Ferrex (Aldi) 20V Battery Adapter

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 t