its built-in 'illumination function'

).
Here's a translation of the German text:
'For small batteries, a simple test you can do is to use a 1.5mm steel wire. If the wire burns through, the battery is likely dead. If it glows brightly immediately, the battery is still usable. If it only slowly heats up, it's nearing the end of its life.'
with a 120Ah tractor battery, that's not going to work

. the wire will still melt even at 10.2 volts .
Oh, so... of course, don't do the test, because the stress can generate hydrogen, and the sparks that draw it to the battery terminals will inevitably create... well, something like a nuclear bomb... blah blah blah... at your own risk.
edit:
'@dieselmartin wow, is that really how well a TDI can start at -6°C? (referring to the BMR). Our BMN engine is really struggling and no one knows why... Sometimes it takes a lot of cranking to start, and other times it fires up on the first try but runs 'rough' for the first few seconds.'