Start with simple isolation tests. Try a different battery, including both Canon OEM and third-party packs if available. If possible, test the same battery in another body, or try a known-good battery in your 7D. Remove and re-seat the battery several times, and if you have a battery grip, test with and without it. Updating the camera to the latest firmware is also worth doing.

You can also try a deeper reset by removing both the main battery and the small date/time backup battery, then reinstalling them. Be aware this may reset some camera settings.

If the camera will let you dismiss the warning, see whether the battery still powers the camera normally. That helps determine whether it is only a communication issue.

Based on the shared answers, if multiple known-good batteries show the same error only in this body, the problem is likely in the camera rather than the battery. One reported cause is an internal hardware fault such as a loose ground connection, which would require repair. Anecdotal tricks like freezing the camera are not reliable and are not recommended.