Battery Backup light comes on for no apparent reason

Every so often the light on the battery backup unit (BBU) that indicates the unit is using battery power comes on even though there was no power failure and there is AC power to the unit.  The first time this happened I called FIOS tech support and they indicated that if the light was on, the unit was operating off of battery power even if AC power was available, and that I would need to reset the unit.  They then took me through the procedure for resetting the unit:

1) Open the case to the BBU.

2) Unplug the battery.

3) Unplug the AC power.

4) Wait at least 3 minutes.

5) Plug the battery back in.

6) Plug in the AC power.

This seems to do the trick and I assume it also resets the ONT.

I have 2 questions:

1) Why would the unit decide to operate off of battery power when there was AC power available and there had been no loss of AC power to begin with?

2) Is it true that if the battery light is on the unit is operating off of battery power even in the presence of AC power?

Any thoughts on these question would be appreciated.

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