Hi @hawkisdead ,
As a first step I suggest that you check if the battery is charging properly when the charger is connected.
If not the battery may need replacing.
Here’s a link to the [https://download.lenovo.com/consumer/mobiles_pub/ideapad_510s-13isk_510s-13ikb_hmm_201608.pdf|service manual] for your model Ideapad.
Scroll to p.21-22 to view the procedures for checking the charging and the battery pack.
Scroll to p.35 to view the necessary pre-requisite steps and then the procedure to remove/replace the battery pack.
On p.70 item #11 you’ll find the part numbers for all the batteries available for your particular model Ideapad. Searching online using the ''part number only'' will give results for suppliers of the part.