What happens if I disconnect the 12v battery?

unfortunately, there have been different experiences. for me i lost trip memory. radio and windows were fine.

there have been various combinations for unknown reasons. you can always put a 12v source on the front jump point.
some seem to have lost mpg's until the computers relearn something or other, i don't understand it.

 

On mine when i installed my power setup for my sub amp and had disconnected for a few hoursi lost my radio presets, clock, and mpg, and upon starting it it didnt start and just showed all the alarm indicators which cleared and started up fine upon a second press of the power button

 

I haven't done it on my PP yet, but from reading others' reporting, the behaviors are different on the PP models compared to older Prius. Even for a PP, 7" HU used on the base trim (either Plus in pre-2019 or LE for post-2020) and 11.6" HU used on the higher trims may behave differently.

OP has a 2017 PP Plus. There is no front jump post in Gen4 and PP.

 

Why are you disconnecting? If temporary, say for a battery replacement, there are memory-saver strategies, gizmos that provide voltage to the car while the battery is disconnected.

 

in another thread, i read that there is a drain on the battery, so the charger has to be powerful enough to overcome it or something, i forget the exact details.

 

I’m constantly doing that.

 

That's correct; you can only get the battery to about 90-95%. IMHO; that's good enough. If you install an automatic maintenance charger; make sure it outputs at least 2 amps. As stated by Bisco; you must overcome the constant drains of the cars appliances - that are running in the background (ie. Entune, key scan, self diagnostic, clock and timers).

I hook-up a charger on mine for a few hours every quarter; while I'm doing yard work. The battery is usually around 11.8 - 12.3 VDC, when I hook it up. When I unhook, it reads 12.8 - but I'm sure it's actually 12.6 or lower.

 

I have the same question as MikeDee... I want to charge the battery with an automatic maintenance charger (I have an adjustable Schumacher charger that will charge up to 15 amps).

@Mendel Leisk and @BiomedO: Do you charge the battery in the vehicle, with the ground wire still connected? Or do you disconnect it (as the manual says to do)? I've charged other vehicles, like our Honda Odyssey with the ground wire connected with no problems, but of course, it's not a hybrid/plug-in hybrid.

Thanks in advance!

 

yes, you can charge it with ground connected. a few have reported trouble charging because the car is pulling current at the same time. but with a proper charger, it should work fine.

 

Thanks @bisco - I checked some other posts here about 12V battery issues and saw that many others were using external battery chargers / maintainers, and NO ONE wrote "make sure you disconnect the negative/ground wire!!!"

I'm guessing that the manual says to disconnect the ground wire when using an external charger to cover their a$$es in case someone uses a faulty or really old-school manual charger.

I also just ordered a BM2 Bluetooth battery monitor today to keep a constant eye on the battery voltage. I've had other cars with batteries that went bad really quickly, only to leave me stranded (or at least having to call AAA to bail me out).

Thanks again; I'm hooking up that external charger now!

 

That, pretty much constantly.

 

Some chargers try to be very smart, and detect how the battery charge is progressing by noticing small changes in the current-to-voltage relationship as a battery gets toward full charge.

Those chargers can have their fancy judgment thrown off by being connected to the battery in the car, so the charger is not just charging the battery, it's also powering all the ECU memories and smart-key transponders and all the other powered-when-OFF stuff in the car. It'll probably still charge the battery, but its smart behavior could be somewhat off. An old-school dumb charger will just go ahead and do what it was going to do anyway.

If you're using a fancy charger and if you really want what it tells you about the battery condition to be accurate, for that you might want the battery isolated.

 

@ChapmanF Thanks for the additional info. I just installed a BM2 bluetooth battery monitor, so even though the PP is drawing power for for all the electronics (while the car is "off" / not in READY mode), I think the monitor will give a pretty good data about the battery's condition.

I'm coming up on 2 years of owning the car (and a lot of miles!) so let's see how the OEM battery holds up.

 

how many amps is the ctek?

 

that is a long time. will be worth trying it with the neg disconnected next time the car sits that long

 

If it took 24 hours probably because the battery is deteriorating. I use the same charger; it’s usually complete in 4~5 hours. Have the battery assessed with an electronic load tester, typically for free at some battery retailers, or DIY with something like Solar BA9.