You do not need to manually iterate through the batteries, and you are already 99% there, really.
Get-WmiObject win32_battery | Where-Object {$_.DeviceID -match "JS07"} | select estimatedruntime
That should do exactly what you want, assuming all DeviceIDs contain JS07 as part of their identifier.
If you want more information, like the device ID or Name:
Get-WmiObject win32_battery | Where-Object {$_.DeviceID -match "JS07"} | select Name, estimatedruntime