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The manual is terrible so I’m confused about how many batteries the plug-in has. Based on guesswork, I believe there are 3.

1. The hybrid battery
2. The plug-in battery
3. The 12V battery in the trunk

Is that correct? As a bonus question, what the heck is Aux Battery Saver? The manual seems to indicate it help keep some battery charged (the 12V)? Why would I want to turn this setting off?

T
 

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One traction battery (360V), and one 12V battery. Yup, that keeps the 12V battery charged. Perhaps you might want to leave it off if you disconnect the 12V because you are not driving for a few months. Otherwise, can't think of a good reason.
 

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The manual is terrible so I’m confused about how many batteries the plug-in has. Based on guesswork, I believe there are 3.

1. The hybrid battery
2. The plug-in battery
3. The 12V battery in the trunk

Is that correct? As a bonus question, what the heck is Aux Battery Saver? The manual seems to indicate it help keep some battery charged (the 12V)? Why would I want to turn this setting off?

T
The car has two battery packs, as @yticolev said, the Traction Battery (360V) one and the Aux Battery (12V) one.

The Aux battery saver is a function that will charge the aux battery (12V) with the Traction battery. The only scenario to turn it off is if you have to park your car for a while (let's say months) and can NOT keep it plugged in. In this case if you don't turn the function off it might drained your traction battery as well.
 

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The PHEV limits the number of times it "saves" the battery between drives. So that is not an issue by itself. But if you park the car for several months, it is still a better practice to turn the option off and disconnect the 12 V battery. That will maximize the 12 V battery lifespan. The only thing better would be to attach a good trickle charger - but I'd still disconnect the battery from the car for safety of car electrical systems.
 

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How can you be sure there is only one batterie for traction ?
I noticed there is one under the back seats and one in the back trunk. They are directly connected to make a big one ?
 

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2020 Ioniq PHEV Limited
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I believe that what you are seeing under the cargo floor is just the battery cooling system.
 

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Are you sure there is no battery in the back trunck under the cargo floor ?
Because I can see a fan , you're right but also a big box with a big orange plug that I guess is for a high power voltage.
My version is for Europe but I guess it is the same for all.
I can send some photos
 

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2020 Ioniq PHEV Limited
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Hey, you're right! From this article when the Ioniq was first introduced, it says, "The larger battery pack extends rearward from the rear set, taking some of the storage space from the rear cargo area." The article contains the following image. I wish there were a larger version, but this is full resolution. BTW, that link is a great article, chock full of info!

32890


Here's the battery pack from the hybrid version that's under the rear seat:

32891


So, from the diagram above, the PHEV battery pack appears to be the standard hybrid battery, plus more cells connected to it in a separate unit under the cargo area. Thanks for pointing that out.
 

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IONIQ Electric, SE (w/ CCP) 2018.
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The layout of the PHEV battery is shown slightly more clearly on page 9 in this document PHEV Document. Surprisingly it does look like the PHEV is built on the HEV (battery under seat) with a second battery installed in the cargo area. Intuitively, this doesn't make sense since, in theory, the two battery systems are of different chemistry's and nominal voltages. Or perhaps they replaced the HEV battery with a lithium battery. I'm not too clued up on battery tech. Perhaps somebody else can fill in details.
 

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Why do you think the type of battery is different from hybrid only and Plugin ?
Both are Lithium-Ion polymere battery.
IMG_2756-668x376.png ioniq-1024x576.png
But different voltage : One is 240 V and the other is 360 V. And different power of course : 1,56 vs 8.9.
I don't believe that the extra battery (in the cargo area) is only a 120 V battery in serial with the battery under the seats (240V) because the global power would not mach with 8.9 kwh.
In the document you mentioned, there are 4 cells in 24 modules in PHEV.
I think that those modules are dispatched in the two locations.
And I don't want tu unplug the battery to see the voltage with a Volt meter !
 

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The layout of the PHEV battery is shown slightly more clearly on page 9 in this document PHEV Document.
Great find, thank you! Pages 14 and 24 make it pretty clear as well.

It appears there may be two battery fans, the one at the back we see when we remove the cargo floor and one at the right side of the under-seat portion of the battery. There doesn't appear to be ductwork for the back fan to pull air through both chunks.

I'm kind of surprised that the forward chunk doesn't have its own safety plug to split its cells, as I would assume that it's a separately removable battery case from the rear chunk. I wonder if the front chunk is wired as two legs, one inbound and one outbound from a U in the rear? Now I want to take mine apart...
 
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