Better 12V Car Battery Tester, under load??

Hi.
I want to be able to test a car battery (think its called load test).
I have a few batteries i keep swapping between the car and boat. (both have 2) and a couple spare. I purchased another battery today and they used this small box, connected the clips to both terminals and it showed the voltage, then they flicked a switch on the bottom for a few seconds and it tested the battery under load. I want something like this to test my batteries.
I also have a DMM if its possible to use that with a shunt or something? (you may need to explain how to)
I have a parts store nearby so should be able to get all the parts needed.

Thanks
Stu

I have one of these that I use to determine if a battery has sufficient Cranking Amps. This does not measure the battery capacity in AH, however.

I built a computer-controlled battery capacity tester that discharges a lead-acid battery at 30A to a preset cutoff voltage (~11.5V), and logs the discharge/recharge curve into an Excel file. Post if you are interested in the details.