I sometimes drive only 5 miles a week and the 12v battery goes dead. When it is dead is it possible to turn on the system and let the HV battery charge the 12v? Like leave it on for a few minutes?
welcome! unfortunately, no. if you don't drive much, you should get a 12v battery maintainer, and leave it connected when not using the car.
Dead ICE battery in front end of 2018 Prius Plus plug-in is a design problem that manifests itself if you are to successful spending most of your time driving on traction battery and seldom running the ICE (internal combustion engine) in Charge Mode. I went thru 4 ICE batteries and Dealer tech mystified because they all died from a bad cel about every 18 months. Design flaw is that charging traction battery (backend of car) does not charge ICE battery (front end of car). Conclusion appears to be that ICE battery only charged when car is in Charge vs. EV mode while driving. So, you need to manually monitor or have a monitoring device (why doesn't Prius have an ICE Battery indicator) to check the state of charge of ICE battery and/or put a battery charger or solar charging panel on it to maintain its charge or the ICE battery self-destructs. My son bought a plug-in British Mini (their beloved little square car) and I told my son this and he asked the Mini dealer who told him the Mini ICE battery is not charged when the traction battery is charged and can fail the same way. There's a research project for someone to find out how many of the Plug-in brands have the same problem.
This is easier to do if you don't wait until it's dead—because you need it not-dead to turn on the system. Assuming you catch it when it's sufficiently not-dead to still start the system, then yes, you can do this, but you have to make it a pretty regular chore. "A few minutes" may be adequate if it has only been sitting a few days. Maybe turn on the radio and catch up on your day's news, then shut it off. When it gets close enough to dead for you to feel lucky it started, the charge time from that state is measured in hours, not minutes. If there is a convenient plug for a battery maintainer near where you park, that's a much lower-hassle solution. Provided you don't drive away with the maintainer plugged in.
