MASTER THREAD: Powering house or other things with Model 3

[n2mb_racing]

A diode would waste too much power and get very hot. You'd want to wire it so that the relay turns off when the PCS turns off, disconnecting your buffer battery from the PCS.

The pre-charging with the buffer battery might be more challenging, since you have to pre-charge the buffer battery to match the voltage of the PCS, otherwise it will trip the PCS by drawing too much current or it will charge the buffer battery with too much current.

Also, is the PCS voltage too high to float charge a lead acid battery? I thought I saw it was 14V.

This is rather complicated. Maybe we should just work on designing a chademo inverter and get 400V from the charge port.

 

[FalconFour]

A diode would waste too much power and get very hot. You'd want to wire it so that the relay turns off when the PCS turns off, disconnecting your buffer battery from the PCS.

Schottky diodes exist. Using a MOSFET as a diode might be even better. Both of these options reduce the power loss (heat issue) from "very" to "some" and may not even need a heat sink, let alone a fan. Considering how electricity flows, there's no way I can tell to turn off the relay when the PCS turns off. When the relay is closed, the output and input are shorted together... so it becomes a feedback loop and the relay would stay closed from the buffer battery if the PCS turns off.

The pre-charging with the buffer battery might be more challenging, since you have to pre-charge the buffer battery to match the voltage of the PCS, otherwise it will trip the PCS by drawing too much current or it will charge the buffer battery with too much current.

Doesn't tend to happen. Yeah, there's a bit of a surge sometimes as the PCS just dumps near-unlimited current into the poor battery... and that could be a concern for sure. But in my experience the worst I've seen was when the PCS was at 14.4v and the battery was at 12.5 (50% charge or so). It saps the PCS for a while... but has never tripped it. Using Scan My Tesla, I see it briefly gulps 1400w, then within seconds 1100w, 800w... and back to normal. Scan My Tesla is essential for this kind of hackery, so you can see how the car is reacting to things.

Also, is the PCS voltage too high to float charge a lead acid battery? I thought I saw it was 14V.

No, the PCS voltage is constantly adjusted in order to maintain/test/manipulate the front 12v battery. The PCS and the 12v lead battery are connected by an E-fuse (which is also software controllable), not by another level of charge control. So the car tweaks the PCS voltage to keep the 12v battery charging at 10 amps, then once the battery reaches 14.4v, it backs off to idle level. So in effect, the attached lead 12v buffer battery is just "along for the ride" with the main 12v battery, at voltage levels designed to keep a lead battery very happy. Just with hundreds of amps available at that voltage.

This is rather complicated. Maybe we should just work on designing a chademo inverter and get 400V from the charge port.

Done, lol. Vehicle To Home V2H 6kW | SETEC POWER - I used this with my Leaf long ago, and it's a massive piece of crap, but it does what it says on the tin (at least for a little while before its crappy built-in solar inverter throws a permanent fault that makes it a expensive brick). The problem isn't that "nobody's done it", it's that Tesla specifically refuses to allow power export through the DC port in any way. I guess because they have Powerwalls to sell and every one of their customers has infinite money or owns their home. (you know, it's a damn shame we can't use emoji here!)

 

[PianoAl]

In the past, during power outages, I've run some low-wattage stuff (like the 20 watt woodstove fan) from our truck's 12V battery. We sold it, and have only one car now (our M3).

I realize that the 12V in the car is too wimpy, so I'm thinking of getting these things for a standalone backup (separate from the car):

https://www.amazon.com/dp/B000MSBUA...olid=3UWNQLEQNSTG4&psc=1&ref_=lv_ov_lig_dp_it

https://www.amazon.com/dp/B06XWDZ2K...olid=3UWNQLEQNSTG4&psc=1&ref_=lv_ov_lig_dp_it

https://www.amazon.com/dp/B00JOY6U7...olid=3UWNQLEQNSTG4&psc=1&ref_=lv_ov_lig_dp_it
​

But maybe it would make more sense to replace the M3's battery with something more robust??

​

 

[jjrandorin]

There is a stickied thread in this section that is dedicated to charging options using the car:

MASTER THREAD: Powering house or other things with Model 3 12V battery

 

[animorph]

Or something like a UPS which combines a 12V battery and an inverter.

 

[PianoAl]

Instead of this battery:


I bought this one ($100 cheaper but not as pretty):



I'll report on how well it works.

I know this is off on a tangent, but:

1. Would it be safe to charge this in the house rather than in the garage (probably won't do that)?

2. Would it be safe to use this in the house instead of with a long extension cord from the garage?

 

[miimura]

Instead of this battery:

View attachment 607257
I bought this one ($100 cheaper but not as pretty):

View attachment 607260

I'll report on how well it works.

I know this is off on a tangent, but:

1. Would it be safe to charge this in the house rather than in the garage (probably won't do that)?

2. Would it be safe to use this in the house instead of with a long extension cord from the garage?

Yes, an AGM battery is safe for indoor use. Just make sure you use a charger that has a proper AGM mode and won't over charge it. The float voltage for an AGM is lower than a flooded lead acid battery.

 

[PianoAl]

I got my toys and here they are being tested:

The Amazon description for the charger/tender mentioned AGM batteries.

The battery weighs a ton (okay, not literally).

I may buy a heavier duty inverter.

Yes, an AGM battery is safe for indoor use.

It would make sense to keep it/charge it in a cabinet in the living room, but I don't know if I can bring myself to do that. Hmm. I had thought I'd keep it in the garage and bring it inside during outages, but that would require multiple hernia repairs.

Actually, the most important use will be for the gas on-demand water heater, which only requires a few watts of electrical power. That sits in the garage.

 

[PianoAl]

My testing showed that the above inverter shuts down when the battery voltage falls to about 11.5 V.

My understanding is that that battery can get much lower than 11.5 V (how low??). I'll need to find an inverter that won't shut down.

Note that I realize this is somewhat off the topic of powering things from the M3 battery, but I'm thinking that this is useful info for those who want to use the M3's battery then learn that isn't a good idea.

 

[PianoAl]

My understanding is that that battery can get much lower than 11.5 V (how low??). I'll need to find an inverter that won't shut down.

Further research shows that my understanding was wrong. Even deep cycle batteries shouldn't be discharged too much:

Why No Inverters for Deep Cycle Batteries?

 

[miimura]

Yes, shallow discharge will prolong cycle life of lead acid batteries. They should be charged as soon as possible after discharge and left on regulated float charge if possible. If not possible, it should be left disconnected and topped off at least every 6 months.

 

[PhysicsGuy]

The more I think about this issue, I think it is best solved with a dual battery system. This is common in the overlanding community (Jeepers). The basic idea is that your loads are connected to a dedicated battery. That "load" battery is charged by the vehicle's systems when the "engine" is running. There are devices available that will only connect the two when the voltage is high enough to indicate that the alternator is charging the vehicle battery.

For the Model 3, I would instead use a PWM charge controller to charge the load battery from the vehicle. The best and most convenient place to safely tap the Model 3 electrical system is the supply for the Subwoofer. It's a 30A 12V circuit. The charge controller will limit the draw to the rated amperage, even if the voltage difference would normally draw significantly more amperage. I have done this with a small 12V AGM battery for a my electric cooler. In that case, I used a 10A charge controller so that it would not overload a "cigarette lighter" 12V power point in my other car. If you're going to use a 120VAC inverter to power a refrigerator and other things in an emergency, 120W (10A 12V) is not going to be enough to keep the load battery charged, hence the 30A subwoofer tap.

I agree. Having a second battery and a good inverter is a sensible way to approach this. For example, I have used a Jackery 1000 Watt-hour battery, which comes with its own built-in inverter, to run my refrigerator at home, and one can use the model 3 battery to charge the Jackery and run the fridge from the Jackery at the same time. The Jackery has enough high-end to start the refrigerator (about 1200 watts briefly). The fridge uses only about 1.4 kWh per day, so on paper this would seem to very promising since 2% of the model 3 LR battery is enough power to run a normal size refrigerator for a day or so.

Unfortunately, there seems to be a show stopping problem, which is basically the overhead associated with having the contactors closed and the model 3 "on". I believe that in order to draw power from either the "cigarette lighter" 12V port or the subwoofer tap, the car has to be in some sort of camp-type mode, which I believe leads to a drain of around 15 kWh per day. I would love to be wrong about this, but as far as I understand it, running any sort of appliance engenders this quite large "overhead" cost essentially independent of the load which you are actually using. For example, running an ordinary home fridge, a freezer or even a portable Dometic-type refrigerator, you could be actually using an amount of energy corresponding to less than 2% of your battery per day, but you will end up losing an amount closer to 20% of your battery per day due to overhead.

Maybe I am wrong. I hope so. Does anyone know a way to tap into the power of a model 3 battery without wasting battery energy at a rate of about 500 watts or so, i.e., almost 1% per hour. Would love to hear if people disagree or agree with this perspective.

 

[miimura]

Yes, you would have to keep the Model 3 awake, which increases background power consumption. However, I think your figure of 15kWh/day seems high. You should be able to keep it closer to 400W which should remain below 10kWh/day. I'm sure if you search you should be able to find some good figures somewhere here on TMC.

 

[Watts_Up]

Does anyone know a way to tap into the power of a model 3 battery without
wasting battery energy at a rate of about 500 watts or so, i.e., almost 1% per hour.
Would love to hear if people disagree or agree with this perspective.

You are right, unless you use the Camp Mode, after about 5 minutes
the car will go in sleep mode, making the 12 V plug unavailable.

Why not tapping directly the 12 V battery? (with a reasonable load, like 15 A max)
This is something I do to run accessories like a portable tire air inflator.

I have a Bluetooth monitor so I can check when the 12 V battery get charged by the internal DC charger.
Typically, the DC charger runs for two hours every 36 hours without waking up the car.

If you plan to have constant load connected to the battery, I would recommend to use
a voltage detector like the one used for dashcams, to disconnect the load with a relay.

 

[Watts_Up]

I got my toys and here they are being tested:

This is the type of system that I am considering to build,
to run my refrigerator during the day while charging the battery at night using ToU rate.

 

[rrolsbe]

My 2018 sleeps somewhere between 16 and 24 hours before waking up to top off the 12V AGM battery. The time awake is between two to four hours. I loose about four miles of rated range on average daily. To be clear, when the DC-to-DC converter is active and charging the 12V battery the car is awake. I think the awake power usage is between 200 and 250W. Since my car is 2 1/2 years old the battery might discharge while sleeping faster; therefore, requiring more often top offs?

 

[PhysicsGuy]

Yes, you would have to keep the Model 3 awake, which increases background power consumption. However, I think your figure of 15kWh/day seems high. You should be able to keep it closer to 400W which should remain below 10kWh/day. I'm sure if you search you should be able to find some good figures somewhere here on TMC.

Thanks. I agree. But then you would be wasting about 10 kWh per day (13%) in order to use 1.5 kWh (2%) to keep your refrigerator cold. I guess you could do it for a few days, but it does not seem that appealing or sustainable to me anymore.

 

[cali8484]

You can enable Sentry Mode to keep DC-DC converter running and my testing shows ~200W power consumption. Still significant overhead but much lower than Camp Mode. Another way is to have a small continuous load directly on the 12V battery terminals that keeps the battery from getting fully charged but it doesn't seem to be much better than Sentry Mode in terms of overhead and definitely inconvenient. In general, running DC-DC converter is pretty inefficient for small loads (<1KW) but may still be worthwhile in emergency situations.

 

[miimura]

Thanks. I agree. But then you would be wasting about 10 kWh per day (13%) in order to use 1.5 kWh (2%) to keep your refrigerator cold. I guess you could do it for a few days, but it does not seem that appealing or sustainable to me anymore.

As stated above, Sentry Mode could cut the overhead by half. Regardless, this only makes sense for emergency use, not for routine use.

 

[PianoAl]

I got my toys and here they are being tested:

View attachment 609285

Update: I tried discharging it until the inverter shut off (due to low voltage). That happened after about twelve hours with a 60-watt bulb plugged in (those numbers approximate).

I was surprised to find that it took 2-3 days for the charger/maintainer to finish recharging the battery. That's okay for my usage, as long as the power outages don't stack up.