The Case For DC Slow Charging

I really want to build a DC electric vehicle charger. Most people hear DC and think “DC fast charging,” because all the fast charging systems out there are DC, but that association is sort of backwards: fast charging must be DC, but DC charging need not be fast. In fact, I think there’s a very important use case for slow DC charging that’s totally unaddressed.

Some background: how does EV charging work? There are 3 components, really: the supply, the charge controller, and the battery itself. People tend to think of the box on the wall or the cable that came with your car (EVSE – electric vehicle supply equipment) as the “charger” but this is not so much the case: it’s basically just a plug adapter to attach household AC wiring on one side and the car on the other, with switches in the middle to make sure you can’t zap yourself at any point while handling the pieces. In reality, the charger is a bigger, more complicated box that serves two functions: rectification, and charge control. It rectifies the AC we pipe around our electric grid to high-voltage DC that can be consumed by the battery, and controls how much current is available for the battery to consume.

To support level 1 and level 2 charging (<2kW and <10kW AC), a charge controller is built into every EV on the market. A charge controller’s high-level job is to keep track of the power available on the supply side, the maximum safe power the load (battery) can handle, and pump the lower of those two numbers from input to output. Without too much detail, this charge controller box does rectification, power factor correction, DC-DC boost, and high-voltage DC switching. None of those steps is 100% efficient, and even 1% inefficiency at 10kW is 100W, so you can see how the physical size of the box would need to scale up to handle increased power beyond 10kW of charging.

That on-board AC charger is kinda big as it is at 10kW, so if we want to charge that same car at 100kW or even 300kW, it’s clear we can’t simply stuff a 10x or 30x larger charger in the car. A 30x larger box needs to go somewhere else not-on-wheels, and supply battery-ready high-voltage, high-current DC to the car instead, which is exactly how superchargers work. The charge controllers all live in that massive, loud gray cabinet off in the corner of the charging facility.

So now we know why DC charging is necessary if we want to charge fast. But why would we care if we want to charge slow?

Solar panels produce DC current

Firstly, solar panels generate DC current in the sun. Secondly, depending on how much sun hits them, they generate mostly the same VOLTAGE, but with extremely variable CURRENT available. For example, a 3kW solar array will be able to produce, say, 300V 10A in direct noon sun, but maybe only 5A a couple hours later. That’s fine if we attach a load to those panels that’s smart about detecting and consuming exactly as much power as they’re able to generate in-the-moment. Grid-tie inverters do just that – they pump into your AC mains exactly what the solar panels can generate at a given moment, and anything excess above what your household is using gets “exported” to the grid as current flowing out of your house. In other words, the grid is used like a big battery to make up the difference, positive or negative, and if you’re lucky your utility will charge or pay the same rate either direction.

What if it won’t, and you need to use the energy you generate or lose it? This is one use-case for Tesla’s powerwall and its various competitors: charge up during the daylight with your solar panels, then use that power later on when you need it. The powerwall has a charge controller that can keep track of what’s available from the solar panels and use it all.

But you already have a big battery that you’ve already paid for: your car. Why can’t your car do that same thing – take all the power your panels can generate, to use later [when you need to drive places]? The issue is that, if you do the traditional thing and use an inverter to make AC from the solar array, then the onboard charge controller of the car, there’s an information disconnect in the middle. The car’s charge controller doesn’t know how much power your panels are making: it just assumes a number for available power based on what kind of plug its attached to. Similarly, your (off grid, I’m assuming, since we don’t want to export) solar inverter doesn’t know anything about the load attached. If we consume less power than it can make, all is well, but we’re wasting some. If we consume more power than it can make, the system will quickly shut down to self-protect, and the car stops charging.

What we want is a charge controller WE have full design control over: one that can take DC from the panels, figure out exactly how much power is available, then push that exact amount of power into the car. As a bonus, high voltage is relatively easy to design for, where high current is what requires lots of physical space and cooling. We wouldn’t need to do any power factor correction (no AC involved), and DC-DC conversion could cover a very small voltage delta, making the circuitry compact and efficient as well. Simply stringing 10 or 12 panels together gets us close to the pack voltage at 400V, so our DC-DC converter really only has to control the current into the battery.

In other words, if we could design a box to “DC slow-charge” from a solar array, it would enable extremely compact, cost-effective, and energy-efficient solar harvesting straight into the car. Pretty neat, huh?!

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