One of the main reason is concerns about degradation. Battery capacity and performance degrades as the number of discharge cycles increases. And EV users are particularly sensitive to that degradation (see issues with early Nissan leafs for example). And replacing an EV battery is currently really expensive (often exceeding the entire value of the used vehicle).
It does look like with good battery management, the lifespan can be considerably lengthened however. Particularly, if you cap the max charge to 80% and min charge to 20%. This, plus active cooling is probably how Tesla has managed to achieve such good battery longevity. I wouldn't be surprised if in a few years, EV owners get more comfortable with it.
Another thing to consider is that right now the 'peak' grid usage (when EV discharge would be most useful) usually occurs in the evening, probably when a lot of folks are still using their EV for commuting. So rolling out the infrastructure to do two way charging is probably not worth the cost because utilization might be pretty low. I.E. What % of EV drivers would have enough spare battery capacity to discharge a non-trivial amount right after their commute. Probably not many, at least not until we either see a big increase in EV capacity (which will probably happen as batteries get cheeper).
> What % of EV drivers would have enough spare battery capacity to discharge a non-trivial amount right after their commute.
It would probably still be pretty high. Total ranges are in the hundreds of miles, but the average commute is 16 miles, so most of the capacity would still be there when you get home.
The real question is going to be whether it's cost effective. If you do that and then have to replace the battery twice in ten years instead of once, is that actually more profitable than having one battery for your car that lasts ten years and another which is purpose built for grid storage and also lasts ten years? Basically a question of time value of money vs. whether being built for purpose is sufficiently more efficient.
The point I was trying to make was that 2 way charging infrastructure is not cheap[1]. And right now battery storage is really only cost competitive as a peak flex provider largely because you need to pay for the electricity that goes into the battery, i.e. a battery does not generate electricity, it merely allows for price arbitrage. So you would have to try and make your money back by only selling during a 2-3 hour window each day, and that window happens to coincide with prime commuting hours. So even if you weren't commuting during that time, ever, you would be making about ~$1 per day[2] and $365 per year. Assuming $5k in infrastructure costs you are looking at 13 year payback, and this ignores battery degradation entirely, among other things[3]
[1] Lets say $5000, which is pretty reasonable considering you would need a new charger, an inverter, and would probably have to modify your vehicle.
[2] You are arbitraging, so lets' say you but at $0.08/kwh and sell at $0.16/kwh. And in 2 hours you can discharge 10-15 kwh, which puts you in the $1 dollar per day range.
[3] Also assume the utility will pay you retail rates for you kwh.
> Lets say $5000, which is pretty reasonable considering you would need a new charger, an inverter, and would probably have to modify your vehicle.
That would presumably be a lot less if the cars start having that designed in. They're already using AC motors, so they already have inverters, the question becomes how feasible (and efficient) it is to have them produce mains power.
(The obvious step before they do that would be to add a "number of charge cycles" maximum to the battery warranty, if they don't have that already.)
> So you would have to try and make your money back by only selling during a 2-3 hour window each day, and that window happens to coincide with prime commuting hours.
This is also likely to change somewhat with the rise of solar. Right now the demand peak starts around 4PM, but the sun is generally still out then. If a significant fraction of generation capacity becomes solar then that means no lack of supply at that time and the real price surge happens as the sun sets, i.e. once most people are already home. So the number high demand hours decreases, but that moots the commuting issue. Meanwhile the remaining demand is even higher because not only do you have high demand, you have a reduction in supply due to the loss of solar capacity, which means the price differential to arbitrage may increase from what it currently is.
It does look like with good battery management, the lifespan can be considerably lengthened however. Particularly, if you cap the max charge to 80% and min charge to 20%. This, plus active cooling is probably how Tesla has managed to achieve such good battery longevity. I wouldn't be surprised if in a few years, EV owners get more comfortable with it.
Another thing to consider is that right now the 'peak' grid usage (when EV discharge would be most useful) usually occurs in the evening, probably when a lot of folks are still using their EV for commuting. So rolling out the infrastructure to do two way charging is probably not worth the cost because utilization might be pretty low. I.E. What % of EV drivers would have enough spare battery capacity to discharge a non-trivial amount right after their commute. Probably not many, at least not until we either see a big increase in EV capacity (which will probably happen as batteries get cheeper).