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Comment by Rychard | Hacker News Reader

I wonder what the efficiency losses become in this configuration; using the grid to charge a battery to charge a vehicle.

A quick google query says that efficiency of L2 residential EV chargers is somewhere in the neighborhood of 85%, and for DC-DC chargers the efficiency is in the order of 90%. I don't know nearly enough about this to state anything definitively, but it sounds like the efficiency losses start to accumulate pretty quickly.

I wonder what the break-even point is in terms of cost compared to an average ICE vehicle.

Apply a healthy dose of "Don't let perfect be the enemy of good enough" of course, but I am curious about the economics at play.

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It sounds like you almost have enough numbers to calculate the ball-park efficiency.

Grid -> condo battery ~85% (basically an L2 charger)

Condo battery -> L3 charger let's say ~96%[0] because it's a high discharge rate

L3 charger -> car ~90%

It's simple math now: 0.85 * 0.96 * 0.90 = ~74% end-to-end efficiency.

[0] https://www.researchgate.net/figure/Energy-efficiency-map-of...

I don't think the efficiency can be partitioned like that. An L3 charger is supposed to output the same electricity that goes directly into the car's battery, with no additional conversion. An L1 or L2 charger has no conversion inside the charger - it's just a wire and a little electronic tag that says what the amperage is.

The losses are different, but still exist.

In the L3 case there is still a voltage conversion because the input voltage needs to be higher than the battery voltage in order to put energy into the battery. There will be a loss at that stage.

But a big part of the loss during L3 charging is in the battery chemistry. The relatively high charging rate is less efficient than slower charging rates. The high powers involved also generate heat, which often requires active cooling which is an additional reduction in plug-to-wheel efficiency.

In the L1/L2 case there is a conversion from ~240v AC to DC but it's inside the vehicle. Plus the battery chemistry charging losses, but they are much lower than in the L3 case.

These are all ballpark numbers anyhow, so you are welcome to suggest difference values which you believe are supported.

Considering ICE is less than 40% thermal efficiency of the fuel, I'd say it works out just fine.

But fast charging in general should only be used when there's no voice - it's a lot harder on the battery. Better to slow charge overnight.

With something like this, throwing in solar makes a lot of sense (IMO). There are already batteries involved so you could use solar as the primary recharge mechanism with the grid as a backup.

A single apartment complex could almost certainly not cover enough space with solar panels in order to charge a battery to accommodate this.

That's the neat thing, it doesn't have to. Every kWh that goes into these batteries is a kWh not purchased from the grid. The payback for the solar panels would very quickly add up.

In fact, trying to 100% charge the batteries via solar would not be the most economical way to operate as then you are just wasting money sending power back to the grid.

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