Hello TomT13,
Thanks for reaching out to Microsoft Community. I understand how you wanted to replace the battery.
Unfortunately, there is no replacement for Surface batteries. If your battery would no longer work, the device needs to be repaired or replaced entirely. To get your device serviced and check for further options, I'd highly suggest contacting our Microsoft assisted support.
Go to Contact Us - Microsoft Support and select Devices.
Choose your preferred support method.
Type in the problem you need help with.
Look at any available solutions offered and select Contact Support.
I hope this helps.
Kind regards,
StevenK