[Elecraft] KX3 Batt Err

[gt-i at gmx.net]

I get an "bat Err" message if I want to charge my battery pack on the KX3.
The battery (NiMh) seems to be very low, as the KX3 doesn't start with 
them. The LEDs light up shortly then nothing.
But if powered externally, I'd expect I get my battery recharged.

What can I do?
Tnx,
Gernot, DF5RF