The original Inductrix has a battery plug with 1.25mm spacing between the two pins.
This connector is not designed for repetitive connection. It will fail after 20-100 connection cycles.
The most common part is the FC side connector to fail. So you can usually fix it for a while by just replacing the 1.25mm pigtail.
A better, longer lasting fix is to replace the power plug with a PH-2.0 connector. It has 2mm spacing between the prongs, is significantly more robust and passes more current.
So if you buy batteries, always check if they have a 1.25 or 2.0 plug.
And if you have a mix of batteries (or pan to transition), you can solder two pigtails to the same quad.