Electrical issue at 10% HEV level?

Went into the Rocky Mountains over the weekend. While drive east on I-70 up to the Eisenhower/Johnson tunnels (11,000 ft altitude) with DR lights on, the HEV battery ran down to about 10% (but did not go to "0") and the dash electrical system brightness suddenly dimmed/dropped to almost off.

I continued up the hill, through the tunnel and down the east side. While going down hill the HEV battery recharged to 100% and the dash lighting returned to normal.

Has anyone else had this happen?

Regards,
GEWB