What's the difference between a conductance tester and a carbon-pile load tester?

Conductance testers (most of the digital units here) send a small AC signal through the battery and estimate CCA from internal resistance, while carbon-pile testers apply an actual heavy resistive load. Specialist mechanic subreddits note carbon-pile gives a truer real-world load reading, but conductance testers are faster, safer, and don't require a fully charged battery to give a usable health estimate.