Today I received some parts I ordered. I bought two 9 volt batteries (one 200mAh and one 250mAh) hoping one of them would fit. They both do. After that I connected both battery terminals to the 250mAh battery and clamped the battery to the PCB. Next I finally soldered the terminals to the PCB. The last thing to check is the battery charging circuit.
Also in my parcel was a DC-plug & cable assembly to charge the Mini³ battery with. A check with my Velleman P1832 PSU @ 15 volts output shows that the battery charging circuit checks out OK. Charging current is ~18.5mA. For the 250mAh battery to be fully charged it would take about 13½ -15 hours. Modern speed chargers laugh at this but slow charging is better for the battery.
The battery on the left is an old 200mAh that doesn't fit inside the case. The right one is the new 200mAh that does fit. The left one is a 9.6volt (8x1.2v), the right one is 8.4volt (7x1.2v).