Here is an example of what you are trying to achieve....https://www.picoauto.com/tutorials/battery-discharge-problem.html
This was performed with the PP264 60 A Current Clamp that came bundled with my Automotive kit.
I used the same "fly lead" method that Rob mentioned.
When I doubt or question my test results, I try to challenge the equipment. In the case of battery discharge testing, I find it useful to turn on a "consumer" such as the headlights, if only to confirm that my measuring device is working correctly.
"Test your test methods" is something that I have learned the hard way.