I was reading an article on the prospects of using electric vehicle batteries to help smooth out grid demand, especially in California. That got me thinking.
For this to work, the car charger would have to control somehow if it was providing power to, or asking for power from the battery in the car. How does this happen? I’m not asking how the electronics talk to each other, I mean how do we control which way the electricity goes.
My first thought is the electricity would flow towards the side with lower resistance (or voltage or something like that, I’m no electrical engineer.) But then I thought about my wireless headphones. They are an example of one battery charging another and the charging cradle will happily drain its battery down to a very low state in order to charge up the actual headphones. Perhaps the wireless headphone thing is a bit of smoke and mirrors and the battery in the charging cradle is actually “stronger” even when it says it is nearly depleted…
Anyway, how do we control which way the electricity flows?
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the simples answer is, if you give more voltage than a battery has, it charges, so this way you can take the voltage from one side and increase it to the other side
For dumb batteries power flows through potential levels, like how two water puddles will level off if connected.
However, car batteries (and presumably your charging cradle) use sophisticated charging circuits which manage the voltages (which will also manage the current flow). They can use things like “DC-to-DC boost circuits” to change the potentials and change which way current and power flows. There’s also circuits that do DC -> AC -> higher voltage AC -> DC, lots of ways to change up voltage.
These are active electronics not passive devices. The electricity isn’t going to follow its normal rules and flow according to resistance because there are active electronics in there changing the paths available.
The actual implementation in the car charger is going to be complicated but a [Boost Converter](https://upload.wikimedia.org/wikipedia/commons/thumb/3/39/Boost_conventions.svg/2880px-Boost_conventions.svg.png) should cover similar points for you.
A boost converter takes a lower DC voltage and bumps it up to a higher voltage. Normally current won’t flow from low to high voltage only from high to low so there’s some trickery required here. That switch in the bottom center is a transistor and is controlled by a tiny processor which uses it to charge up the inductor then opens up forcing the inductor to ram its current into the capacitor raising the voltage, then it closes again to repeat. It does this tens of thousands to millions of times per second to get just the right voltage on the output
If I want to be able to bump up the voltage from a 3.7V battery to charge a phone at 5V but also receive 5V and bump it down then slap a [buck converter](https://upload.wikimedia.org/wikipedia/commons/thumb/5/52/Buck_operating.svg/1280px-Buck_operating.svg.png) pointing the otherway and put a switch in front of each one
Now with two fairly simple circuits you can take 5V in and buck it down to charge a 3.7V battery or take 3.7V in and boost it up to charge a phone at 5V
There would need to be communication to the processor controlling these two circuits so it knows what you want it to do, but it is able to achieve it.
Doing it with car chargers likely involves fancier inverter circuits as they’re going from AC to DC or DC to AC, but the core concept of “use inductors and transistors to make electrons do what *you* want” remains the same as is critical for basically everything you use that isn’t a toaster
Other people have already given good answers, but I’ll also add that at the most basic level, Kirchoffs laws state that you can basically combine power sources together based on the net flow. So if you have 10 amps going left, and 5 amps going right, and 2 more amps going right, you can treat that source like (10-5-2) 3 amps left. But in this case you have to use other devices to force the electricity to flow in the direction you want.
Many EVSEs (electric vehicle servicing equipment aka car chargers) are connected to the internet or the cars themselves are. Depending on where you live, currently, almost every car or EVSE has an option to delay charging until a certain time when the grid is under less load. Power companies incentivize this by charging higher rates during peak demand and significantly lower rates late at night. So to control when your car is charging you can either set up a timer which automatically starts charging your car if you have it plugged in at a certain time, or have it controlled the same way some grids can control your thermostat, it must hte connected to the internet.
Contrary to popular belief, power companies don’t want you do use as much power as possible. When they design the grid, they have to design it to supply power during the hottest heat wave and during the coldest winter storms, they can’t design for what the average load would be during the year so there is a shit ton of excess capacity late at night. That’s why many electricity intensive industries operate at night and that’s when they want you to charge your car.
So I believe the answer is the net current flows from the side of the circuit providing higher voltage to the side of the circuit providing lower voltage.
You use active electronics to influence the voltage presented by each side of the circuit and therefore control the direction of the current. From those buck and boost converter diagrams, it appears you get clever with capacitors, diodes, and switches to build up higher voltages on one side of the circuit or the other. I’m sure there’s other ways as well.
Is that correct?
– edit :a word
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