12v battery tech

[Tim-in-CA]

Are the jump points accessible from the outside of the vehicle? On Tesla its a PITA to jump the vehicle (at least in my 18 MS) if the 12V battery dies as you need to gain access to the trunk. Luckily service just replaced mine (unprompted) last time I brought it in for a warranty issue.

Sponsored

 

[godfodder0901]

Are the jump points accessible from the outside of the vehicle? On Tesla its a PITA to jump the vehicle (at least in my 18 MS) if the 12V battery dies as you need to gain access to the trunk. Luckily service just replaced mine (unprompted) last time I brought it in for a warranty issue.

Easy peezy. No hood access required.

 

[Tim-in-CA]

Easy peezy. No hood access required.

YEA!!!! Thanks for the quick response!! One of the benefits of already having your truck!

 

[Zoidz]

Disappointing that they went with primary (12V) batteries at all. No need for those with modern 90%+ efficient buck converters that can draw off the huge traction battery till the cows come home and would never need replacing.

I suspect it has to do more with safety/redundancy. If the fusible link blows on the traction battery at 70 mph at night, it would be nice to still have headlights, steering, etc,. to control the car safely and come to a stop.

 

[NY_Rob]

I suspect it has to do more with safety/redundancy. If the fusible link blows on the traction battery at 70 mph at night, it would be nice to still have headlights, steering, etc,. to control the car safely and come to a stop.

Agreed, but even that can be accounted for with a secondary circuit and small backup buck converter.

AFAIK, the Rivian isn't drive by wire and the brakes are still hydraulic so each will still work without electric power (albeit with more force required).

 

[Zoidz]

Agreed, but even that can be accounted for with a secondary circuit and small backup buck converter.

AFAIK, the Rivian isn't drive by wire and the brakes are still hydraulic so each will still work without electric power (albeit with more force required).

Agree that they could, but that still doesn't provide optimum safety for occupants and first responders in the event of a crash. Hyundai engineers give their opinion here, I'll defer to their expertise.

"We asked Hyundai's EV engineers why the 12-volt battery persists, and Ryan Miller, manager of electrified powertrain development, responded. "All the ECUs in the vehicle are powered from the low voltage, as well as the power relays that separate power from the high-voltage battery pack and the rest of the high-voltage network in the car," he said. "That separation allows us to safely disconnect the high voltage from the low voltage when the vehicle is not being driven or in the event of a crash."

 

[zefram47]

Disappointing that they went with primary (12V) batteries at all. No need for those with modern 90%+ efficient buck converters that can draw off the huge traction battery till the cows come home and would never need replacing.

The idea is that when you power down the vehicle the contactors disconnect the HV battery so there shouldn't be any high voltage energized anywhere. So you need the low voltage, 12V, system to re-energize the system. It's kind of a safety thing. But I wondered the same when I got my first EV. Funny enough, I heard recently that the Ford Mach-E apparently stops charging the 12V battery below 15% SoC, which seemed odd.

 

[astonius]

Just got a call back from Rivian about this question. They are two 6v sealed AGM batteries wired in series.

 

[crashmtb]

Just got a call back from Rivian about this question. They are two 6v sealed AGM batteries wired in series.

Sounds like they couldn’t source a 12v battery the right size/shape for where it needed to go

 

[astonius]

Sounds like they couldn’t source a 12v bsttery the right size/shape for where it needed to go

My guess too. Packaging!

 

[crashmtb]

My guess too. Packaging!

Hopefully my only interaction with those batteries is when mobile service has to come replace them in 5 or 6 years after initial delivery ?

 

[IPTV65]

Disappointing that they went with primary (12V) batteries at all. No need for those with modern 90%+ efficient buck converters that can draw off the huge traction battery till the cows come home and would never need replacing.

That surprises me as well...you could even put a buck on the "back side" of the 12V battery if they were really hell bent on putting a 12v battery in. Maybe for maybe redundancy? Would suck if you ran the main battery to 0%(something close to it) and the doors would not open. Efficiency on a buck is quite high, as you note, and cost for a single unit is ~$200. If I can buy one for $200 I am sure they can do it cheaper.
An AGM battery can cost about $200 and sounds like Rivian may have 2? I am assuming they are AGM but who knows. Even replacing one with a buck would seems to be a good decision from weight, economic, and real estate perspective. I am sure they had their reasons...just thinking out loud. Would be cool to try but not sure I am too keen on tapping the 400V system myself.

 

[Zoidz]

The Owners Manual vs. Customer Service - different information - so which is correct?

 

[crashmtb]

The Owners Manual vs. Customer Service - different information - so which is correct?

Technical writer gets: ”the 12v system is two 6V batteries in series”

technical writer outputs: “there are two 12v batteries“


?‍

 

[Ladiver]

I have an open e-mail with Rivian CS regarding the 12v system. I will add that question and let you know if I hear anything.

Unfortunately, from CS, all I got was a fluff non-answer.

Sponsored