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How many amps can you get from the 12V system for emergency situations?

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The charge circuitry (at least up to the late 2019 one here) has diodes in it. This will specifically prevent the electric current from flowing back into the charger. It's designed to prevent it from happening.
Just to be super clear, are you sure the diodes are on the 400A DC bar? You need some big ass diodes for 400A, and doesn't the video say that the DC current can flow backwards for pre-heating the system when you are driving to a supercharger?
Actually, sorry, 250kW for the new superchargers is 625A. Are you sure that is going through diodes?
 
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Just to be super clear, are you sure the diodes are on the 400A DC bar? You need some big ass diodes for 400A, and doesn't the video say that the DC current can flow backwards for pre-heating the system when you are driving to a supercharger?

Look in the description of the video. He links to a followup video where he traces out the circuit and shows the current flow.

The engineer who put out the original suggestion that the Model Y could output current conceded he was correct in his analysis.
 
Just to be super clear, are you sure the diodes are on the 400A DC bar? You need some big ass diodes for 400A, and doesn't the video say that the DC current can flow backwards for pre-heating the system when you are driving to a supercharger?
It's not 400A it's 400V. It's a charger, so the amperage is controlled by the charger. The charger is 48A 240VAC, so about 11.5kW. By the time that power gets to the battery pack voltage, it's no more than 32A.
The battery heating is using waste heat from the drive unit. The charger is not involved in that.
 
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It's not 400A it's 400V. It's a charger, so the amperage is controlled by the charger. The charger is 48A 240VAC, so about 11.5kW. By the time that power gets to the battery pack voltage, it's no more than 32A.
The battery heating is using waste heat from the drive unit. The charger is not involved in that.
Please don't get things mixed up. I am not talking about AC charging, please re-read what I wrote. I am talking about DC charging which can now be done at 250KW, which at 400V is 625Amps peak (it used to be 400A peak, but that was raised recently).
I'm surprised that they have enough diodes to allow that current flow without blowing up, but if someone says they verified that the DC system is also directional, so be it.
 
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Please don't get things mixed up. I am not talking about AC charging, please re-read what I wrote. I am talking about DC charging which can now be done at 250KW, which at 400V is 625Amps peak (it used to be 400A peak, but that was raised recently).
I'm surprised that they have enough diodes to allow that current flow without blowing up, but if someone says they verified that the DC system is also non directional, so be it.
The diodes are in the charger. Supercharging goes direct to the battery pack. No diodes involved.
 
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I was referring to using DC fast charging capabilities to get a direct DC connection to the battery. That bypasses the on-board charger. There should be no hardware reason why it wouldn't work. You would still need an external HV inverter.

However, the BMS software in the battery might not allow it. It might shut down the contactors (open the circuit) if it sees current flowing out of the battery, when current should be flowing in.
 
I was referring to using DC fast charging capabilities to get a direct DC connection to the battery. That bypasses the on-board charger. There should be no hardware reason why it wouldn't work. You would still need an external HV inverter.

However, the BMS software in the battery might not allow it. It might shut down the contactors (open the circuit) if it sees current flowing out of the battery, when current should be flowing in.
This is exactly the point. You may be able to trick the car into passing the voltage from the battery pack to the charge port, as if it's charging, like with a CHAdeMO adapter. However, Tesla is looking at the charging current, at least for the purpose of displaying the charging power. If it sees a negative charging power due to you taking energy out of the pack, they could shut it down as a safety measure since a charger is not supposed to do that.

On top of that, you would need an inverter designed for the 400VDC nominal pack voltage. Those are not that common.
 
This is exactly the point. You may be able to trick the car into passing the voltage from the battery pack to the charge port, as if it's charging, like with a CHAdeMO adapter. However, Tesla is looking at the charging current, at least for the purpose of displaying the charging power. If it sees a negative charging power due to you taking energy out of the pack, they could shut it down as a safety measure since a charger is not supposed to do that.

On top of that, you would need an inverter designed for the 400VDC nominal pack voltage. Those are not that common.

I feel ignored :) First, 10 messages to tell me that the DC system cannot be bidirectional when it's the AC system that isn't, and then I wrote twice already that you'd need a battery-less powerwall at home.
Powerwalls use 400V-ish, so clearly those inverters exist and are as common as powerwalls with batteries: Tesla Powerwall – Lithium Ion Battery Test Centre
 
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I feel ignored :) First, 10 messages to tell me that the DC system cannot be bidirectional when it's the AC system that isn't, and then I wrote twice already that you'd need a battery-less powerwall at home.
Powerwalls use 400V-ish, so clearly those inverters exist and are as common as powerwalls with batteries: Tesla Powerwall – Lithium Ion Battery Test Centre
The curvy Powerwall 1.0 unit you linked does not have an AC inverter inside. It relied on a paired solar inverter like StorEdge.
StorEdge® Battery Solutions - Maximum PV Usage | SolarEdge US

The Powerwall 1.0 controlled charge and discharge with an internal DC-DC converter.

I am not certain what the internal voltage is of the Powerwall 2, but I'm pretty sure it's not 400VDC.

Anyway, this is all very off-topic for a thread about how many amps you can get out of the 12V system from a Model 3.
 
Hi @marcmerlin , very nice work you've done here.
I'm considering installing an inverter in my (future) Model 3 to power motorcycle tyre warmers. As I understand it they would be fine on a modified sine wave inverter (resistive load). The tyre warmers are rated for 1100W, I was looking at something like that:

https://www.amazon.co.uk/Mercury-Modified-Sine-Inverter-12Vdc/dp/B00J68QIXA?th=1

I'm intrigued by the "slow start" feature mentioned. Any chance that would alleviate the "inrush" when the inverter is turned on, and I would not need the resistor and relay? Or is that on the AC side of the inverter to avoid shorting the inverter itself?
 
Hi @marcmerlin , very nice work you've done here.

I'm intrigued by the "slow start" feature mentioned. Any chance that would alleviate the "inrush" when the inverter is turned on, and I would not need the resistor and relay? Or is that on the AC side of the inverter to avoid shorting the inverter itself?
Inverters have a big capacitor on the 12V side.
Capacitors can take mostly infinite current (1000A or more, whatever your wire will allow). This will cause a massive spark every time you connect to a battery. The battery does not care, but the DC-DC in the car does, and it will shut down.
So yes, unless you pre-charge the capacitor with a battery (with the inverter turned off), and move it over quickly to the car (a pain), you do want the slow start.
 
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Ok I see, so whatever the (decent size) inverter, it will always need some type of setup like the resistor and relay to not trip the DC-DC.
I notice yourself and @Timbo2 had a bit of trial and error regarding resistor size, did you manage to find out which size is safer/more reliable?
 
Ok I see, so whatever the (decent size) inverter, it will always need some type of setup like the resistor and relay to not trip the DC-DC.
I notice yourself and @Timbo2 had a bit of trial and error regarding resistor size, did you manage to find out which size is safer/more reliable?

I'm currently using 150 ohms. I purchased a 2 pack, so for grins I tried them in series for 300 ohms and was unable to get enough power into the inverter to get the relay to close. So at least with my 300W test inverter less than 300 ohms.
 
There is indeed no right answer on the resistor. You want it as big as possible to slow down the current, but not too big that the relay won't close. This is totally specific to your inverter in your relay, That said, 100 ohms is probably a good place to start (I initially had 1 Ohm and it worked with one inverter, but not the other one, this is because the slowdown curve is related to RC (resistor and capacitor which you can't see)
 
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Ok, since I can't edit the first message, and my solution is now hidden in the middle of the thread, I'll post it here again:
Marc's Blog: electronics - Using a Tesla Model 3 as Emergency Power Source In Case of Grid Failure

One thought... You might consider moving the relay to control the positive terminal and have the negative always connected. I assume the car chassis is connected to the 12V negative. If the inverter chassis is also tied to battery negative, then it could short to the chassis and bypass your relay.
 
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