DC to DC converter: what can it reasonably handle in terms of using your battery as a power source?

[PhysicsGuy]

I would like to focus here on a very specific technical issue which is essential to using your model 3 battery as a power source. I think the key thing question for those of us actually thinking of using the car battery as a power source is whether the dc-dc converter can handle this, or whether it is unwise due to potential impacts on the dc-dc converter. Any thoughts from people who have experience with this or some pertinent technical background would be most helpful.

A bit of background and contextualization: What I would like to do is power a refrigerator and a freezer for up to a week or so as needed (in a power outage). The amount of energy that would consume is modest; about 100 watts each at 50% duty cycle implies 100 watts x 24 hours = 2.4 kWh per day. My model 3 battery stores up to 75 kWh, so 4 days is only about 15%. The draw is also slow, which is benign, so I feel confident that the effect on the battery would be of no significance. I am not concerned about the battery warranty; my battery is excellent so far (more that one year) and I keep it between 30 and 70% most of the time (and never below 10%).

I am, however, pretty concerned about the dc-dc converter. I don't want to trip it or to end up having to replace it. Has anyone done this in a sustained manner? (I know Marc Merlin did a demo with the involved running a microwave, briefly I think. I believe his dc-dc converter tripped at least once which cause me some concern.)

I am pretty sure that with the pick-up truck this would be effortless. I think there are 110 volt outlets, probably 15 amp, probably gfi) in the pick-up truck. So I am wondering about trying to retrofit a model 3 to do that same thing. I think one would have to use the 12 volt posts under the rear seat, connected to an inverter with quite large cables. (5 amps at 120 volts coming out of the inverted implies 50 amps going in at 12 volts.) Additionally, a pre-charge loading circuit would presumably have to be used to control the peak current on the dc side as the capacitors in the inverter charge. While this is benign for the battery (as mentioned above) and it is pretty straightforward, the dc-dc converter is a concern. Can the dc-dc converter handle this, or would it likely lead to dc system problems down the road??

 

[smatthew]

The DC-DC converter is capable of producing 193Amps of 12Vish power. Unfortunately, there's no way to access all that power without the computers freaking out. The car measures current delivered to different sources, and if the charging current and current used are significantly different, it freaks out. So connecting directly under the rear seat is a no-go.

If you could ensure that the fridge and freezer don't operate at the same time, you could use a SmartSine 300w inverter connected to the 12V power socket in the center armrest. The limit there is 12 Amps continuous, 16 amps peak.

 

[cwerdna]

I would like to focus here on a very specific technical issue which is essential to using your model 3 battery as a power source. I think the key thing question for those of us actually thinking of using the car battery as a power source is whether the dc-dc converter can handle this, or whether it is unwise due to potential impacts on the dc-dc converter.

Keep in mind that per the manual on page 156: https://www.tesla.com/sites/default/files/model_3_owners_manual_north_america_en.pdf, it says "Caution: Do not use the Battery as astationary power source. Doing so voidsthe warranty."

 

[smatthew]

Keep in mind that per the manual on page 156: https://www.tesla.com/sites/default/files/model_3_owners_manual_north_america_en.pdf, it says "Caution: Do not use the Battery as astationary power source. Doing so voidsthe warranty."

What counts as using the battery as a stationary power source? When I go camping and enable camp mode so I can use the 12V and USB outlets to power things while stationary am I voiding my warranty?

 

[animorph]

There are a couple of threads addressing this question. Searching for "inverter" should bring them up. IIRC the 3 required a direct connection of your DC-AC inverter to the DC-DC converter (for a large inverter) because the wiring to the battery is fairly small. So it's a bit of a project. 200W might be on the edge of what you could do directly from the 12V battery.

 

[cwerdna]

What counts as using the battery as a stationary power source? When I go camping and enable camp mode so I can use the 12V and USB outlets to power things while stationary am I voiding my warranty?

I guess one could try contacting Tesla for clarification, unless one wants to be a test case in the event of some problem/failure/something Tesla discovers.

My guess is that they don't want people running inverters off the 12 volt bus and/or they don't want people doing unofficial V2H or V2G if you can even do it the Teslas that are in customer hands now. (This is as opposed to Leafs beyond a certain model year and Outlander PHEV (well, at least in Japan and elsewhere, not sure about US) which can actually output power via their CHAdeMO ports.)

I've run 1000W 12VDC to 120VAC Pure Sine Inverter | PST-1000-12 off the 12 volt bus of my Bolt while leaving the car in READY mode (and workaround to prevent the stupid 1 hour automatic shutdown) for over 24 hours at a time thanks to PG&E PSPS ("public safety" power shutoff). I ran my fridge, some LED lights, my security camera NVR system, laptop and charged my phones.

The 1000 watts is not enough to run my microwave oven though. If I experience another PG&E PSPS, I may have look for another microwave that uses less than 1000 watts from the wall. I've seen some 700 watt output ones but I'm not sure how they pull from the wall.

 

[SSedan]

I wonder about adding a second 12volt outside the car to take the big draw and slowly recharge so the car never sees peak draw?

 

[rrolsbe]

The DC-DC converter is capable of producing 193Amps of 12Vish power. Unfortunately, there's no way to access all that power without the computers freaking out. The car measures current delivered to different sources, and if the charging current and current used are significantly different, it freaks out. So connecting directly under the rear seat is a no-go.

If you could ensure that the fridge and freezer don't operate at the same time, you could use a SmartSine 300w inverter connected to the 12V power socket in the center armrest. The limit there is 12 Amps continuous, 16 amps peak.

I read another thread on this forum where a member tested an inverter connected to DC-to-DC connections under the rear seat at 1500W. Maybe Tesla got wind of this and changed the firmware to look for power out approx equal to power used by the cars normal 12V subsystems? If I ever tried using an inverter, 1500W or less would be the max power.

 

[smatthew]

I read another thread on this forum where a member tested an inverter connected to DC-to-DC connections under the rear seat at 1500W. Maybe Tesla got wind of this and changed the firmware to look for power out approx equal to power used by the cars normal 12V subsystems? If I ever tried using an inverter, 1500W or less would be the max power.

So Tesla knows how much power the DC-DC converter is producing. And because every circuit in the car is protected by an eFuse, the car knows exactly how much power is being used. When you tap into 12V under the rear seat, that ends up being an unmonitored power draw. When the car sees the power produced, and power consumed diverge significantly, it assumes something is wrong.
That's why using the 12V accessory circuit works. As well as hooking into the amplifier power circuit. If you're not listening to music there is 30 amps of power available there. Moderate volumes will easily leave 20 amps - or about 250 watts.

Either way, I'd still recommend the SureSine inverter. Very efficient pure sine-wave 300w inverter.

 

[SSedan]

Last summer when we had a prolonged outage I bought a 1500watt inverter and hooked it up to the battery on an old truck I keep around for hauling.
It powered an OLD upright Manitowoc residential freezer and a modern fridge no problem.

I know you don't want to idle an ICE bit it is a better solution.
Fridge and freezer can be run for a few hours then shut down, long as they are not opened they will stay cold awhile.

 

[Rocky_H]

Don't need a new thread for this. Check out this stickied thread at the top, which has been around longer and has a lot more discussion.
MASTER THREAD: Powering house or other things with Model 3 12V battery

 

[PhysicsGuy]

The DC-DC converter is capable of producing 193Amps of 12Vish power. Unfortunately, there's no way to access all that power without the computers freaking out. The car measures current delivered to different sources, and if the charging current and current used are significantly different, it freaks out. So connecting directly under the rear seat is a no-go.

If you could ensure that the fridge and freezer don't operate at the same time, you could use a SmartSine 300w inverter connected to the 12V power socket in the center armrest. The limit there is 12 Amps continuous, 16 amps peak.

Thanks. Good info and good ideas. I think the problem with using a 300 Watt inverter is the start. A freezer draws about 5 or 10 amps (at 110 v) at start for about 1/2 second*, and I don't think the 300 watt inverter or the 12v power socket can handle that.

 

[PhysicsGuy]

Don't need a new thread for this. Check out this stickied thread at the top, which has been around longer and has a lot more discussion.
MASTER THREAD: Powering house or other things with Model 3 12V battery

I have read those threads and they lead nowhere, except for Marc Merlin's demo project at somewhere around the 400th post out of 1000 or so. And that is a no-go for reasons that SMatthew elucidates here. MM is a certified highly skilled hacker who can do things for fun that one may not wish to try at home. Those threads are educational and interesting (I have read them), but they don't lead to an actual useable way to use power, which is what I am hoping to find. Something in between: "it is impossible" and, "it has already been done so why talk about it".

 

[PhysicsGuy]

So Tesla knows how much power the DC-DC converter is producing. And because every circuit in the car is protected by an eFuse, the car knows exactly how much power is being used. When you tap into 12V under the rear seat, that ends up being an unmonitored power draw. When the car sees the power produced, and power consumed diverge significantly, it assumes something is wrong.
That's why using the 12V accessory circuit works. As well as hooking into the amplifier power circuit. If you're not listening to music there is 30 amps of power available there. Moderate volumes will easily leave 20 amps - or about 250 watts.

Either way, I'd still recommend the SureSine inverter. Very efficient pure sine-wave 300w inverter.

Could you say more about the amplifier power circuit?

 

[PhysicsGuy]

Last summer when we had a prolonged outage I bought a 1500watt inverter and hooked it up to the battery on an old truck I keep around for hauling.
It powered an OLD upright Manitowoc residential freezer and a modern fridge no problem.

I know you don't want to idle an ICE bit it is a better solution.
Fridge and freezer can be run for a few hours then shut down, long as they are not opened they will stay cold awhile.

I am amazed that that worked but I guess with the cold-cranking amps of a truck 12 volt and a large inverter it makes sense. The cold cranking amps must have been sufficient to get you through the brief high current draw at the start.

 

[PhysicsGuy]

Either way, I'd still recommend the SureSine inverter. Very efficient pure sine-wave 300w inverter.

I am thinking you would need a larger inverter to get you through the first second when the freezer starts. At that moment, when it starts, I think the current peaks at about 5 to 10 amps on the AC side, which means, of course, 50 to 100 amps on the 12 volt side.

I happen to have a pretty good 1500 Watt inverter lying around, but I am a little hesitant to plug that in to the 12 volt cig lighter port for fear of tripping something during the initial charging of the inverter capacitors. Can I just plug that in to the 12 volt aux port? Will something bad happen or will it just charge slowly due to the internal limits of the 12 volt current that you mentioned earlier?

 

[smatthew]

I am thinking you would need a larger inverter to get you through the first second when the freezer starts. At that moment, when it starts, I think the current peaks at about 5 to 10 amps on the AC side, which means, of course, 50 to 100 amps on the 12 volt side.

I happen to have a pretty good 1500 Watt inverter lying around, but I am a little hesitant to plug that in to the 12 volt cig lighter port for fear of tripping something during the initial charging of the inverter capacitors. Can I just plug that in to the 12 volt aux port? Will something bad happen or will it just charge slowly due to the internal limits of the 12 volt current that you mentioned earlier?

Honestly, new-ish freezers and fridges do not have the starting current peak you're thinking of. I know a lot of people who use that inverter with their freezer.

If you go over the current limit the e-fuse will shut the power off.

 

[SSedan]

I am amazed that that worked but I guess with the cold-cranking amps of a truck 12 volt and a large inverter it makes sense. The cold cranking amps must have been sufficient to get you through the brief high current draw at the start.

I should have been clear I idled the truck, 135amp alternator

 

[thefrog1394]

Having trouble finding more recent information on this. This blog post indicates that connecting directly to the DC-DC bypasses any checks on current draw. Some posts in this thread seem to indicate that large draws from the DC-DC directly are an issue. Anyone have updated info as of current software?

 

[David99]

Having trouble finding more recent information on this. This blog post indicates that connecting directly to the DC-DC bypasses any checks on current draw. Some posts in this thread seem to indicate that large draws from the DC-DC directly are an issue. Anyone have updated info as of current software?

The M3 doesn't have fuses. It monitors the power draw and turns it off if it's beyond what it should be. That's no different than a traditional fuse would handle too much current. The advantage of using FETs rather than a fuse is that you can close them again instead of having to replace a physical fuse. I assume there is no check of the total power drawn. So if you hook up the inverter you should be fine. I don't have the specs of the Model 3 DC-DC converter. The Model S is rated for over 2 kW. I'm running a 1500 Watt inverter directly off the DC-DC on my Model S. So far absolutely no issues running high power draws like a vacuum or microwave off it. I rarely use high power draw, though. And if I use them, only for a short time. My main purpose is to charge the batteries of my equipment when I'm on the road and power laptops and other smaller devices.