I've had some issues with the charging system on my Tre-K not doing its job. I'd be on a ride, everything seemingly OK, arrive at my destination without any clue that anything is wrong. No "BATT" warning on the display. But when I go to re-start the bike there's no juice in the battery.
Last time it happened I was at a mate's place. When I went to leave, there was not even enough power to actuate the starter relay. We put a voltmeter on the battery and it read 8V. Checked all the fuses - all good. Put it on the charger for a few hours and the voltage came up to over 12V, the bike started normally and was making about 14V when I revved the engine, so I rode it home. Checked it when I got home and it was still behaving normally. Starting and charging OK.
Anyway, that's the second time it's done that to me, so my confidence in the charging system is somewhat diminished. I need to be able to monitor it when riding and do something about it if it goes belly-up. To that end, I bought a cheap little waterproof voltmeter on eBay and stuck it to the dash mounting bracket with double-sided tape. It's wired so it powers up when I switch on the ignition. The display is bright enough to read in direct sunlight. So far The charging system has been behaving itself, so still no clue as to what the original problem was.