"Service Battery" appears when rMBP (2013) is connected to HDMI Display - vanishes on disconnect.

As described. Normal battery life, no swelling. However, when I plug my rMBP into an external display via HDMI, I get a "Service Battery" warning. Not immediately, after about an hour. It happens much quicker when watching netflix - even if Netflix is on the laptop's monitor. There isn't an excess of heat and the laptop is on a hard flat surface for best ventilation.

As I said, when unplugged, battery life is normal and status is normal. What could be going on here?

Thanks!

MacBook Pro (Retina, 15-inch, Early 2013)

2.7 GHz Intel Core i7

16GB 1600 MHz DDR3

OS X Yosemite Version 10.10.3

External display is an Asus VE245 Display connected via HDMI cable.