[optimusprime]

I'm working on an electrochemical experiment and I'm a little short on equipment.  I do have a car battery charger lying around, and I wonder how insane I'd have to be to work it into a power supply?

[ARGOS++]

Dear Optimusprime;

You don’t have to be insane in any kind!,   ─   because it is already a power supply and it is of DC nature too.

But you need a little bit to know about your device, such as:

• a.)   What is its Voltage? 12 or 24 Volts? (maybe always a little more, because charging.)
• b.)   Has it already a possibility for regulation the Current? (Regularly or only max. Current?)
• c.)   Maybe it has already an Instrument for Current and/or Voltage?

In the best case you need only an additional adjustable Resistor and an additional Voltage instrument.
So, please tell us something about your device (Maybe also about your experiment).

Good Luck!
                    ARGOS⁺⁺

[optimusprime]

a) 12V nominal.
b) Selectable current 6A or 2A
c) It has an analogue 0-8A ammeter but I have a multimeter I can use as needed.

I thought I would only need a resistor or two... in worst case, I could hook it up to a car battery as a big resistor of sorts.

[ARGOS++]

Dear Optimusprime;

It’s not all as simple as most people may think at the first moment!

From your description I assume that you really want do only electroplating and not additionally more or less analytical work too. That’s good, as otherwise it requires a lot more.

Because the charger has only a current-selector switch and an current meter, I “oracle” that on one hand you may own a very smart device for current regulation (maybe too expensive) or a quite simple “mechanical/resistor” limited device. Maybe you can find it out by its manufacturer.

In both cases you have a “real” Current Source and that you have to think it (be) replaced by an “ideal” Current Source combined with a build-in (by Mother Nature) Resistor. But this build-in resistor is not really constant, especially not in a smart charger.

Now you have also to think about that the charge is build for a 6-cell Accumulator (6 times 2.0 Volts), but such a cell has a variable Voltage from 1.75 V till 2.4 V depending on its state of loading, and that means that your charger will deliver 14.4 Volts till 16.0 V,   ─  that’s much too much for electroplating.
On the other hand your electroplating cell is in the sense of  a current also a very variable Resistor too, and you have to follow certain rules for the used Voltage during electroplating to get a good plating and to not have water/electrolyte discharging in concurrence or instead.

So the simplest, but only under visual control!, is to add an additional powerful variable Resistor (Take care it will get hot!), all in serial order!, to adjust the Voltage to the allowed value. And that’s why you should use your “Multimeter” for control. It can be a little more complicated in case of a smart device.

It is very unwise to use another Accu as a Resistor, because in serial order you have two possibilities.
If you insert it the wrong way, then your final device will deliver 16.0 + 12.0 = 28 Volts, and if you do it the other way, your final device will load the Accu instead and ends with a delivery of 16.0 – 14.4 = 1.6 Volts.
In case you insert it in parallel to your charger in correct kind it will only work as a “Buffer”, and in the other way   ─   Don’t try!!

Maybe the best is you draw “A Picture” for you and think about.
Also any search can be useful as for an Example:   "Charge Controller”
(- with its additional Links.).

I hope I have been of help to you.

Good Luck!
                    ARGOS⁺⁺