R31 says battery is low - it's not.

[Keithj]

When I switch on without the mains PSU connected, I get beeps and warnings that the battery is at 5% (9 mins) and I should connect power or shut down immediately.

My R31 has done this for as long as I can remember, and I assumed both my batteries were past their lifetime (both show the 5%).

Today, I decided it shouldn't be doing that, so ignored the low battery warning. An hour later, it's stopped beeping and still says 5% (9 minutes) remaining. I conclude something, somewhere, is feeding me misinformation.

Is this a known behaviour, and is there anything I can do to get to be honest about the power remaining?