Why the fixation on measuring the battery's impedance? If you are, like you said, trying to evaluate whether the battery is high drain enough for your intended purpose or not, measuring ESR is not the way to go. The right way to do it is to actually load it up with the current you expect in your target application and see how/if it delivers and for how long. One way to do that would be by using a device called Electronic DC Load. As you seem to be a beginner lacking much experience with this kind of stuff, I don't recommend you start with high capacity Li-Po packs. These things, when mishandled, can easily set you and your house on fire. Please be careful and choose a project more appropriate for your experience level. Get the basics right first.