[stymaar]

> batteries next to these chargers that they charge "off peak"

I don't think that's what they'll do. Charging off peak means being able to store the entirety of the energy demand for the power station in a battery, which is going to be very expensive (assuming 20 cars charge during peak hours every day, that'd mean having to swallow the cost of 20 cars worth of battery per charging station. Good luck getting a good ROI with that).

Instead I think they'll just use the battery so that they never drain the full power of a charge when a car is charging. Drawing a megawatt of current 5% of the time is putting lots of pressure on the local grid, and it can be mitigated by having a battery with the capacity of a car battery that you charge slowly during the whole day (including during peak hour) and discharge fast when a car is charging (for instance, if in average you have 2 cars charging for 5 minutes every hour, you can draw 166kW continuously instead of having bursts of 1MW consumption).

[alkonaut]

> if in average you have 2 cars charging for 5 minutes every hour, you can draw 166kW continuously instead of having bursts of 1MW consumption)

You definitely need to have that to not load the grid with 1MW, but the question still remains what the capacity of the battery is. A charger that promises a 5 minute 1MW charge BUT which can only do it once per hour and then falls back to 200kW doesn't seem as special as a charger that actually charges a car every five minutes.

It's convenient to get going in 5 minutes. But the time you REALLY want the charger to be quick is when you are third in line to charge at that charger.

[stymaar]

I was definitely using simplifying assumptions to get my point straight here.

Setting the actual parameters for such systems is an engineering job, I just wanted to illustrate that the goal isn't going to have the charging station off the grid during peak hours thanks to the batteries, and more about managing the burden you put on the grid.

[alkonaut]

Yeah that's why it probably needs to be more than 1 charge in the battery. Unless you do N back-to-back charges during peak time, the charger isn't utilized enough. And to do N back to back charges you need about N car batteries as buffer.

[stymaar]

If you have full usage of your charger, then batteries are pointless anyway because you have steady usage no matter what.

But it's not a realistic assumption, at the very least the driver has to park, get out of their car, plug the car, spend some time on the payment interface, then unplug the car and leave.

So even in the maximum theoretical scenario where drivers are lining up at the charging station, your charger isn't going above 80% utilization. Using a single car battery, you can save 20% in terms of connection to the grid (you “just” need a 800kW connection instead of a 1MW one), and you aren't nearly as much of a nuisance to the grid as if you were having constant ups and down of 1MW.

In practice there will a be a trade off between how much you save in connection infrastructure to the grid and how much you spend on batteries, and this calculation will depends a lot on the usage pattern.

[alkonaut]

Yes it can't be used 24/7 for 5-minute charges because then the buffer does nothing.

But if there is never back-to-back charges then I'd argue it's also kind of pointless because when the speed most needed (when there is a queue) is when the charger starts going the slowest. The balance is to have N charges (say 3, 5 or 7) in the battery. That way you can churn through the peak with N charges (say 07-09) and then charge for several hours until the peak hour returns at 16-18 when you can once again at least serve the first few cars without falling back to whatever you can suck from the grid continuously.

[stymaar]

> then I'd argue it's also kind of pointless because when the speed most needed (when there is a queue)

The thing is: with 5min charging, it reduces the amount of situations where there's a queue at all.

> That way you can churn through the peak with N charges (say 07-09)

Again, 9 batteries is just an hour worth of energy if car are lining up. The goal of such battery is just to smooth the demand spikes, not to get though peak hour.

> The balance is to have N charges (say 3, 5 or 7) in the battery.

I suspect this will depend a lot on the expected usage of the charging station (an maybe adapted later on if the usage doesn't match their expectations), as I said above it's going to be a full time job.

[soco]

So do those batteries support fast charging AND fast discharging?

[Tostino]

Generally fast charging has been a much harder nut to crack than fast discharge. If you have fast charging you necessarily have fast discharge in my experience.

[mekdoonggi]

Yes. IIRC they are the same batteries in the cars.

[jannyfer]

Oh then that's like the battery swap idea but without the swapping!