Ev used as battery backup, help I have questions?

Solar, wind, hydro and other non-fossil sources of electricity.
Post Reply
hal2000   1 W

1 W
Posts: 57
Joined: Sep 20 2012 11:04am

Ev used as battery backup, help I have questions?

Post by hal2000 » Sep 15 2017 12:34pm

Damien Maguire has posted a couple videos using the high power D.C.battery output from his converted bmw to run many of the loads in his house straight from dc power without running the power through an inverter.
Damien says that many electronics accept a variety of voltages and use switching power supplies to do this. He demonstraights charging a cell phone, a flat screen tv an angle grinder, cordless tool chargers, etc. I am vary confused. why do i need an inverter in my off the grid cabin when I could just run on D.C. ? Damien demonstrated many items that would function on D.C. power, however the fan on his wielder failed to function while his vacuum cleaner ran just fine. I am left both excited for the possibility of creating a way to use my ev as backup power but mystified as to the theory and limitations of such a system .https://youtu.be/QlmmerXOkuE?list=PLPHK ... mlgI3u8BOQ I assumed an angle grinder and vacuum cleaner are ac motors, can you run ac motors on dc? is this only applicable in an emergency short term application? Is D.C. more dangerous in your house?
I am hoping the members here can provide me with enough background theory for this to make any sense to me, thanks!

billvon   1 MW

1 MW
Posts: 1779
Joined: Sep 16 2007 9:53pm
Location: san diego

Re: Ev used as battery backup, help I have questions?

Post by billvon » Sep 15 2017 2:35pm

>why do i need an inverter in my off the grid cabin when I could just run on D.C. ?

Because many devices (basically anything with a transformer or an AC motor) will not run on DC.

>I assumed an angle grinder and vacuum cleaner are ac motors, can you run ac motors on dc?

No. You can run universal motors, or brushless motors with inverters, on AC or DC. You cannot run any type of induction motor on DC.

>Is D.C. more dangerous in your house?

Yes. You cannot use standard breakers; they will start a fire if they trip. (Several Youtube videos about what will happen there.) In addition, there is a much greater electrocution hazard due to the different effect that DC has on your body.
--bill von

craneplaneguy   10 kW

10 kW
Posts: 758
Joined: Nov 27 2015 9:15am
Location: S.E. Idaho USA

Re: Ev used as battery backup, help I have questions?

Post by craneplaneguy » Sep 15 2017 9:20pm

With the proliferation of relatively low cost, and highly efficient, inverters, why mess with it? I was off grid for 28 years, back before much of the equipment now available was around or even dreamed of, so I had a mish mash of DC and AC things, and wiring. It was a PITA in retrospect, if I ever quit being grid tied (best of both worlds, as I already had power lines near my property) and went back to being off grid, I'd do it 100 % AC, through an inverter and keep it simple. Though just for grins, a few weeks ago my utility had a scheduled outage for 3 hours, while doing some major work. So I ran my computer off my Prius traction battery using a little inverter during the outage, just so I could say I still had power.

User avatar
spinningmagnets   100 GW

100 GW
Posts: 10652
Joined: Dec 21 2007 10:27pm
Location: Ft Riley, NE Kansas

Re: Ev used as battery backup, help I have questions?

Post by spinningmagnets » Sep 15 2017 9:38pm

AC breakers can be much smaller than DC breakers that are rated for the same number of volts and amps. This is because the AC has a sine-wave that crosses a zero point 60 times a second, so the "arc" is broken 60 times a second as you are moving the contactors apart.

A high-amp DC breaker will have a large arc the entire time you are pulling the contacts apart.

hal2000   1 W

1 W
Posts: 57
Joined: Sep 20 2012 11:04am

Re: Ev used as battery backup, help I have questions?

Post by hal2000 » Sep 18 2017 1:18pm

thanks a lot, how do you know if you have a universal motor? It sounds like the easiest safest option would be to route the traction battery through the same combiner box and dc breakers as i would already have for my solar panels then it would route through my inverter and on to my ac panel. What do you think is inside the Nissan vehicle to grid box that makes it so expensive?

billvon   1 MW

1 MW
Posts: 1779
Joined: Sep 16 2007 9:53pm
Location: san diego

Re: Ev used as battery backup, help I have questions?

Post by billvon » Sep 18 2017 6:48pm

hal2000 wrote:thanks a lot, how do you know if you have a universal motor?
It has brushes.
It sounds like the easiest safest option would be to route the traction battery through the same combiner box and dc breakers as i would already have for my solar panels then it would route through my inverter and on to my ac panel.
You need to drop the voltage. If you are talking about a Leaf, for example, that's 400 volts you need to drop to 12/24/48 volts somehow.

Probably a lot easier to use a 12V inverter on the 12V battery.
What do you think is inside the Nissan vehicle to grid box that makes it so expensive?
The CAN bus controller, high voltage relays and potential tester that is required by the ChaDeMo spec before it's considered safe to close the relays and expose the 400 volts to the outside world. I believe their V2G box also has an inverter (expensive.)
--bill von

User avatar
liveforphysics   100 GW

100 GW
Posts: 13951
Joined: Oct 29 2008 1:48am
Location: Santa Cruz, CA, USA

Re: Ev used as battery backup, help I have questions?

Post by liveforphysics » Sep 19 2017 2:15am

DC works great for about everything but refrigerators and table saws and AC units, but I've only been using ~116vdc, not leaf voltages which may get gnarly.

Likely most anything with a modern PFC stage front end would be fine, but just normal house switches and breakers on DC tend to fail quickly and sometimes impressively.
Each carcinogen vapor exposure includes a dice roll for cancer.

Each mutagen vapor exposure includes a dice roll for reproductive genetic defects in your children.

Each engine start sprays them into a shared atmosphere which includes beings not offered an opportunity to consent accepting these cancer experiences and defective genetics life experiences.

Every post is a free gift to the collective of minds composing the living bleeding edge of LEV development on our spaceship.

Matt Gruber   100 kW

100 kW
Posts: 1472
Joined: Feb 08 2007 11:02am
Location: New Smyrna Beach FL

Re: Ev used as battery backup, help I have questions?

Post by Matt Gruber » Sep 24 2017 9:45am

I don't have an E-car. if i did, i'd find out how many watts it will deliver to the 12v battery. if that is enough, i'd buy a 12v inverter. I'd run things as needed, not the whole house at once. Don't run a 3,000 watt oven to reheat a slice of pizza! Buy a $10 oven at walgreens that uses ~300 w. ETC. etc.

craneplaneguy   10 kW

10 kW
Posts: 758
Joined: Nov 27 2015 9:15am
Location: S.E. Idaho USA

Re: Ev used as battery backup, help I have questions?

Post by craneplaneguy » Sep 25 2017 8:43am

My PluginPrius can offer up about 1000 watts out of the smallish 12 volt accessory battery. That seems to be the limit of the converter, that works off the big traction battery. I have not bothered to investigate accessing the big battery directly, don't need to and besides you can do a lot with 1000 watts. When I plug my 12 inverter (Harbor Freight 400 watt continuous/800 surge, mod sine wave piece of crap, but it works perfect, pure sine wave not required for my Luna chargers) into the beefed up 12 vdc outlet (most car "cigarette lighter" sockets are 15 amp, I draw a bit over 17 amps when charging my 52 volt battery at 3 amps) as long as the car is "on", not running the ICE engine but just turned on, the big battery keeps the small one topped off. Eventually the ICE engine auto starts, runs at a most efficient RPM for a few minutes while it recharges the big battery, and then shuts down again. Prius geeks have reported that this process and the fuel it requires, shows that is MORE efficient then any other small gas generator, Honda etc. It's also the quietest and least amount of hassle.

I only use this capability when on the way to a ride, with ebike along, and when camped out. Once we had a scheduled power outage by my local utility and I needed to use my home computer (my grid tie system is battery less by design, so my power goes out when the grid is down, after being off grid for 28 years I like not having them and the outages are so rare it's not an issue) so I backed the PIP near the house and ran an extension cord into my office. The car never started in 2 or 3 hours, so yeah it works great and is a widely unknown advantage of driving a hybrid. That and being able to camp in them while having heat or AC all night while using very little fuel.

BTW: I just bought another HB inverter, same as my first one, for my work truck, and they are on sale right now, less then $20.00!

Post Reply