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Comment by xattt | Hacker News Reader

Can you talk about why DC fast charger cabinets are so large and what’s inside of them?

They contain huge AC/DC inverters. The car has a small, usually 10 or 20kw inverter built in that handles level 1 and 2 charging. Usually about the size of two textbooks per 10kw.

When you're at a DC fast charger they still only have access to AC power from the grid, but you want 250kw, 350kw of DC power to charge the car. Not only do you need 20x the inverters, but because they don't have to fit in a car and move around, they're fitted with larger, more efficient cooling.

I wonder why cars don't just use the large 3-phase inverter they already have to drive the motor as a charger.

One of the early Renault Zoé with the Continental power train did. It had up to 43kW charging on AC, which was a bit unique. So unique that very very few chargers offer more than 22kW AC.

I also heard that it was very noisy during charging.

Level 2 connectors are only rated to ~19kw[1] so I imagine that must have had it's own custom connector, and I don't particularly see the 80amp chargers becoming particularly common for home use, newer homes generally only have a 200amp main breaker, so 2 of those would take 100% of your continuous power load [2]. I have a 48amp charger and if I need faster than that I go to one of the local fast chargers and worst case wander around a store for 15/20 minutes.

[1] 80amps at 240v, at least in the US, not sure about the type 2 connector used in EU

[2] code in the US says you should only draw 80% of the rated current if the load is continuous

There are a handful of them at businesses and it's wonderful if your car can support it. We get about 60mph on those chargers, so even a quick food stop gets you 20-30 miles of range. 50% charge in about 2 hours!

Most public AC chargers around me are 6.6kw and it's truly a useless amount of power. It's about 20mph, so a 20 minute food stop is giving you ~7 miles of range. To get a 50% charge takes about 6 hours. Even in a 9-5 full work day you're not able to get a full charge.

Yeah, I generally assume the AC chargers at business's are for employees cause it's just not worth it to plug in if you're running in to shop for less than an hour.

I had one of these and indeed it was noisy. But only marginally noisier than a more recent Zoe.

I've wondered this too. You could set the gear in neutral, connect the AC power directly to the motor terminals, and then basically run the inverter in regenerative mode.

It looks elegant on paper, at least, but I'm sure there are reasons why it's not done. Maybe due to safety considerations, maybe because the inverter can't handle that much continuous power, or maybe it's just cheaper to have a few big supercharger inverters than to slightly increase the cost of connectors on every car.

It seems like thermal dissipation is the issue based on the grandparent post. An EV duty cycle using the built-in inverter won’t drain the battery in the same amount of time as high-speed charging would charge the battery.

Perhaps there are also weight optimizations being made that limit bidirectionality of the inverter.

Two great videos on the subject:

https://youtu.be/RMxB7zA-e4Y

https://youtu.be/TcoLCTkM0ys

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