This would be perfect little PIC project. Here is an outline of what I would do:
1. Make or get a load capable of dissipating the power needed. 3V at 600 amps is 1800 watts and 0.005 ohms. You could use a coil of wire in a bucket of water as a poor man's load. Or you can get 6 pieces of 0.03 ohms 220W resistor for $17 each:
2. Get a 500 amp 50 mV shunt, or as low as 250 amp for a 2x overload for a couple minutes. $25:
3. Get a Microchip (or other) development board, like this, for $100 or less:
or this for about $22:
or this for $17:
plus a PIC16F684 for under $2:
and the PICkit3 for all PIC projects, $42:
4. Get some high current MOSFETs or IGBTs that can be paralleled for safe handling of 500 amps at 30 volts or more. Like this which is good for 75A continuous and 30V. Use 6 in parallel, $4.65 each:
5. You will need some other parts like capacitors and resistors and op amps and gate drivers. Maybe $10 worth of parts.
6. Connect the 6 power resistors to the six MOSFET drains, to the positive battery test terminal. Connect the source pins together to one side of the shunt. The other side of the shunt will be the negative battery test terminal and your circuit ground.
7. Set up the PIC to provide a 0-100% duty cycle drive to the MOSFET gates, at something like 1kHz to 10kHz.
8. Make a differential amplifier to read the shunt voltage and multiply by 50 so you get 0-2.5 VDC from 0-50mV for 0-500 amps, and connect to one of the A/D pins.
9. Make another amplifier to read the battery voltage up to 24 volts, divide by 10 for 0-2.4V to another A/D pin.
10. Connect a pot to another A/D pin for current set control.
11. Add START and STOP buttons.
12. Set up the serial port to send the A/D readings to the computer. You can just use HyperTerm or other similar terminal software, and just send all three readings 4 times per second.
13. Power up the board with 5VDC from an adapter or the USB port. It should start with output turned off 0% duty cycle. The pot can be turned to set, say, 400A for 2VDC. The Hyperterm display should read this voltage as the setpoint.
14. Connect your battery. The voltage display should read the voltage, and current should be just about zero. The A/D converter is 10 bit, so you should be able to read four full digits to better than 0.1% accuracy.
15. Press the START button, and the PWM should increase until it supplies the requested 400A. The software can be set up to ramp it up over one or two seconds or as quickly as 100 mSec or faster.
16. The controller should hold that current within a couple amps, and it will be monitoring the voltage and sending readings to the computer. You can have it automatically shut down at 50% of initial battery voltage or whatever you like.
17. The possibilities are endless. You could set up a test sequence at several pre-programmed levels. You could have the current turn off every couple of seconds to get one or two open circuit readings, which can be used to determine internal resistance. You can add a temperature sensor which will track the rise over time and shut down at some point. You could add an LCD display and keypad for display and programming.
18. For a finished product, you could use a USB PIC and have a Windows GUI which will nicely display the data and show graphs and spreadsheets and save the results to a database. You can even get an Ethernet enabled PIC and set it up as its own mini web server and access it with an HTML web page anywhere in the world, or from an iPad or Android App.
Probably more than you need to know, or are willing to do. But to me this would be a cool weekend project and I already have all the parts I need. If it would be a valuable commercial or hobbyist product, I think it could be built for under $100, especially using surplus parts. Of course the devil is in the details, but if you would like to build something like this, I'll gladly help. If you just want a poor man's version you could do it with a surplus 500A IGBT module and a 555 timer and a few odd parts, and a makeshift load. Let me know...