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

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Question:
Is there a better 12V bus I should tap into that gets more amps from the DC-DC converter than what is being sent to the battery (only a few amps it seems)

Details:
I'm interested in getting a decent amount of amps from my car while it's parked at home if I'm in a power emergency situation (i.e. no power) for maybe 2-3 days (as sadly is now a new "normal" in california).

I've read a bunch of threads about this (given at the bottom of this post), please do not tell me that:
1) I should get a powerwall. I'd need to spend $30 to $40K in powerwalls to get the same amount of energy I already have in my car and I already paid for. Sorry, but I'm not spending that much for something I may need once every few years. Load shifting my solar in california is otherwise a straight loss, and PG&E does not allow using powerwalls for time of use power arbitration, so powerwalls are useless to me. I would buy one if it only had the electronics and it plugged into my car to use its lithium that's already there.
I know a powerwall will give me more than the 1000-2000W my system will give, but honestly I don't care. I want 2-3 days of runtime which I won't get if I'm using more than 1kW anyway. I'm ok with an extension cord plugged into an inverter and not feeding my whole house.
Elon, if you are reading this, build this car assisted battery-less powerwall and I'll buy it tomorrow.

2) this voids the warranty. This has been discussed at length. If I break something, I'll pay for it, that's fair.

3) that the battery in the car is way too small to power a proper size inverter (I have a 3000W one to be safe but honestly I only plan on using 1000W peak for my fridge and an average of 200W otherwise when the fridge is on. I already measured this).


What I'm doing
a) I have a huge ass GLA 12V battery, it can power 3000W right now (my fridge and my microwave) through the 3000W inverter. It's already tested, it works. It just won't work super long if I don't recharge it

b) I've connected it directly to the car battery to tap into the 12V system. I don't expect to need or pull 100A or even 50A from the car since my buffer battery can take that load

c) I already have verified that the car will sense the 12V system attached to it (both batteries wired in parallel), are getting low and use the DC-DC converter to recharge. What I read online says it's rated for 2000W (i.e almost 200A), so it should be more than plenty

Everything works great, except I cannot get more than maybe 5A from the car to its 12V system. It's not even what the cigarette lighter adapter is rated for (15A peak). In other words, it charges my batteries slower than they discharge if I plug just a 100W light in my inverter.
I've tried putting the car in drive and even that way, it still doesn't recharge the 12V system faster than it discharges.
Is there a better 12V bus I should tap into that gets more amps from the DC-DC converter than what is being sent to the battery?

Thanks, Marc

Previous threads:
Whats the largest Power Inverter you can use in a Tesla?
Model 3 electric outlet options | Tesla
Can Model 3 keep 12v on without going to sleep?
note that camper mode is not what I need because I'm tapped into the main 12V and it doesn't charge fast enough even if the car in drive:
Eight Hours Overnight In A Tesla Model 3 In Freezing Temps: Video
 

eprosenx

Active Member
May 30, 2018
2,076
2,554
Beaverton, OR
Question:
Is there a better 12V bus I should tap into that gets more amps from the DC-DC converter than what is being sent to the battery (only a few amps it seems)

Details:
I'm interested in getting a decent amount of amps from my car while it's parked at home if I'm in a power emergency situation (i.e. no power) for maybe 2-3 days (as sadly is now a new "normal" in california).

I've read a bunch of threads about this (given at the bottom of this post), please do not tell me that:
1) I should get a powerwall. I'd need to spend $30 to $40K in powerwalls to get the same amount of energy I already have in my car and I already paid for. Sorry, but I'm not spending that much for something I may need once every few years. Load shifting my solar in california is otherwise a straight loss, and PG&E does not allow using powerwalls for time of use power arbitration, so powerwalls are useless to me. I would buy one if it only had the electronics and it plugged into my car to use its lithium that's already there.
I know a powerwall will give me more than the 1000-2000W my system will give, but honestly I don't care. I want 2-3 days of runtime which I won't get if I'm using more than 1kW anyway. I'm ok with an extension cord plugged into an inverter and not feeding my whole house.
Elon, if you are reading this, build this car assisted battery-less powerwall and I'll buy it tomorrow.

2) this voids the warranty. This has been discussed at length. If I break something, I'll pay for it, that's fair.

3) that the battery in the car is way too small to power a proper size inverter (I have a 3000W one to be safe but honestly I only plan on using 1000W peak for my fridge and an average of 200W otherwise when the fridge is on. I already measured this).


What I'm doing
a) I have a huge ass GLA 12V battery, it can power 3000W right now (my fridge and my microwave) through the 3000W inverter. It's already tested, it works. It just won't work super long if I don't recharge it

b) I've connected it directly to the car battery to tap into the 12V system. I don't expect to need or pull 100A or even 50A from the car since my buffer battery can take that load

c) I already have verified that the car will sense the 12V system attached to it (both batteries wired in parallel), are getting low and use the DC-DC converter to recharge. What I read online says it's rated for 2000W (i.e almost 200A), so it should be more than plenty

Everything works great, except I cannot get more than maybe 5A from the car to its 12V system. It's not even what the cigarette lighter adapter is rated for (15A peak). In other words, it charges my batteries slower than they discharge if I plug just a 100W light in my inverter.
I've tried putting the car in drive and even that way, it still doesn't recharge the 12V system faster than it discharges.
Is there a better 12V bus I should tap into that gets more amps from the DC-DC converter than what is being sent to the battery?

Thanks, Marc

Previous threads:
Whats the largest Power Inverter you can use in a Tesla?
Model 3 electric outlet options | Tesla
Can Model 3 keep 12v on without going to sleep?
note that camper mode is not what I need because I'm tapped into the main 12V and it doesn't charge fast enough even if the car in drive:
Eight Hours Overnight In A Tesla Model 3 In Freezing Temps: Video

So you are saying you have attached your big battery directly in parallel to the 12v battery in the Tesla, but then when you put load on your battery, the Tesla seems to only charge your battery at 5 amps, while you think the DC to DC converter should be good for closer to 200a? What makes you think that? The wires for that would be HUGE.

I suspect the DC to DC converter is smaller than you think as it really does not need to charge the 12v battery that fast. It really just needs to be able to power all the 12v loads in the car, plus slowly charge the 12v battery.

I have not seen the specs on the battery in the Model 3 but I suspect it is quite small and so it would damage it to put too high a charge rate on it.

Also, I wonder what kind of battery it has compared to what you have. Different kinds of battery technology need different charge voltages. I wonder if you have not let your battery discharge enough to hit the Tesla threshold for recharging?

Btw, I love the research you are doing! While this is not “Tesla Approved”, it is great research to have available out in the community. The only issues I can think of with it would be overheating your DC/DC converter due to over use or otherwise damaging the Tesla 12v battery due to cycling it to fast/deep/frequently. Also, if you parallel another battery in, the two will try to equalize voltages and that could result in large flows in or out that may damage the battery.
 
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So you are saying you have attached your big battery directly in parallel to the 12v battery in the Tesla, but then when you put load on your battery, the Tesla seems to only charge your battery at 5 amps, while you think the DC to DC converter should be good for closer to 200a? What makes you think that? The wires for that would be HUGE..
You are correct, 200A is a power bar, not a wire anymore :) To be clear, I am not looking at getting 200A from the car, but my inverter is indeed designed to draw over 200A, that's the only way you can get 3000W from 12V. you need thick wires, and they're around the thickness of what good jumper cables, use.

I suspect the DC to DC converter is smaller than you think as it really does not need to charge the 12v battery that fast. It really just needs to be able to power all the 12v loads in the car, plus slowly charge the 12v battery.

I've been told 200A, and been told that power steering can use 100A, which clearly isn't sourced from the battery that is way too small for that. The AC and heating also must use non trivial amounts of power.

Also, I wonder what kind of battery it has compared to what you have. Different kinds of battery technology need different charge voltages. I wonder if you have not let your battery discharge enough to hit the Tesla threshold for recharging?

Btw, I love the research you are doing! While this is not “Tesla Approved”, it is great research to have available out in the community. The only issues I can think of with it would be overheating your DC/DC converter due to over use or otherwise damaging the Tesla 12v battery due to cycling it to fast/deep/frequently. Also, if you parallel another battery in, the two will try to equalize voltages and that could result in large flows in or out that may damage the battery

All good questions I gave thought about. Answers:
- I forget what lead acid battery is in the car, but I read it's a motorcycle type of lead acid battery. Definitely not meant for big amps.
- overheating DC/DC converter, if I were drawing 50 or 100A from it continuously, I agree, but realistically I'm looking for 20A or less. My loads can be high amp for a short time but over time (a day), it will even out, and the load spikes will be sourced from my big battery, not the DC/DC system
- voltage matching between batteries, this is also a good point. It's mitigated by making sure that the batteries are close enough in voltage when you connect them, and then they stay in sync. Also, if you use wires that are not too thick, the wire resistance will limit the amount of amps that can flow between them to something reasonable (but obviously my wires are big enough to allow more than 8A)
- cycling the battery frequently, you are correct here too, but because the battery I have in parallel is about 10 times bigger in capacity, the cycling will be limited and also I'd want the DC/DC system to be on so that ultimately most of the power comes directly from it and not from discharging the batteries.
 
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Sophias_dad

Active Member
Supporting Member
Jul 29, 2018
2,179
2,466
Massachusetts
Is there some reason you don't skip finding the bus and connect your buffer battery and load to the 12V battery directly?

Just for the record, 20 amps or less on a 12V source is <240 watts, assuming 100% efficiency. I think you are underestimating what you are really going to use. Have you attached an energy meter to the things you plan to power?

I don't think you should be relying on the resistance of the wires to reduce current transfer to your buffer battery .... almost by definition that's going to be overheating the wires.
 
Just FYI, the AC compressor and the cabin heater are both high voltage and powered off of the main battery pack. One possible source of higher current might be located under the rear seat cushions in the battery pack's penthouse. There's an output from the DC to DC converter that people tap into for aftermarket amplifiers and subwoofers. You just need to make sure there is not a lot of power draw as the car turns on or it will disable the converter temporarily. Look in the sound system threads for more info.
 

AlanSubie4Life

Efficiency Obsessed Member
Oct 22, 2018
11,687
15,169
San Diego
Seems like you are probably right about the approximate DC DC spec. I am sure Google search will confirm. Looks like 2500W for the Model X.

Tesla Model X (2015-2018) OEM GEN 3 DC/DC Converter Assembly Part # 1060985-00-B | eBay

Seems that Tesla limits the charge rate of the 12V AGM as you do have to be careful about these rates to maximize the lifetime.

So presumably there is some circuit component between the 12V and the DC DC output, so you just need to connect to the DC DC output directly...if possible. And figure out how to reliably keep it turned on.

Just guessing based on the description of symptoms. No idea really. I figure someone must have worked this out by now. I’m interested in this and doing it safely for all the same reasons I suppose.
 
Just for the record, 20 amps or less on a 12V source is <240 watts, assuming 100% efficiency. I think you are underestimating what you are really going to use. Have you attached an energy meter to the things you plan to power?

I don't think you should be relying on the resistance of the wires to reduce current transfer to your buffer battery .... almost by definition that's going to be overheating the wires.
250W is what my fridge uses when it's on, and it's running maybe 33% of the time, so I do think estimating my averaged emergency power need to 250W (let's say even 500W) is not unreasonable at all. I just bought a 3000W inverter because all those inverters don't work well when you get close to their capacity, and I need the inverter to take quick load spikes like when the fridge starts (1000W or so).

Yes, I have 120V energy meters as well as 100A DC energy meters already wires (using a shunt). This is how I know that my inverter draws 8A at 12V when I have a 100W lightbulb and that the battery is discharging quicker than it's recharging from the car.

Overheating wires: actually no. I thought about it, it's self regulating. When the wires (which actually are still thick), start limiting current, they'll heat up a bit, but as they do their resistance goes up, then there is a voltage drop until the voltage matches because the voltage mismatch is equal to the voltage loss through the wires, and current stops flowing. The only way for current to resume is by being lower, causing the voltage drop to decrease just enough for current to flow at a lower pace.

Have you considered tapping directly into the high-voltage system and using your own DC converter?
I have not. It's 400V, enough to kill me and using equipment I don't have and don't wish to have. 12V is safe, 120V AC is safe enough if you get zapped. 400V DC is not safe at all (as a reminder DC will kill you much more than AC :) ).


Seems like you are probably right about the approximate DC DC spec. I am sure Google search will confirm. Looks like 2500W for the Model X.

Tesla Model X (2015-2018) OEM GEN 3 DC/DC Converter Assembly Part # 1060985-00-B | eBay
Seems that Tesla limits the charge rate of the 12V AGM as you do have to be careful about these rates to maximize the lifetime.

So presumably there is some circuit component between the 12V and the DC DC output, so you just need to connect to the DC DC output directly...if possible. And figure out how to reliably keep it turned on.

Just guessing based on the description of symptoms. No idea really. I figure someone must have worked this out by now. I’m interested in this and doing it safely for all the same reasons I suppose.
Yes, thanks for confirming what I'm seeing and your guess as to why it's going on, I agree that the car seems to have a circuit that limits current going to the battery so as to improve its life by not charging it too quickly.

Now I need to find a better 12V DC source, be ready for the fact that it will only be on when the car is awake and hopefully not hit some other limiter. Maybe I should just find where the power steering is connected in the fuse box, and tap into that.
 
Just FYI, the AC compressor and the cabin heater are both high voltage and powered off of the main battery pack. One possible source of higher current might be located under the rear seat cushions in the battery pack's penthouse. There's an output from the DC to DC converter that people tap into for aftermarket amplifiers and subwoofers. You just need to make sure there is not a lot of power draw as the car turns on or it will disable the converter temporarily. Look in the sound system threads for more info.
Thank you for that info. Ok, then it's only the power steering that may be using 12V at 100A peak from what I've been told. Still, there should be a 12V bus that has more than 8A, I will look at the sound threads, thanks, and then worry about keeping the car awake so that it doesn't shut down.

It's around 10 or so amps and then the plugs will shut down.
If you pull directly off the battery, the car will get upset. It may start throwing alarms and stop the car from operating.
The car is NOT a UPS
Thanks for the replies.

So, I know the cigarette lighter adapter is limited to 15A peak (10/12A continuous), which is clearly more than what's being sent to the battery.
You are correct that when I'm pulling from the battery, the car gets upset and start throwing low battery alarms, which is why I understand that approach is not going to work and I'm looking for a better source.

Now, plugging a small inverter directly into the 12V system somewhere else in the car, should be fine, but I'm not sure how I feel about plugging my 12V buffer battery into that system since the car's 12V bus may not be 13V and maybe exactly 12V, causing my buffer battery to actually discharge into the system, which would be bad. Also if it's 14V, it would constantly push too high of a voltage with almost unlimited current (only limited by my wires), into the buffer battery I have outside the car, and that would not be good either. This would "just work" in an ICE car as the alternator keeps the 12V voltage to the right level to charge but not overcharge the lead acid battery. Clearly the model3 is more fancy than that :)

Conversely, getting rid of the buffer battery isn't great either since it would connect the 3000W inverter directly into the car and cause draw spikes that may not be worse than the existing power steering, but I'm just not sure if it's ok or safe.
Still, at this point, it seems that my best course of action is:
1) remove the buffer battery, it just makes things more complicated and error prone, and doesn't help protect the car's small battery since I won't be connecting to it anymore
2) find a 100-200A capable 12V power source in the car (fuse box? 12V bus bar somewhere?) and connect the inverter directly into it
3) use one of the camper mode hacks to keep the car awake and the 12V bus bar powered.

Would you agree?

As for "car is not a UPS", that's an artificial statement. It's not designed to be one, but it can be used as one if done properly in my opinion. Competing cars already do this fine right now, including toyota EVs. If it makes you feel better about it, let's just say that I want to have a 50 to 200A 12V draw on the car in a safe manner. It's already been done in at least a model S: The modified Tesla in 'Racing Extinction' that's worthy of James Bond
 
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Rocky_H

Well-Known Member
Feb 19, 2015
7,845
9,870
Boise, ID
2) find a 100-200A capable 12V power source in the car (fuse box? 12V bus bar somewhere?) and connect the inverter directly into it

Would you agree?
I'm still curious what kind of cabling you think you are going to be able to find in the car that can carry 100A - 200A. It takes about 6 gauge pretty thick wire to do short term 50A. You would need a lot thicker than that to do over 100A, and I don't think any of the car's 12V systems run anything that is that high of a current.
 

KenC

Active Member
Sep 4, 2018
4,230
3,931
Maine
While in general, I like the idea of multipurposing your car's battery, I'm a little reluctant to want to use a $50k car, in case I screw something up! Why not just get a $300 inverter generator, and hook your fridge up to it? At a light 200w load, I imagine one gallon can run 11 hrs or more.
 
I'm still curious what kind of cabling you think you are going to be able to find in the car that can carry 100A - 200A. It takes about 6 gauge pretty thick wire to do short term 50A. You would need a lot thicker than that to do over 100A, and I don't think any of the car's 12V systems run anything that is that high of a current.
The 3000W inverter I have is technically supposed to do 250A at peak use. It came with thick (and short-ish) cables. I agree that getting 100A or more on any distance requires thick cables. To be honest, I'll cross that bridge when I get to it (thicker cables), for now I'd be happy with even 20-30A which is trivial to do on the cables I currently have.

While in general, I like the idea of multipurposing your car's battery, I'm a little reluctant to want to use a $50k car, in case I screw something up! Why not just get a $300 inverter generator, and hook your fridge up to it? At a light 200w load, I imagine one gallon can run 11 hrs or more.
Not a bad question. Gas generators require non trivial maintenance and if you use them once every other year, even with fuel stabilizer, the fuel will be no good.
Otherwise, it's also because I'm an engineer, and why not? :)
 
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Why wouldn't you just get a gas generator?

Hell, you could even charge your car off it in an emergency as well.
I just replied to this :)
Spoiler alert, when PG&E kills power to your entire neighborhood, don't expect your local gas station to be working either, and then you're back on your crappy 2 year old 5 gallon gas container of which a 4th manage to evaporate and leaving you for stale gas that is scary to put in any ICE (I know, I went through this back when I had a lawnmower)
 
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FlatSix911

Porsche 918 Hybrid
Jun 15, 2015
7,472
7,689
Silicon Valley
The 3000W inverter I have is technically supposed to do 250A at peak use. It came with thick (and short-ish) cables. I agree that getting 100A or more on any distance requires thick cables. To be honest, I'll cross that bridge when I get to it (thicker cables), for now I'd be happy with even 20-30A which is trivial to do on the cables I currently have.

Not a bad question. Gas generators require non trivial maintenance and if you use them once every other year, even with fuel stabilizer, the fuel will be no good. Otherwise, it's also because I'm an engineer, and why not? :)

I decided to do the same thing with a smaller 750W inverter attached to the 12V/50A jump posts in the frunk. It worked well during the recent PG&E outages and I am now interested in a direct connection with a larger 3000W inverter.
Let us know how it goes with your proposed design... I would be interested in seeing your final installation as I am nearby in Los Altos. :cool:
 
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I decided to do the same thing with a smaller 750W inverter attached to the 12V/50A jump posts in the frunk. It worked well during the recent PG&E outages and I am now interested in a direct connection with a larger 3000W inverter.
Let us know how it goes with your proposed design... I would be interested in seeing your final installation as I am nearby in Los Altos. :cool:
Do you have a model S or model 3? If model 3, where are those 50A posts in the frunk?
 
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