I did not say anything about heat. I was just pointing out that the Gen 4 12v battery is located in the engine bay. You wrote about the 12v battery being in the rear of the car and about high temperature back there.
I've found the gen4 is pretty stingy about sending power to the 12VDC battery and prioritizes the current to the traction battery. These cars don't behave like a regular ICE, where the alternator is ALWAYS pumping 14VDC into your battery as long as the engine is running. Someone here in the past has charted the voltages going to the battery during several hours of operations. Sometimes it's a 14VDC and other times it's as low as 12.25VDC. Bottom line; you can't count on intermittent operations of your gen4 to maintain your battery at a decent level. If this was a daily driver, this wouldn't be an issue. Go to harbor freight and get a <$10 digital volt meter. Check the battery voltage on occasion, and charge it when necessary. It only needs a couple of hours @6A to get back up to 12.8VDC; a bit longer if your charging at a lower amperage. The problem with the cigarette lighter voltage meter, the car needs to be turned-on for it to operate - that's going to mess with your results. The lighter is on a relay and turns-off with the car, so you'll get invalid results. Hope this helps and slows down the cars' battery consumption.
I agree with you for a Gen 3 Prius. I do not know enough about the Gen 4 Prius to know if that is true or not.
Paul, Most of the discussion here is about the 12v battery's ability to store energy as it ages. You voltmeter in the auxilliary outlet is measuring the charging system voltage as the auxilliary outlet is not active when the charing circuit is not running. JeffD
I have only experience with a gen 3 and a gen 5, so my two cents might be worth zero cents for a gen 4.
Okay, battery installed, all is well. The only thing I could not figure out is the black plug that came with the battery. It was in a little plastic bag and stuck under the black terminal plug. Is this important? I could not find anything in the manual. See attached.
Is it a battery with a port that allows connecting a hose for venting (say, if it were installed in the cabin, like other Prius generations do)?
Ah, that must be it. There is talk of a vent plug, but this is sitting under the hood so I think I'm fine to leave it as is. Thanks!
The only way to know more about your battery is with a monitor. Here is five day graph of voltage activity on my two cars with Ancel monitor. The purple is an alternator vehicle, green my 2020 Prime. There is a period of external charging the 12 v. Prime battery, showing what the charging curve is for that device.
I discovered a loose connection on my 100Ah lithium auxiliary battery, branded, "Li-Time." Because it was running the cooler all night, it had gone completely flat. When I connected my charger to it, it would not take a charge, the charger displaying an "err" message. This was disappointing, because the battery is only about 1 year old, and it was supposed to have battery protection to prevent such things. Later in the day, the battery began to take a charge. I suppose this is due to a bit of regeneration that can occur when the battery sits idle for a time.
