Solution to 12V issues? Disconnect 12v negative terminal sensor plug

Did you make a typo for the year? Did you mean 2026?

 

Yes I did. Thanks for picking it up. I was able to change it.

 

Yes, that can be done (idling in READY for an hour). You would have to do that every week. A battery maintainer is the way to go though, but you need a garage.

 

vehicle battery isolator quick disconnect, amazon et al:

 

Ok; I just did a quick test. This is by no means a real test, don't have a BM2 to do a long term test.

1. my battery was 11.8VDC, so I placed a 6A battery charger on it for two hours and disconnected the sensor
2. battery check was 12.8VDC now, so placed the car into Ready mode; meter shot up to 14.2VDC - No CEL, 5 minute test.
3 . Shut-down and reconnected the sensor, battery @ 12.4VDC - Got the car back into Ready mode - 14.1VDC for another 5 minutes.

I'll need to get a BM2 to chart the charging voltage as the car is running, but disconnecting that sensor doesn't trip the CEL. Since there wasn't a significant change in the DC-DC converter over either 5 minute test interval - I can't really say if it's working or not; just that it doesn't trip the CEL on a gen4.

 

It could also be possible to check the voltage while running with OBD Fusion or Car Scanner over CarPlay (Gen5). Possibly charting as well.
edit: This video might not be definitive. Perhaps I want to log on a timed interval.

 

Never dropped below 14v for me after disconnecting. Stayed between 14-14.1v.

No CEL.

I'm thinking I'm gonna leave it disconnected as my OEM battery shat the bed after a year (11.6v resting voltage on a 20F day after no driving for 2 days; car still started fine and no errors but I didn't like that and swapped to an AGM). Maybe it was a bad battery, maybe it was the BMS shenanigans, don't know don't care. Going with the safe route: BMS disconnect + AGM.

 

Wondering if effect on mpg will be seen?

 

I estimate it to be about 0.5 mpg in the ballpark. The extra power consumption in watts is (14.1 V−12.89 V) times the current in amperes drawn by the 12-V system.

 

I wonder if the constant 14.1 volts to the battery could potentially be worse for a sealed AGM battery or the vented flooded type?

Electrically, I'm no whiz, but if the battery is getting charged with 14.1 volts, once it gets to 100% it's not accepting any more juice, ie, current, correct?

With a lead acid battery, you want to keep it at 100% as much as possible, no?

 

Its own voltage at 100% charge is less than that, so your 14.1 will still be forcing current backwards through it, just without any further increase in charge. You're just making heat and bubbles at that point.

 

That would depend on how long the car sits (battery discharge) and how long your in Ready mode. ie. if your a long-haul highway driver, you'll be overcharging the battery (blowing bubbles and generating heat); whereas if your a short-hop person and only drive the car once in 4 days, those shot hops probably isn't going to put back in the same amount of juice that was burned sitting idle and starting up the car. That's why the battery on a daily driver will normally last longer than one that just sits. If that daily driver is routinely used for short hops, you would simply slowly chip away at the battery - because you'll be running at a deficit on each trip. That deficit will add up over time and eventually kill the battery, if you don't place it on a charger either quarterly or semiannually.

 

Plus a conventional alternator behaves differently than our Prius' arrangement? There's a voltage regulator involved with them, right? Like I said, I'm no whiz.

 

Depends on how old the conventional alternator is. The "voltage regulator" at one time was a 'vibrating-reed' type, where the output voltage was applied to a magnet coil that would move a reed away from a contact that supplied too much alternator field current and toward one that supplied too little (with a resistor network applying some in-between amount when the reed was between the two contacts). It was a "voltage regulator" in a very coarse sense, and the resulting voltage would be bouncing around between not-quite-13ish and usually-not-15ish, also depending on what you were doing at the time, and anything in the car that really needed a predictable voltage to work right, like the fuel gauge, would have its own gauge regulator to provide that. Which might, itself, just be a heat coil around a bimetal that opened and closed the circuit so the average voltage was right, and the gauge would also be a bimetal and respond slowly enough that the pulsation didn't matter. Ah, old car stuff.

Our Prius' arrangement has a voltage regulator much more like the kind in computer power supplies: rock solid, and when the ECU wants it to be 13.6 it's 13.6, and when the ECU wants it to be 14.7 it's 14.7, and the ECU makes those decisions according to deliberate rules, like whether the car is moving or not, and how charged the battery is.

But it sounds like that isn't much different from how 'conventional' alternators and their regulators are working, the last 15 years or so.