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2021 Refresh: Model S (and X?) Changes to 12v System

Phlier

Bluebird
Jun 12, 2019
2,143
3,968
Utah
Referencing the video in the OP...

The author of that video has suggested putting two (or even possibly three) diodes (rated for the appropriate amperage) in series to the "12v" accessory outlet to drop the voltage a bit.

This would be a much better idea than using some sort of buck convertor, as most of the available buck convertors are of very poor quality.

It wouldn't be difficult at all to buy a 12v plug extension cord and wire a couple of diodes into it.

That's the route I'm going to go when I finally get my Plaid.
 

Snowstorm

Active Member
Dec 8, 2016
1,568
1,505
Ontario Canada
Referencing the video in the OP...

The author of that video has suggested putting two (or even possibly three) diodes (rated for the appropriate amperage) in series to the "12v" accessory outlet to drop the voltage a bit.

This would be a much better idea than using some sort of buck convertor, as most of the available buck convertors are of very poor quality.

It wouldn't be difficult at all to buy a 12v plug extension cord and wire a couple of diodes into it.

That's the route I'm going to go when I finally get my Plaid.
How much current can these diodes handle? Is it OK to pull 15A through them? Say each diode drops 0.7V, that would be 12W per diode. That may work through the accessory port, but you won’t be wiring a 1000W inverter that way. Anything that can step down 1.4v or so at 100A is going to be very beefy and expensive.

Strange for Tesla to use a non standard 16V LV system. Might have well gone to 24V instead.
 
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Phlier

Bluebird
Jun 12, 2019
2,143
3,968
Utah
How much current can these diodes handle? Is it OK to pull 15A through them? Say each diode drops 0.7V, that would be 12W per diode. That may work through the accessory port, but you won’t be wiring a 1000W inverter that way. Anything that can step down 1.4v or so at 100A is going to be very beefy and expensive.

Strange for Tesla to use a non standard 16V LV system. Might have well gone to 24V instead.
Luckily, I have no plans to run an inverter through the accessory plug, so the diode solution will work.

But yes, for those that want to run an inverter, a better solution is needed.
 

4SUPER9

MSLR #RN11510 6/7/21
Jun 6, 2013
3,084
2,919
California
Referencing the video in the OP...

The author of that video has suggested putting two (or even possibly three) diodes (rated for the appropriate amperage) in series to the "12v" accessory outlet to drop the voltage a bit.

This would be a much better idea than using some sort of buck convertor, as most of the available buck convertors are of very poor quality.

It wouldn't be difficult at all to buy a 12v plug extension cord and wire a couple of diodes into it.

That's the route I'm going to go when I finally get my Plaid.
Thank you. I actually never watched that video and found it very informative. I am psyched about using diodes. I must have 50 of them lying around and will give it a whirl. My radar detector was expensive (9500ci) and I would like to protect it.
 

n2mb_racing

Active Member
Jun 14, 2014
1,309
1,136
durham, NC
Any good automotive electronics should be designed for 24V max, -12V min (reverse battery protection). They don't have to function with -12V, they just should not be destroyed by -12V. Because people hook up their batteries backwards... and everything has to survive. Also, sometimes people jump start cars from 24V trucks, and they need to survive that too.

Almost everything should work ok at 16V, if it was designed well...
 

Cal1

Member
Sep 22, 2013
603
465
Battle Ground WA
I just measured the voltage across my 2012 Tundra. 13.08v with the truck turned off. 14.23v with motor running. My battery trickle charger keeps it charged at a pretty high level. As just stated your alternator keeps you lead acid battery charged by putting out more than 12v. My old NSX used to charge at 15.1v.

I'd be surprised if this higher voltage affects anything as all ICE vehicle charging systems put out more than 12v. In fact if your ICE system is only putting out 12v you'll discharge the battery overtime because that means it's not being charged.
 
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Snowstorm

Active Member
Dec 8, 2016
1,568
1,505
Ontario Canada
if you are plugging in a cell phone charger or something then 15 or 16v may be Ok. But if you are plugging in an inverter, they are going to much more sensitive. I have yet to see a “12V” unit that won’t shut down past 15.5V
 

KevinY

Member
May 27, 2017
156
101
Hawaii
if you are plugging in a cell phone charger or something then 15 or 16v may be Ok. But if you are plugging in an inverter, they are going to much more sensitive. I have yet to see a “12V” unit that won’t shut down past 15.5V
No looking hard enough, I found two brands on Amazon rated to 16v. Can't speak to the quality
 

mba83

Member
Jan 10, 2019
128
59
South Carolina
While I haven't tested this, I doubt a typical "12V" inverter will shut down at 15.5V. But I'd love to be proven wrong if any of you guys can find one that does :)
 

Phlier

Bluebird
Jun 12, 2019
2,143
3,968
Utah
You guys are overthinking this. All cars produce 14V *constantly* when the engine is running. And voltage can be higher (as much as 16V on some cars) for a few minutes after first starting the engine.
Most cars electrical systems, including Teslas when the DC/DC convertor is powering the LVS, power it at 14.2 volts. But the high end of the new Refresh cars LV battery is 16.8 volts (when the DC/DC convertor is *not* powering the LVS, but the battery is it'll do so at whatever the LV battery's voltage currently is, which can be as high as 16.8v). That's enough to cause a problem with some devices. Tesla even calls it out in the Refresh car manual, stating that "Inverters must be able to accept an input voltage of 16.8 volts."
 
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jstjohnz

Member
Sep 7, 2020
105
72
Indianapolis
3 diodes in series will drop about 2 volts. Something like 1N5402. Probably get 3 for $1. This would be fine for small electronics but not for inverters due to high current requirement.
 

n2mb_racing

Active Member
Jun 14, 2014
1,309
1,136
durham, NC
Most cars electrical systems, including Teslas when the DC/DC convertor is powering the LVS, power it at 14.2 volts. But the high end of the new Refresh cars LV battery is 16.8 volts (when the DC/DC convertor is *not* powering the LVS, but the battery is it'll do so at whatever the LV battery's voltage currently is, which can be as high as 16.8v). That's enough to cause a problem with some devices. Tesla even calls it out in the Refresh car manual, stating that "Inverters must be able to accept an input voltage of 16.8 volts."

I hope they aren't charging the 12v to 16.8. that would wear it out faster. It's going to go through a lot of cycles, so they should charge to 80%....
 

Phlier

Bluebird
Jun 12, 2019
2,143
3,968
Utah
I hope they aren't charging the 12v to 16.8. that would wear it out faster. It's going to go through a lot of cycles, so they should charge to 80%....
If it's an LFP based battery, it can be charged to 100% with every duty cycle with normal degradation. But then the question becomes how well will it perform in cold weather? Guess we're all about to find out!
 

n2mb_racing

Active Member
Jun 14, 2014
1,309
1,136
durham, NC
If it's an LFP based battery, it can be charged to 100% with every duty cycle with normal degradation. But then the question becomes how well will it perform in cold weather? Guess we're all about to find out!
It seems likely to be Li-ion because it's being charged to 4.2V, LFP max is usually 3.65V. Also, I think it was labeled as 4s1p lithium ion. But, I think it was 9.8 Ah, which is a strange capacity. If it were 2p, then 21700 cells would make sense...

LFP would have made more sense, since the voltage range is more suitable to 12V, and has more charge cycles, etc. But, maybe they don't have good supplies for LFP batteries? Strange.

Anyway, I have to assume Tesla added a small heater to the 12V battery. That way the main pack would heat it up before starting charging, if the temp is below freezing. Even a bunch of 12V Lifepo4 batteries have built-in heaters now. The BMS just diverts the charging current to the heaters until the battery is up to temperature, then it allows charging automatically.
 
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n2mb_racing

Active Member
Jun 14, 2014
1,309
1,136
durham, NC
Ah, good catch.



It's only 6.9 Ah(!). Seems really small for the use case, but without knowing how much current the car draws when it's asleep, it's hard to know for sure.
Yeah, that's a strange size. If it were 2p, that might make sense that they are using 18650s that are 3450 mAh, probably similar to the model S cells.

I guess it is going to cycle a lot. Presumably they will keep it in the 80-20% range, so it gets a lot of life. But, still. They might only be designing it for just over 4 year lifespan?? Just get to the end of the warranty period?

I guess the DC/DC stays on if Sentry mode is enabled, because that's almost 300 watts, the 12V would be dead in ~20 minutes. Maybe the new car does a really deep sleep with the contactors off, and 100 whr lasts a long time?
 

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