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12v battery tech

Cosmacelf

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The "why do we need a 12V battery at all" question has been raging on the Tesla forums since 2012. And got very heated when Tesla had several year's worth of bad batteries, necessitating emergency swap outs. The answer is always safety. When you get into a crash, the car will blow the high voltage pyro fuse, so even a buck converter would be dead. And you want to have the 12V system still operating post crash. So...

The big innovation Tesla finally, like in the last 6 months, did was to start using a lithium ion battery for the 12V system rather than a lead acid AGM. It should be longer lasting and more reliable. But, it took Tesla many many engineering manpower days to figure it out, and it certainly isn't a cheaper solution. Like all things automotive, it has to be extremely reliable across a very wide temperature range - the main battery pack has a sophisticated Battery Management System along with a heating and cooling system for the battery. The equivalent must be engineered for a lithium ion 12V battery (lead acid batteries are a lot tougher and don't need to be coddled as much).
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electriclove

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I can confirm 100% the 12V battery configuration is 2 12V batteries in parallel. Lead acid. Ohmmu (www.Ohmmu.com) will be working on offering a Lithium replacement!
 
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astonius

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I can confirm 100% the 12V battery configuration is 2 12V batteries in parallel. Lead acid. Ohmmu (www.Ohmmu.com) will be working on offering a Lithium replacement!
Rivian and others have stated they are AGM, not lead acid.

Lithium would have been nice though. Only modern car I haven’t had idle drain issues with is the lithium-equipped X5.
 

electriclove

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Rivian and others have stated they are AGM, not lead acid.

Lithium would have been nice though. Only modern car I haven’t had idle drain issues with is the lithium-equipped X5.
AGM means “Absorbed Glas Mat” which describes the electrolyte interface between the lead plates of the cell. AGM is a lead acid cell. Flooded lead acid is the type of cell with actual liquid electrolyte in it. When I say lead acid I am not saying FLA/AGM, I’m being more vague and just sating the chemistry.

Tesla took 10 years to deploy a 12V Lithium battery, in due time Rivian will do that as well, for now will have to do your own upgrade aftermarket.
 

Cosmacelf

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Tesla took 10 years to deploy a 12V Lithium battery, in due time Rivian will do that as well, for now will have to do your own upgrade aftermarket.
As long as the batteries work, I'm fine with lead acid. Tesla had a horrible time with theirs early on since they ended up with a crap supplier (it was an American supplier, but they subcontracted supply out to some cheapest bidder). And then really early on their charging software wasn't up to snuff so the batteries would get drained. Porsche is actually having a similar battery charging problem right now in their Taycan - under certain conditions, the 12V battery can get drained. And Porsche is using lithium ion, so that problem can occur no matter what chemistry you use.

Unlike Porsche, I'm glad Rivian does real OTA updates that can fix problems like the above, should they have any.
 

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zigzagzap

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So it's my understanding that these TWO - 12V batteries act as a charge reserve of last resorts when you drain your regular battery pack. I believe they provide about 40 miles depending on load/wheels/weather AND require replacement when drained. Cost $800. Yikes.
 

electruck

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So it's my understanding that these TWO - 12V batteries act as a charge reserve of last resorts when you drain your regular battery pack. I believe they provide about 40 miles depending on load/wheels/weather AND require replacement when drained. Cost $800. Yikes.
No, the 12V batteries are for the 12V sub-systems (same as with an ICE vehicle). They are not capable of powering the 400V motors. The high voltage pack is used to keep the 12V batteries properly charged.
 

zigzagzap

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No, the 12V batteries are for the 12V sub-systems (same as with an ICE vehicle). They are not capable of powering the 400V motors. The high voltage pack is used to keep the 12V batteries properly charged.
That information was provided to me by a service center tech about a vehicle in for repair with this diagnosis. The R1T was used for moving and filled to capacity causing the milage estimates to go deeply off a cliff and brick. I would be nice to determine if he was right or wrong.
 

electruck

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That information was provided to me by a service center tech about a vehicle in for repair with this diagnosis. The R1T was used for moving and filled to capacity causing the milage estimates to go deeply off a cliff and brick. I would be nice to determine if he was right or wrong.
Well, the aspect of the 12V providing a charge reserve is correct in the sense that it ensures things like the lights, door locks, and - most importantly - the contactor for the high voltage battery remain operable without power from the main pack. The 12V batteries do not, however, power the motors or serve as any kind of range extender.
 

Autolycus

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That information was provided to me by a service center tech about a vehicle in for repair with this diagnosis. The R1T was used for moving and filled to capacity causing the milage estimates to go deeply off a cliff and brick. I would be nice to determine if he was right or wrong.
There are 2 12V batteries in the R1T. They cannot power 400V systems, but even if they could, they would give only a tiny range boost draining them from full to empty. As best I can tell, they're 18Ah or 216Wh each. That's 432 Wh combined total. That would get you about 1 mile.
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