MacBook Air sudden black screen when on battery

Hi,

I have a MacBook Air 13", early 2015, macOS Mojave 10.14.6. Battery still supports 1 hour+ operation, but I have a Service Battery warning in the battery menu.

I am now getting random black screens when I work on battery without power cable, even if the battery is still mostly full, for example at 70%. This is not a screen saver, this might happen while I'm actively working on the machine. When I have a black screen, the machine is not powered down, and the Caps Lock light still reacts to changing it. I have not been able to get rid of the black screen other than reboot by holding the power button. Hooking up external display didn't help. Machine is still booting without power cable.

Is this a known issue, is there a fix other than servicing the battery? I'm hesitant to spend money on the battery if the black screen issue is a larger, unrelated problem.

Thank you!