find a new way to connect heating bed to power

[opatiny]

Current method (using two bent copper wires touching copper pads) is unreliable:

• the copper is oxidizing
• good contact with the pads is not ensured
• the wires take a part of the weight of the tank -> up to hundreds of grams depending on the conditions

We want to replace this system by a connector.

For example, we could use XT60 connectors with 12AWG (cable quality reference), which resist to very high currents.

Some of these were ordered on 2020.06.17, and will be tested when they arrive.