Dead 12V Battery 🙃 What now?
This happened once before Christmas, too. Car is dead, 12V battery reads <3V. Internet tells me anything under 10.5V is a damaged battery - but in December we had the breakdown service out, he charged it up and said it should be OK to go and it obviously held charge for a few more months so I don't know what to believe on that front.
In the short term, I've tried "jumping" it with a small portable jump kit. This allows the car to turn on, but the jump kit will only apply current for something like 20s at a time. Clearly during this time although the car's electronics come on, it does not start charging the 12V battery from the traction battery. Eventually the jump kit discharges, at which point (just now) the battery was reading about 3.25V, but presumably will settle somewhat lower. This seems an unfeasible way of proceeding. The kit charges via USB (at 1.5A only!) so this is effectively charging the battery at 7.5W, requiring (presumably) many cycles before the battery is charged enough to keep the car on long enough to make up its mind and use its vast 800V battery to charge the small one.
We can get the breakdown service to come again and charge the battery again, but I'd like some more ideas. Would it in fact be better to just go to a parts store and pick up a replacement battery? Do I need to buy a proper battery charger anyway, regardless of what I do with this battery, in which case, should I just buy one and try it on the battery in there?
Does anyone know what logic governs the car charging the 12V battery from the main battery - in an ICE car as soon as the engine is turning under its own power, you're good, but this is clearly more complicated.
EDIT: ordered a new AGM battery. I'm still interested in knowing whether it's advised to get a battery charger or whether they make it unnecessary.