Thanks for the replies. I'll get an electronic one then.
I was going to buy a 100A one and after looking around I noticed that all the banded 100A testers (Sealey / Draper etc) only went to 500 CCA on the scale, but the China ones (identical design) went to 1000 CCA. Higher is better obviously as it means being able test a greater range of batteries. The scale caught my eye though. See attached.
The green section of the scale on the right with the CCA markings on it is stepped. It marks the point at which a battery is Good or Weak while under load on the tester. I got the Sealey BT91/7 scale from their instruction manual. The China copy scale came from Ali and they're all the same. Both units pull the same current ~100A.
The CCA scale on the China copy sits in
exactly the same place on the voltage scale as the Sealer unit, even though the CCA ratings are different, so they both can't be right as they both pull the same current. On the Sealey unit, 11.2v is the limit for a 500 CCA battery, but on the China unit 500 CCA equates to around 10.4V & 11.2v is the limit for a 1000 CCA battery.
Given both units pull the same current it looks as though they've just changed the CCA numbers on the scale but kept everything else the same including the stepped part between Good and Weak . The giveaway comes from the instructions on the back of the copy unit for temperature adjustment. The instructions talk of 50 CCA steps for differing temperature bands - which is correct for the Sealey \ Draper originals, but wrong for the copies as they're stepped at 200 CCA.
If I'm correct then anyone buying these copies has a scale that's wrong and weak batteries could show up as good.