At work, we test batteries by loading them with a heavy duty resistor. A pc browsing the net will average at around X current, while heavy load will make it consume Y current. You can simulate X and Y situations by using different resistor configurations. Not exactly a real time benchmark, but you do get exact results this way. Downside, you might kill the battery when running it completely empty, you need an automatic stop circuit, for example when the battery voltage drops below 11V (no idea how much is really need to make the PC shut down). Nothing too hard to build, costs nearly nothing too.

drop me a line when you're up to the challenge;)