In the world of electric cars, some people talk about an idea called "vehicle to grid" or V2G. Renewable energy's biggest challenge is storage -- wind and solar only come at certain times of the day, but we need electricity all day. The V2G hope is to use all the batteries in electric cars as a means of grid storage.

Initial V2G plans just involve electric cars that are sitting plugged into charging stations. They would fill up, as they always do, but mostly during the night when power demand is lowest. They would also charge up, if plugged in, after working the morning commute.

Then, if the grid had a shortage of power, it could send the message out, and ask cars to send power back out of their batteries back into the grid to make up the shortfall. This could help handle the top peak loads, for which there is simply not enough generation capacity available, or it could also reduce the need to fire up "peaker" plants that burn fossil fuels.

Storage for the grid is challenging, and many are working on using grid-based batteries as they get cheaper. Other options include pumping water back into reservoirs (one of the top choices) or fancier approaches like compressing gas in underground chambers. V2G suggests using the big fleet of roving batteries.

Robocars add another dimension to this. Unlike regular cars, if there is a major power outage and they are not in use, they can drive themselves to a connection point to sell their energy. All the idle cars can get in the game if there are enough connection points in the right places. Energy can be bought at night for 6 cents/kwh and sold at high peaks for 30 cents/kwh or more.

So many people have mistakenly identified this as a possible win.

Car batteries vs. stationary batteries

A number of factors work against this idea:

First, car batteries must be as small and light as practical. All weight must be carried around. This is why they are all Lithium batteries today. Grid batteries don't care about weight or size or safety in a collision. They care only about cost, and in particular the cost per lifetime kwh. Lifetime kwh is the number of kwh you can put into and get out of the battery before it has to go out of service.

It turns out that even though weight is unimportant for grid batteries, lithium batteries are still the most cost effective technology thanks to their falling cost. In cars, cooling in a compact space is a big issue. (Even worse for cars that are not moving to gain air cooling.) Some other technologies should be better for large facilities, such as flow batteries (for predicted peaks,) zinc batteries and advanced lead-acid. There are also some interesting technologies in the lab which promise much lower cost.

In cars, we care not just about weight and size, but also range. Range is precious, especially in human driven cars. Having paid to put it there, we are very reluctant to give it up.

Batteries are, for now, a consumable resource

The clearest problem for V2G is that all batteries today are a consumable resource. They have a lifetime, because they degrade slightly with each kwh that goes in and out. Their capacity reduces until it gets too low to be acceptable. In a regular car, the effect is immediate, because people crave range and feel their car is much less valuable with lower range. In a robotaxi, the lowered range reduces the amount of time it can operate in a day. With a grid battery, it barely matters until capacity is so low that the real estate to house it is no longer justified.

Degradation is not completely linear. The faster you put energy in and out, the more degradation there is. If you discharge a battery all the way to the bottom, or charge it all the way to the top, lifetime is shortened. Car driving, particularly with jackrabbit starts and Level 3 supercharging, causes more degradation than grid use, so the real answer is complex but the main point remains true.

If your car provides power to the grid it uses up some of its lifetime. Unless the battery has "surplus lifetime" because it sits in a car that is very rarely driven, V2G is hard to justify.

Do some simple math. Imagine a car with a battery good for 160,000 miles. This means it will roughly store and provide about 40,000 kwh in its life. Typical human driven cars will go about 10,000 miles/year (2,500 kwh) while taxis will go 60,000 miles per year (15,000 kwh.)

Say the car is providing power to the grid at 6kw for 2 hours/day for 208 days -- I have chosen that because that's also 2,500 kwh per year which makes the calculation obvious. Now the battery is wearing out twice as fast. If the one battery would last the life of the car, now you need to buy two batteries.

Instead of buying two batteries, it makes more sense just to buy the one battery, and put the money of the 2nd battery into a grid battery -- or more likely, a share of one. Because of the lack of need to worry about weight, volume and crashes, you can get more for your money in a grid battery. Yes, two batteries requires more interest on capital cost, but that should be offset by the lower cost of grid batteries and the cost of swapping.

(There is one factor here in the other direction which is harder to predict. The future battery might be significantly cheaper than it is today. That has been the pattern so far, though most expect a floor to appear based on raw materials cost, but it could also do well. If you're sure of this, using up your battery faster at a profit could make sense.)

A battery in a car is not yet a resource that is just sitting there unused, wasted if you don't take advantage of it. It's a consumable resource, and it does not make sense to burn it up on grid energy when it's designed for something very different.

Some day, we hope to have battery designs that don't degrade with use. When we do, then a car battery just sitting there can be a wasted resource waiting to be exploited. We'll want all our batteries to be of this new design. This does not alter the other issues discussed.

By the way, a great use for depleted car batteries is transfer out of the car to grid storage. A half capacity battery is a big waste in a car, but no problem in a grid facility, until it gets so bad it's cheaper to recycle it.

In the taxi, which is using up far more battery capacity per day, there is even less incentive to use up "spare" capacity on the grid. For a taxi fleet operator, building a grid storage unit is something more practical than it is for an individual. As battery lifetimes increase, once they exceed the life of the car, there would be "spare" capacity to sell off at a profit. For taxis, robotaxis in particular, the battery will not last as long as the rest of the powertrain, and so using it for V2G means replacing it sooner.

If that's not enough, though, it gets worse.

Time of selling the power

The next problem is that the peak demand on the grid tends to run from 3:30pm to 8pm. That's the combination of the built-up heat of the day needing air conditioning, and the homes coming online while the offices are still running. After 8pm it drops quickly until about 7am. The only time V2G is likely to be wanted is in that peak zone, especially the top period from 5pm to 6:30pm.

Maybe you've noticed the problem. This is the peak of rush hour. This is the time when the cars are all out on the road. And the time before it is the time where people want their cars to be fully charged because rush hour is coming. It's the last time you want to drain your car to help the grid.

Yes, there will be some cars that are able to sell electricity during the peak:

• Vehicles that are offline for maintenance
• Long range cars which are sure they will not need their range that day
• Privately owned cars which know they won't be driven in that rush hour
• Cars that have gotten home from the rush hour drive and have extra power left for the later section of the peak.

Of course to do so, they must be plugged in to a charger which has the necessary inverters to convert their power to grid power (220vac in the USA.) I haven't even talked about the cost of all that special equipment at the charge/discharge stations.

A decision to sell the energy in a car's battery is a decision to decrease its range for that day. It has to be rare, otherwise you put a wastefully large battery in the car.

Super peak

It could make a lot more sense to use V2G during the "super peaks" -- those few hot days when the grid runs out. Building capacity to use just a few days a year is expensive, be it peaker plants or grid batteries.

Here the problem is that it's also expensive to put a grid-quality inverter at a lot of charging stations to only use a few days a year. Here's where robocars are a win. You don't need to build a lot of fancy inverters. Instead you build a few garages with the right grid intertie equipment (possibly using the DC direct from the compatible cars) and then you dispatch the robocars there on demand. It does mean longer wait times at rush hour, but if the price offered for the electricity is high enough, it could be worth it.

Time of charging the battery

One last issue -- if the rest weren't enough -- is a future one. Our dream is of a mostly solar grid, with so much excess solar that we need to store that extra energy for use in the peak and at night. While cars don't drive all day, they day is when they do most of their work -- and that's the only time excess solar will be available.

Cars instead prefer to charge at night, when the power comes from baseload -- nuclear, coal, geothermal and some wind. Hydro also runs at night as needed, but since hydro and natural gas can be turned up and down on demand, they usually are best for the loads of the day.

Update: Super-long lifecycle batteries

My friend Carl points out that battery systems are getting better, and eventually will have extremely large numbers of recharge cycles possible before end-of-life. This would change the equation enough to make V2G practical, but not completely. In that world, there is "spare" capacity in an existing battery because it will not lose much lifetime by feeding the grid, and so cheap night energy can be sold at a profit at 5pm. But only, as noted above, if the car is not planned for use in that evening rush.

In today's world of human cars, a surprising number of them are not in the evening rush. With enough profit incentive, people might give up their range on days when they are pretty sure they don't need it.

The bad news is that in the robotaxi world, there aren't many "spare" cars. While I don't think everybody would convert by giving up car ownership and using only robotaxis, for those who did, the fleet will be in full utilization driving at the rush hours. I say full utilization because even though you only need enough cars for the annual peak, and the daily peak is a bit less, the size of your "idle but on call" fleet is what governs how quickly you can serve a passenger -- their wait time. Short wait times, which come from having every car ready to serve, are a big competitive factor once a city gets competition.

So there's no such thing as an idle robotaxi which is ready to give up its energy, unless you have a majorly overprovisioned fleet. However, as the peak goes on and traffic starts going into decline, a fleet can safely take cars out of service and let them sell their extra energy. This would probably happen around 6:30pm or so, but that's just a guess.

On the plus side, out in the country, there will be more owned cars and no robotaxis, and these cars, once it is known they are not driving any more than evening, could sell their energy.

So why the excitement in V2G?

So what factors keep people thinking about V2G? The arguments I made about cost are based on a more mature world of grid batteries. Today, car batteries are the subject of volume production and high research, which is making them cheaper than other batteries. Factory capacity is in place and it's a (high end) consumer technology, while industrial technologies can end up costing more even though their raw cost is cheaper. This has driven the cost of Lithium batteries down so that they now beat lead-acid in lifecycle cost. (Lead acid, due to its maturity, is recycled extremely well compared to other technology.)

There may be some attraction for those who want to use an electric car as storage not for the grid, but their house. People have a strong (mostly irrational) desire to be free from the grid. If they have solar panels on their houses, they delude themselves into thinking they are running their car on solar energy.

A storage battery that disappears during the day is not a very good solution. It's unlikely to be there at the peak times when your power needs are high (air conditioning) and energy is expensive. I mean, it could help, but it's a sucky solution.

Perhaps of more interest is the ability to use the car as emergency backup power for a home when the grid is down. That's more reasonable, though it requires inverters and a complex transfer switch to protect the grid. If you do this, you will be shortening the lifetime of the car battery, but this time it's well worth it.

The real efficient solution is not yet practical. That's neighbourhood grid storage. Blocks of houses could pool the money they are not spending on replacing their car batteries faster to install a neighbourhood grid battery that would offer both cheaper energy during the expensive part of the day, and emergency backup power. By sharing the cost of inverters and installation, it would be the win -- if the power companies would facilitate it.