Tesla is using a new 12V Lithium-ion battery in the Model S Plaid

[JRP3]

I wish someone would measure it so we could get an idea of the cell size. I think it almost has to be 4680 to get 6.9Ah at 15.5V, (3.875 per cell) in a 1P 4S configuration. I also assume it doesn't get cycled unless there is a failure of a DC/DC converter, which also means this battery will not work with older Tesla vehicles, along with the obvious capacity difference.

 

[CyberGus]

Do they have a separate low power output DC-DC that is always on (maybe in the battery pack?) and supplies a constant 12V to this lithium pack? Has it always been that way on Model S (two DC-DCs?)? I've heard claims about that but no idea.

There are practical considerations to using a 12V battery instead of just taking 12V off the HV pack. For instance, the HV contactors are disengaged while in "Park" for safety reasons. Also, if the HV pack is being actively discharged, then it must also employ thermal management which is yet another electrical load.

I agree it's weird that the 12V battery is needed, but there must be a good reason for it if Mr. "best part is no part" cannot eliminate it.

 

[AlanSubie4Life]

pack. For instance, the HV contactors are disengaged while in "Park" for safety reasons

There is no reason really why they could not have a small DCDC in the pack that passes a fused 12V out and is active when the contactors are open, as long as it has minuscule standby drain and appropriate behavior to shut down to avoid bricking the pack at very low SoC. It would be safe; 12V is safe. It could power down if the pyrofuse blows (far side of pyrofuse, near side of contactors, as seen from the pack).

Also, if the HV pack is being actively discharged, then it must also employ thermal management

I don’t really think that is necessary for a ~5W drain. Discharge of the pack at those levels should be fine both when extremely cold and extremely hot.

I agree it's weird that the 12V battery is needed,

I don’t think it is weird that it exists. It is absolutely needed for emergencies, after the pyrofuse blows, or if the DCDC fails, etc.

As mentioned, there may be another DCDC as well. Hard to know.

 

[AlanSubie4Life]

no reason really why they could not have a small DCDC in the pack

Probably more like “on” the pack…so that it can be replaced if it fails, without pack disassembly.

 

[mark95476]

The new Shanghai LFP cells add substantial weight, but nothing compares to how resilient they are. Likely cheaper too.

Depth of discharge, charging, discharging, temp sensitivity, ...etc... I think you're well aware of the advantages/disadvantages of LFP vs lead-acid. Bjorn's done a lot of testing and the LFP cells from Shanghai are awesome for cold temps and low SoC.

 

[JRP3]

The specs don't add up to LFP for the "12V" in the Plaid.

 

[AlanSubie4Life]

The new Shanghai LFP cells add substantial weight, but nothing compares to how resilient they are. Likely cheaper too.

They could be used here. Odd that it says 1P4S though. As was asked above, dimensions would be nice to know. The capacity is roughly consistent with a 2P4S 18650 configuration. 2170s are only 45% higher capacity or so, so that does not work either.

I think it almost has to be 4680 to get 6.9Ah at 15.5V

Just look at the volume of that cell, though. 4680 cells are HUGE (longer and more than 2.5x diameter, capacity proportional to square of that…). I would expect they would have about 8x the capacity of an 18650 battery, at least. So a single cell would be 100Wh, if I am calculating correctly. Not including overhead so it probably will be higher (18650 cells are about 12.5Wh - 8256 of them make a 103kWh pack, prior to Plaid - they could be slightly higher capacity on the Plaid, no idea).

So I am guessing either 18650 “2P” called “1P,” for some reason, or they are pouch cells of some sort. As you say, dimensions could probably clear this up - or be inconclusive.

 

[JRP3]

Can't imagine any reason for mislabeling the battery. Could be a 26650 size which is a standard cylindrical cell size.

 

[AlanSubie4Life]

Can't imagine any reason for mislabeling the battery. Could be a 26650 size which is a standard cylindrical cell size.

Yeah that would be pretty close.

 

[Dolemite]

There are practical considerations to using a 12V battery instead of just taking 12V off the HV pack. For instance, the HV contactors are disengaged while in "Park" for safety reasons. Also, if the HV pack is being actively discharged, then it must also employ thermal management which is yet another electrical load.

I agree it's weird that the 12V battery is needed, but there must be a good reason for it if Mr. "best part is no part" cannot eliminate it.

I saw an engineer post about his work on eliminating the 12V battery entirely - can't find it, and now it's even harder with all the BS noise about the new li-ion.

The basic conclusion was that it's basically not possible. Something about needed a secondary power source to power the main one on, and also have it be serviceable in case of issues. Not to mention there are a ton of established accessories built to a 12V standard.