Make sure that both sides of the clamp are making good contact with the battery terminal. Many AVR battery testers use a heavy cable on one jaw, and a small voltage sensing wire on the other jaw. If one of them does not make good contact, strange readings or no readings may result.
Observe the clamps when applying a load. Smoke and arcing at the clamps and battery terminal indicate a poor connection. Clean or re position the clamps and try loading again.
Carbon piles are not indestructible. Repeated heavy loads (200 to 600 amps) without adequate cool down will melt the insulation off of cables, generate excessive heat inside the AVR and cause the carbons to become more brittle and crack. Allow the unit at least 5 minutes to cool down between loads.
If you are checking charge current on a vehicle with the engine running, ensure the amp clamp is positioned safely away from rotating components (fan, belts, etc). The amp clamp is one of the more expensive parts of the AVR, and in some cases, in older units, a complete replacement is no longer available. A good AVR starts at about $1500.00, so treating yours right is a good idea.
Most of the new hand held (and some larger units) battery testers are an “electronic” tester. As the battery ages, its internal resistance increases meaning less current is available to start or run the vehicle. The new testers are very accurate BUT they will diagnose a bad connection between one of the clamps and the battery post just as easily as they will diagnose a bad battery. Keep this in mind when using them, they won’t give any warning like a carbon pile (smoke and arcing at battery post). Always make sure both sides of the clamps are making GOOD contact with the battery post.