This is a battery adapter for Ferrex (Aldi) 20V and 40V (2 x 20V) Litium battery packs to supply an arbitrary 20V load with up to a few amps. I am powering my TS100 soldering iron almost exclusively by my 20V Ferrex battery pack as it is so convenient.

The battery's built-in charge state monitor is quite basic. Therefore my design includes a digital voltmeter to continuously check the battery voltage and avoid deep discharge. To save power you may turn the voltmeter (but not the load) off by means of a built-in switch. The proposed switch is not robust enough to interrupt an external load of several amps. 

The B2+ contact on the battery delivers nominal +20V, B2- is 0V (ground).

The 40V battery version is composed of two internally separated 20V "half-packs", independently accessible through the B1 and the B2 contact pairs. On the 40V version my adapter draws power from the B2 half-pack only, i.e. the nominal supply voltage is still only 20V and the usable capacity is only half of the 40V battery pack's total capacity.

Caution:

• Be aware that Litium batteries require sophisticated care to operate them safely. In case of mishandling the battery pack may catch fire and/or explode. The electronic circuitry for this care is normally located in the genuine Ferrex tools and the genuine Ferrex charging adapter. By using the Litium battery with my adapter you are on your own to follow the necessary safety measures.
• Exclusively use the genuine Ferrex battery charger and follow the Ferrex safety instructions! Never charge the battery through my adapter. Charging with an excessive current or overcharging or charging a battery that has been discharged below a minimum voltage (even if only once) or charging a battery at freezing temperatures will lead to fire and/or explosion. My adapater has no active control over charging voltages and current and completely ignores the temperature sensor accessible via the middle contact ("T / ID") of the Ferrex battery. The genuine Ferrex charger incorporates all of these controls.
• Drawing too much current from the battery pack or a short-circuit may lead to fire and/or explosion! I have no profund information about the maybe internal fusing of the battery pack and the maximum allowable current. It may be around 10A, but this is just an assumption. There are no maximum current specs on the Aldi or Ferrex website. Again my adapter completely ignores the middle contact ("T / ID") possibly used by genuine Ferrex tools to sense dangerous battery temperatures caused by high current loads.
• There seems to be no battery discharge protection in the battery pack to avoid discharging the battery below a safe minimum voltage. Such discharge protection is normally located in the Ferrex tools and stops their use before a safe minimum voltage is reached. So it is up to you to stop discharging before the safe minimum voltage level is underrun. A battery discharged below the minimum voltage will catch fire and/or explode if being recharged. Therefore the genuine Ferrex charger may permanently refuse to load the battery after a minimum voltage underrun. For obvious reasons I haven't exposed my battery to a respective test to find this minimum voltage. The absolute minimum voltage per 20V pack may be around 5 x 2.5 = 12.5V, but please do not call me to account for this figure and stop at 5 x 3.0 = 15V as a precautionary measure. During use the voltage will remain high (around 18 volt) for a long time and then drop quickly, so watch the voltmeter, disconnect the adapter in time and recharge.
• Disconnect my adapter from the battery when not in use in order to prevent accidental short circuits.
• If you intend to modify my adapter to use both battery packs of the 40V battery pack in parallel (e.g. to access the full battery capacity of the 40V version still at 20V output to the cable), then you should have profund knowledge about the possible consequences, e.g. consequences of both half-packs having different charge levels, whereupon the half-pack with the higher charge level will try to charge the other half-pack trough your connecting cables at possibly excessive current levels. You may add a high-current diode (> 30A) to each half-pack as a protection against this situation. But what if a diode is becoming a short circuit by accident? I strongly suggest to abstine from such modification.
• Using my adapter may void the warranty of your Ferrex battery pack.
• Do not allow other people use the adapter if they are not fully aware of all of these points. Do not allow children use the adapter.

To make the cable strain relief work as intended, the width of the circular cable trench must tightly match the cable diameter (current default is 3.5 mm) in order to create some friction. You may have to modify the parameter "d_cable" in the OpenSCAD source file and re-create the .stl file to match your cable:

• Download and install OpenSCAD from openscad.org
• Open the .scad file in OpenSCAD
• Modify the value of the "d_cable" variable according to your cable's diameter
• Render with "Design/Render"
• Export with "File/Export"
• Feed the exported file to your slicer and print

The sheet metal screws are 2.9 x 9.5 mm (DIN 7981-C-H). 

The AMP contacts are the red or (preferably) the blue 6.3 mm (0.25") type. 

The voltmeter is the XH-028 V4.0 type supplied by e.g.
https://www.ebay.com/itm/125809771454
https://de.aliexpress.com/item/32828845267.html

I suggest to secure and isolate the red and black voltmeter wires with a drop of hot glue at the voltmeter wire soldering points against a possible short circuit in case the wires break at the solder point.

The switch has a 12 x 12 mm footprint. The contacts are 5.08 (0.2") apart. Various manufacturing variants with these dimensions may fit. Please note the "cross wiring" of the switch contacts: One of the switch pins may short to B2+ upon mounting the cover, whereupon the switch will become "always on". "Cross wiring" circumvents this problem.

When using a differing voltmeter or a switch with differing dimensions you will have to modify the voltmeter() and pushbutton() modules in the supplied .scad file. Upon rendering, these two modules are being subtracted (via OpenSCAD difference()) from the cover in order to create the necessary cutouts. So modifying the two modules will automatically provide a fitting cover .stl file. Changeing the distance between the two mounting holes of the voltmeter (currently 28.5 mm) will require further modifications.

Here you may find more information about the Ferrex battery pack:
https://www.youtube.com/watch?v=H9Mb5_nCVeo
https://www.youtube.com/watch?v=2d6iggXptNc
https://www.youtube.com/watch?v=jBfbqZHedto
I am not the author of these channels.

But I am not the author of these videos, so I cannot take any responsibility. I haven't opened my battery pack, so I cannot confirm or deny anything here. And please consider that Ferrex may change the battery cell type between production runs.

See also my Bosch 18V Battery Adapter:
https://www.thingiverse.com/thing:4891096

TODO:
Next big thing would be to integrate an automotive flat fuse. Probably one day ...