Dead aux battery causes EV, ASC and Brake Service Required

I have had my Mitsubishi Outlander PHEV owner since late 2014 with no real issues.

A few weeks ago a couple of 'EV' and 'ASC' service due errors appeared on my dashboard - they didn't stop the car running and they went away after the car was restarted. Coincidentally I had a full service booked a week later and asked the garage to check it out but they could find no error codes registered.

Then two weeks later (last Friday) I went to start the car and had three full 'service required stop safely ' messages on the dashboard, for 'EV', 'ASC' and 'Brakes' and the car wouldn't start. No amount of switching on and off the car would clear these. I called out the AA who diagnosed a problem with the auxiliary lead acid battery as it was on its last legs (down to 9v) and they replaced that. No warning had appeared on the dashboard about the auxiliary battery at any point but interestingly the AA Car Genie I had in the diagnostics port had told the AA about it - so they AA turned up knowing the issue and having picked up the correct battery on the way. Once the battery had been changed then all the other messages cleared and the car started fine and has worked happily for days since.

Sounds a bit of a design fault to me. Something that may only now be appearing as the earlier cars now begin to need new batteries. Firstly the dashboard console should tell you when the auxiliary battery is dying and secondly there were a totally inappropriate set of scary warnings that could result in un-necessary worry and behaviour!

Exactly the same happened to me when i let my kids watch DVDs in the back and it almost completely flattened the 12v battery.
The main battery was 100%.

Mirrors started closing/opening very slow, all kinds of errors on the dash, the trunklid not automatically opening/closing.
Eventually had to 'jumpstart' it with help of a dirty diesel delivery van.

Took like 5 minutes and i was able to start the car properly and i immediatly pressed the charge button to have the ICE working.

Compared to a ICE-car it wasn't that different.
It would be cool if the 12v could be charged ondemand from the main battery in case of 12v issues.

AFAIK the 12V battery is charged from the drive battery via a DC-DC converter, so running the ICE does nothing to charge it unless your drive battery is exceptionally low.

The problem is that a 12V battery in a normal car has to turn over the engine when cold, so it needs a very high startup current. Once the ICE's running, the battery is kept at a high voltage almost continuously. If the 12V battery is acting up, you start noticing it turning over more slowly on a cold morning, or even failing to start. On the PHEV, the 12V battery is managed by the car's electronics. As it's only needed to turn the car on, it doesn't have to supply a large initial current, so it can get away with being in a much poorer state without it causing a noticeable problem. I guess that having some sort of reading that tells you when it's on its way out would be good, though...