What is Reddit's opinion of Battery Eliminator, Power Supply, AC to DC, 3V, 5V, 6V, 7.5V, 9V, 12V Outputs 2 AMPS Regulated DC Power?

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Sounds like a cool project.

If you know the resistance of the bath (you might be able to measure it with the meter), you could make up a voltage divider like you mentioned. You need to make sure the resistors you use are large enough (in wattage) to dissipate the extra energy without getting too hot.

A relatively inexpensive adjustable power supply like this ($40) would be easier and safer. https://smile.amazon.com/Battery-Eliminator-Supply-Outputs-Regulated/dp/B01M3372KU

For even better control, a lab power supply (search for adjustable DC power supply) would be able to limit voltage and current; it will provide X volts at up to Y amps (you adjust both limits). If the resistance drops below X/Y ohms, the power supply lowers the voltage to make sure no more than Y amps flows. Sounds like this would be just the thing for plating (although I don't know a lot about electrochemistry.) Even if the resistance changes, you can limit the current through the bath.

Here's a decent-looking one for about $67. https://smile.amazon.com/Yescom-Precision-Variable-Digital-Adjustable/dp/B00SWK6M0M