Car Battery Tester??

[Elantramax1]

 Hi all. This may sound like a Stupid Question.

 How does the Midtronics (And other like it) actually Test the Battery? I have a Solar brand one. Tells me the V and Amp Capacity of the Battery. I saw a You-Tube where the guy said it Tests the Conductance. Now I am watching someone use a Harbor Freight Tester. And it shows V and M Ohms. I was always told you cant check the Resistance on a Charged Circuit. Unless that is not entirely True.

 Thanks