. [guide|21499]
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=== Update (04/07/2018) ===
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At this point your logic board will need repairs there are a few possibilities on what will need to be replaced there is a MOSFET that might be bad or the voltage comparator logic is damaged.
First you may need a new MagSafe power adapter. Check this Apple T/N to make sure you have the correct one and the things you need to check: [https://support.apple.com/en-us/HT203207|Apple Portables: Troubleshooting MagSafe adapters] Make sure the cable & connectors are in good shape. Any damage you should replace the adapter.
Next did you calibrate your battery? Here's a good write up: [https://www.newertech.com/batteries/power-calibration-guide/|How To Calibrate a Mac Laptop's Power System]
Lastly, you should download this gem of an app! [https://www.coconut-flavour.com/coconutbattery/|CoconutBattery] If you are still having issues lets get a screenshot of the main window and post it here for us to see. [guide|21499]