So I recently found this while clearing out the workshop of someone:
It is a charging indicator for mining lamp charger racks, in partcular for Oldham-Arras lamps. It still had the bus bar mounting clips attached to it. So of course with this sitting around, I decided to built my own mining light charger.
NiCd mining lights were charged using constant voltage, since they were used all ~20h with the charging requiring around 14-16h, so the time of the cells being charged and venting was kept fairly low.
I decided to use the same approach and simply put in a 5V/3A SMPS supply with a 3R3Ω 50W resistor in series in. The 3R3Ω resistor limits the current to around 1.5A, as to not overload the indicator, I went through some manuals for mining lamps and they usually have a not-to-exceed charging current of 1.75A. As such this fits perfectly.
I would have liked to built this more old school by using a linear supply and regulating the voltage down. However since this thing has to run 16h+ at a time to charge a lamp, I decided to go with the way of the least resistance, as in the least amount of heat dissipation.
Update 07.03.2022
I added a 1N5820 diode in series between the +5V from the power supply and the positive output terminal. This should avoid the lamp battery to discharge over the protection circuitry in the SMPS if line power is disabled. The leakage current through the diode is negligible, as the internal battery resistance is lower than the leakage current through the diode. Meaning the battery will rather discharge itself than get discharged through the leakage current.