Original factory 12V battery dead with zero warning

Yes. Most of them have a simple quick disconnect to make using it easy especially if you don't drive every day.

 

Consider hooking up a cable with SAE connector to 12v battery and then route it to behind the tail light cover for easy access to charge. While you're at it. Attach yourself an AirTag or Smart Tag 2 (Samsung) so you can track your car if lost or stolen Sometimes your car might get towed way when you think it was stolen.

 

I use CTEK battery conditioners on ‘the fleet’, with LED battery condition indicators integral to quick connects.

 

We’re currently putting about 3k kms yearly on our 2010, typical usage a 20~50 km drive, once a week on average. Keeps the engine healthy, but not good for 12 volt battery longevity. I’ve wired in a CTEK quick-connect harness (sim to the pic @Air_Boss posted, but no LED indicators), at the under-the-good fuse box. Have a CTEK charger connected pretty much ALL the time: if we drive one day, I pop the hood when we get get home, connect charger the next morning, leave it thus till we next use the car. Current battery was installed over a decade back.

there a low but inexorable load on the battery, around 20 milliamperes, not “bumped the dome light” load, but still enough to run the battery down with sporadic car usage.

 

'I see these gadgets as helping me to avoid asking my neighbor to jump me, but otherwise I'm not so clear on the reason for this.'

What are convenience and reliability worth, especially on infrequently-used equipment?

We use them on 'the fleet', the standby inverter generator, and other 12V devices.

 

Yes because of two reasons that make sense and were already mentioned.

One is the battery has no high current starter load. In conventional cars this is essentially a serious load test every time you crank it. Capacity is checked by electronics (sometimes) but certainly the driver can tell when it is cranking slow meaning low charge. Prius has no high load.

A second is there are a couple of battery discharging mechanisms that get you. The obvious map light. The obvious headlight turned on manually after shutdown. The less obvious hatch not closed 100% but still latched. The far less obvious and less problematic key fob in range of the car (of which is technically only a problem over many days). Aftermarket radios, obd2 readers, gps, etc sometimes fail to go into sleep mode.

The real problem is an unloaded 12v battery that is many years old can suddenly fail 20 minutes after being driven to the store. You have a fairly new battery but each discharge takes an extra chunk of life off the expensive agm 12v (which incidentally takes discharges pretty well).

My recommendation is carry a 12v lithium jump starter with polarity protection. You won't get stranded and the real killer, reverse polarity jumps won't happen. That can and does cause weeks of downtime and frustration that will make a dash light discharge seem like the good ol days.

Lead a horse to water