Something I have come across with my Midtronics 35p electronic tester, as I have to enter the battery amp/hr before the test takes place I now always look in the battery application book and enter that figure of ah, because I have found many older cars have had a battery fitted with less output than specified. eg, it is no good testing a 200 ah battery as a 200 ah and getting a good result at 190 when the vehicle should have say a 400 ah fitted so I test at 400 and the result of 190 would show as a fail.