The B+ has an under voltage detect trigger which results in the power led going off when voltage drops below about 4.65V.
The signal is also available on a gpio line (GPIO35).
Latest firmware update will display a warning symbol in top right of display when this is detected.
It will also disable turbo mode while the warning is displayed to try to reduce the chances of crashing.
I’ve also added a warning symbol to the existing over-temperature condition (> 85’C), which also disabled turbo mode.
Currently the symbols are:
Red square: over-temperature
Rainbow square: under-voltage
As usual you can override the behaviour in config.txt if you understand the risks:
avoid_warnings=1 removes the warning overlay.
avoid_warnings=2 additionally allows turbo when low-voltage is present.
Reboot after making the change.