So a bench power supply can be quite expensive, and something I already have laying around is a car battery and a car battery charger. Is there any reason why a car battery + battery charger would not be a good bench power supply for stepper motors (given the need for straight 12V power, obviously the complexity increases when you must supply other voltages). I'm looking at the setup with a scope and while it's a little noisy it's nothing out of the ordinary either. I'm in the midst of testing out a RAMPS 1.4 unit with a megaduino and a single NEMA 17 motor. While I'll probably scavenge an ATX unit soon, I was wondering if it was crazy to try such a setup with a car battery. Furthermore, looking at the amps some of these car chargers put out, it seems worthy of looking into for vastly more powerful projects as well.
There is an safety issue that should be mentioned. Car batteries can put out huge amounts of power. If you put together a project and you have a dead short somewhere you are going to start lighting things up. A fuse is a must.
That said, a battery can work if you can't get access to a regular DC power supply. It shouldn't be recommended though as no current limiting capabilities are provided. (this is why a fuse is a must)
Another note, batteries will vary in voltage. Don't be expecting a nice clean 12V supply
A car battery & charger is not a good 12v supply - for a start, it's not 12v. A battery, when idle, will be around 12v but it's not regulated and will drop with load & float back up when unloaded.
Car battery chargers are quite variable, and will not output a smooth or regulated voltage - mainly because they don't need to. Some smart or conditioning ones may spike the voltage to de-sulphate the battery plates, or drop to trickle-charge mode, or charge in cycles... in short, when on charge, you can't really guarantee what the voltage will be or what the charger will do if you start drawing current.
As mentioned above, car batteries (even tiny ones) can provide HUGE currents (250A+) so you absolutely MUST have a fuse on it. A friend has just blown the tracks off a circuit board by powering it from an un-fused motorcycle battery.
ATX PSU's are ten a penny from any discarded PC, I would look at salvaging one or more. For added niceness, use the Dangerous Prototypes ATX breakout board or your own implementation of the circuit.
Oh yes, car batteries release explosive gas when in use. Fine in a draughty engine bay, not so great in your back room. They can also leak acid, or explode if there is a fault. It's a bit like keeping a can of petrol in your workshop - in theory it's perfectly safe, but most of us would rather it was sat somewhere else.
Don't forget that a car battery is rarely exactly 12V ( Wikipedia ) and could be significantly higher (possibly 14V+) when on charge... you'll need to make sure that the possible voltage range is within the acceptable tollerances of what you wish to power with it.
Also it's worth bearing in mind that a battery may not have sufficiently high supply voltage to use a voltage regulator (eg a 7812), as they can need a supply that is a couple of volts higher than the regulated voltage they are designed to supply.