Hacker News Reader

Comment by RatchetWerks | Hacker News Reader

I feel that bidirectional charging is the EV feature that most consumers don't know that they want.

My current vehicle is fine, but when I'm in the market.

I want an EV/PHEV vehicle that has enough power output to entirely power up my home, and serve as energy storage for solar. It avoids the need for a IC generator, and a power wall.

They are a powerwall on wheels. When then battery age out in 10 years, you swap the vehicle.

Gensets and in-home battery storage seem very redundant, when a bidirectional charging PHEV can do both.

I've entertained the idea of picking up a Prius and hacking the electronics to act on this.

View on HN

> I've entertained the idea of picking up a Prius and hacking the electronics to act on this.

All the NiMH prius' are unsuitable for this. Total energy storage is only about 1kWh and usable energy storage is more like 0.3 kWh (ie. it wouldn't even boil one kettle of water).

The hybrid-ness is still useful because it lets you get a boost of acceleration to get up a hill or overtake some cars - and the battery is very powerful - ie. it can get all that power out very fast - it just doesn't store much energy.

They probably meant a Prius Prime

PHEV batteries are still pretty small. 14kWh in the Prius Prime for example. This will power an average US house for about 12 hours.

but remember it also has a gasoline engine - so after 12 hours you can always start the gas engine to power your house for ~a week on a tank of fuel.

The prius will do that by default with its current software if left in "ready" mode. You just need to design and add the ~300V DC to 120/240 V AC inverter.

Vehicle by definition can't be connected to the house constantly. I can imagine this lowers need for house battery size, but not need for it's existence. House power consumption is very random.

There is typically good connectivity between houses. That means even if your EV isn't home, the other EV's in your street can send your house a lot of energy.

Unfortunately anti-islanding rules from power companies generally disallow you using the power lines between your house and a neighbours while the grid connection is disconnected elsewhere.

These rules could be adjusted though - although it would take a lot of systems design work to ensure power system stability and re-synchronizability even with every possible combination of islands.

It's more about safety than reconnection: when repairing lines, it's much more manageable to "lock and tag out" the upstream side than all the distributed generation, especially in emergencies.

Any lines being worked on will be shorted (ie. all conductors joined together and to ground).

As long as that is always done, islanding doesn't present any human safety concerns.

That is normally done anyway, because plenty of utility breakers will automatically try to reconnect every few minutes (allowing some grid scale failures to repair themselves with no human involvement).

Doesn't matter. Homes tend to need power when people are at home. The car is - by definition - at home when its driver is at home.

The point is, during the day while you’re at work your solar panels have nowhere to send their electricity.

But when the vehicle is connected, the user is typically at home. So there's a convenient relation there.

House power consumption is random because it's not designed to be predictable. It's trivial to have a washing machine and heating that prioritize energy consumption whenever the EV is connected, or grid energy is affordable.

Unless you have more than one person in the house…

But in general, yeah, when you _need_ the battery, maybe you just need to figure out that you don't need to drive that day/hour.

The other advantage of a car, is that it would allow you to bring in power "from the outside", go to somewhere where the grid is still working, charge up, then come back. Of course, you'd hope to be back on power before this was needed!

For many people, home power use its highest when they and their vehicle are at home.

Everyone brought up really good points. My general logic is that the average American consumer gets a new car every 10 years just due to maintenance/service burden. You might as well get a "free" genset/powerwall upgrade with it anyways. The car should be viewed as an energy source that happens to move.

For me the powerwall only provides value when restoring/storing power, not for peak shaving. My logic is that eventually once all the legalities/orchestration is figured out. The net value on peak shaving will trend to zero.

Drive to work, plug into grid. You pay the grid for cheap "sun" power at work. While at work, your solar panels at home generate revenue by charging near by EV's via grid (or your second EV). Drive back home. Plug into house and supply the grid with power at night time.

PHEV's are an added bonus, since you can use it as a genset during extended power outage. If you want to go 100% "off the grid", then have 2 EV's that you cycle though. One that acts as storage/energy backup, other as a vehicle.

"you swap the vehicle" -- that's a large cost associated with the battery getting old earlier than it otherwise would have. Have you factored this into your math?

If you have a $40k EV expiring two years ahead of schedule, the added depreciation cost should be in the thousands. Remember that battery lifespan isn't measured only in years, it's primarily measured in cycles.

Dedicated LPO home batteries can be gotten for only $110 USD per kWh if you live in a place with no tarrifs or taxes on it. This makes a lot more sense to me.

The only way I can see it making sense to use EVs for this, is if the battery getting old is rarely the main reason for throwing away old EVs. If that is true, then sure, use EVs as a home battery, since you're not getting a massive depreciation cost from doing so. But is it true?

I don't really think consumers want or need this.

The battery chemistry used in most EV batteries are not really optimized for a lot of cycles, and they're somewhat expensive to replace, so you're much better off just buying batteries that are made for the purpose. You also have fairly limited inverter capacity in most EVs, as the inverter installed is scaled to run the cars AC and infotainment system, which doesn't really need that much.

You also have to deal with either having beefy automatic/manual transfer switch or separate isolated circuits to prevent back feeding the grid electricity in an outage or during maintenance.

There's a lot of people repurposing used Nissan Leaf-batteries for what you're describing.

The numbers in the post are 2.2 and 3.6 kW. Even 2.2kW would cover virtually all of my use.

I don't want to buy a battery for the purpose because it doesn't make sense economically. If I could get a car with this functionality though for a similar price as cars without it, that would be a big bonus.

I'm not going to pretend that there aren't edge cases and personal situations that would make it viable, but my initial comment was on consumers as a whole.

And we're talking about averages. So sure, if you have gas boiler you probably could get by with a 2,2kW or 3,6kW inverter, but just powering a simple kettle would likely trip a fuse. You can get around it with just being a little conscious about what you power up, and when you power things up, but I'd argue that most people don't want to deal with that.

And remember, if you're grid connected you either have to have a transfer switch or separate circuits if you want to use this as a backup when the grid goes down. It complicates things a whole lot.

> so you're much better off just buying batteries that are made for the purpose

Or even just used EV batteries. I personally don't think that battery recycling is as big of an issue as some people say. But, if I'm wrong, a cheap worn-out EV battery with "only" 50% capacity is still very useful as a home backup.

> When then battery age out in 10 years, you swap the vehicle.

For this use-case it'd probably be best to go with LFP so you get more lifetime out of the battery.

Though even with Li-ion, I think a modern EV with a big battery will last much longer than 10 years. My old Soul EV with a tiny 27kWh (thus several charge cycles every month) is soon over 10 years and has only gone through half of its cycles at most.

And I would expect that the price of battery replacements for a car will be reasonable in 10-20 years, so should hopefully not be necessary to replace the whole car.

> They are a powerwall on wheels

This feels like it'll be a problem when your car has less charge than you need one day because it's been powering the house.

is this about backup in a power outage or earning money buying when pores is cheap and selling when it is more expensive? There is a big difference as in an outage you can conserve power and watch the meter to goeto a nearby charger as needed. In a money scheme you don' know when you are low, but it isn't hard to have normal range left but no long trips

The Tesla virtual grid typically sets your minimum charge to 50%, so you always have at least ~150 miles of range.

Teslas "virtual power plant" is only for Tesla Powerwall. There's no V2G capabilities in any Teslas to date, and I don't think there's even hardware support for it. The Tesla Cybertruck has V2L capability, so it might have the necessary hardware to do V2G with some software updates, but I kinda doubt it.

The Tesla "virtual power plant" does reserve some capacity for you, and technically you can charge your car with that. A single Tesla Powerwall is "only" 10kWh, and with inverter losses and such you're likely seeing closer to 8kWh of usable energy if you charge your car with it. You're probably much better off just using it to run your appliances so you mitigate some of the losses with more inverters in the mix.

Which is probably reasonable for most people, but still means if you go on a sudden road trip you are have to add power rather soon after leaving. (or you need to plan a head and go to charge only mode before leaving)

> powerwall on wheels

Yeah, a portable battery: I've heard of people using an old Nissan Leaf to power their off-grid home (for low kWh needs).

Drive to nearest town, charge up, drive home and use car battery to power home.

Reply on news.ycombinator.com