How DC chargers work

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brendon_m
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How DC chargers work

Post by brendon_m »

I've been trying to work out how a DC fast charger works, as in internally not how they charge charge a car.
I've tried googling it but my Google fu seems to be failing me and I just keep coming up with "they charge with dc and not ac and it's fast..."

Do they just pull a heap from the grid under demand? Surely not because you would need massive cables to pull 150kw instantly or more with these new standards, wouldn't you?
Or do they have a high capacity battery inside on location? I don't think they are big enough?
So a bucket load of capacitors?
Or a combination?

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Re: How DC chargers work

Post by Rusdy »

Hi Brendon, 150kW is not hard to get (although not trivial). From Tritium user manual (albeit, 100kW station only), it recommends 50mm2 cable (150A feed).

Bunch of no-gooders broke the world record in Kwinana sometime ago, requiring 800A feed (half Mega-Watt). Too bad, I can't find the detailed info how much it pulled from the grid. I remembered it mentioned somewhere in this forum, but I can't trace it :oops:

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Re: How DC chargers work

Post by MDK »

brendon_m wrote:
Wed, 15 Aug 2018, 10:45
I've been trying to work out how a DC fast charger works, as in internally not how they charge charge a car.
They rectify AC into DC, use the DC to drive IGBTs at a high frequency to create AC and then rectify that back into DC at the desired voltage.

And as Rusdy says they tend to just have a very large connection to the grid, though Tesla is considering using powerpacks alongside their superchargers, and @jonescg has been chomping at the bit to wire a large battery in parallel to a 50kW charger (eg in Fremantle where the grid can't handle it)
Rusdy wrote:
Wed, 15 Aug 2018, 11:11
Bunch of no-gooders broke the world record in Kwinana sometime ago, requiring 800A feed (half Mega-Watt). Too bad, I can't find the detailed info how much it pulled from the grid. I remembered it mentioned somewhere in this forum, but I can't trace it :oops:
There were two 400A three-phase feeds (one per warehouse) and we got pretty close to 400A/phase in one warehouse, and ~330A/phase in the other.

Kwinana-feed-sm.jpg
Kwinana-feed-sm.jpg (526.18 KiB) Viewed 2827 times
As you can see near the bottom of the image we pulled ~287kW in one warehouse and ~247kW in the other.

Balancing a bunch of single-phase chargers across six phases was an interesting challenge!

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Re: How DC chargers work

Post by brendon_m »

OK, so they do just lay large cables in and pull straight from the grid. That's what I was trying to work out but I couldn't find it.
At 150kw I can see it working fine but the new 350kw standard surely will push it, and more so if there are multiple stations (like at a servo on a highway might have in the future). And I was reading the other day about a 800kw standard. That's a lot of electrons...

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Re: How DC chargers work

Post by Rusdy »

brendon_m wrote:
Wed, 15 Aug 2018, 12:14
... And I was reading the other day about a 800kw standard. That's a lot of electrons...
Indeedy. I'm very interested to see whether DC microgrid will come into play due to this reason as well. Even Siemens already mentioning MVDC network for AEMO. Why not even go further down to LV network?

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Re: How DC chargers work

Post by brendon_m »

Simply put, DC connection has higher capital costs but lower line losses.
I'm not sure I trust that articles author

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Re: How DC chargers work

Post by Rusdy »

brendon_m wrote:
Wed, 15 Aug 2018, 15:25
Simply put, DC connection has higher capital costs but lower line losses.
I'm not sure I trust that articles author
Hi Brendon, benefit in using DC transmission is already well documented. For example, here.

Specific to the lower line losses argument, this is due to DC doesn't suffer from reactance losses. The legacy benefit of AC in transmitting higher voltage (hence less loss) is no longer true (since DC technology now can do that). Although, DC equipment is more expensive to date.

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Re: How DC chargers work

Post by brendon_m »

The things you learn.
However I wonder if the benefits of 3 phase out weigh the reactance losses

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Re: How DC chargers work

Post by jonescg »

This is the basic layout of DC fast charger:
DCFC circuit.jpg
DCFC circuit.jpg (71.26 KiB) Viewed 2784 times
1. Input precharge circuit to protect switching gear from high inrush currents.
2. Input full bridge rectifier
3. Input capacitors for smoothing the DC.
4. IGBT array for generating high frequency AC
5. Isolating transformer (can also be used to step voltages up or down)
6. Output rectifier to convert HF AC back to DC
7. Snubber filter
8. Output chokes
9. Output filter capacitor
10. Reverse protection diode
11. Prehcarge circuit for inrush suppression when connecting the battery to the charger.
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Re: How DC chargers work

Post by LouB »

Are these proposed 350kW, and possibly 800kW charging stations, predicated on the general adoption of solid state and/or ultracapacitor batteries? Wouldn't current Lithium-ion batteries be seriously degraded by the heat generated (unless charging rate is drastically controlled)?

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Re: How DC chargers work

Post by jonescg »

Nope - just size them accordingly. 350 kW at 700 volts is 500 amps. As long as the battery was bigger than 500 Ah, that's only a 1 C discharge. Battery buffers for fast chargers makes even more sense when considering the instantaneous loads on the grid.
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Re: How DC chargers work

Post by LouB »

Thanks jonescg. Your figures make sense.
I just asked because of reports like this:

Tesla explains why it limits Supercharging speed after high numbers of DC charges

Link: https://electrek.co/2017/05/07/tesla-li ... r-charges/

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Re: How DC chargers work

Post by jonescg »

I found this thread and thought it's as good a place as any to list my musings.

We are trying to get DC fast chargers installed in the West Australian wheatbelt where distances are vast, towns are small, and power is weak. A charger might be $35k, but the cost of installation and connection to the grid is at least this much again. We are often asked, why not do a solar+battery system instead?!

Because cost.

A great idea is to have a battery of about 160 kWh at about 600 V DC. It can be charged with a hybrid grid-connected inverter like the Goodwe ET series, or a Fronius. These work with battery DC voltages of 400-600 volts - almost perfectly suited to the DC bus of a fast charger. The battery is charged by the hybrid inverter which sources its power from a 13 kW solar array. It's only 10 kW, but that's probably enough for topping up a battery. The best part is the same battery is useful for grid support and customer needs when its not being used as a battery buffer for charging EVs. The best price I can find for large format LFP is about $550/kWh including BMS and an enclosure. So there's about $86,000 in battery. Maybe $8000 in solar array, and $5000 in the inverter. Plus the $35,000 for the charger, and maybe $20,000 for installing it all in a remote location (assuming it's got a three phase connection) and you've got a ~$155,000 system. Granted, it can feed power back into the grid, and offer grid firming in remote locations, but when the cost of a transformer upgrade might be less than $35k, it's a hard sell.

But it's also the only option for high powered charging in some locations, so in my view it's worth trying. We know DC-DC fast chargers are available and cheap, but they have more limited functionality than an integrated system like one described here. Jon Edwards has been successfully running his Setec machine on a 400 V DC battery without a hassle, and they're ready to roll. All we need is a willing location and a stack of money...
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Re: How DC chargers work

Post by antiscab »

what if you were to scale things down a bit? maybe go for a DC not so fast charge

a couple of old leaf batteries would get you to 30kwh, keep the 13kwh of solar (obviously this isn't the expensive part)
you could 50kw fast charge till the battery is dead, then slower charge at the combination of available grid capacity and solar output (hopefully not at night)

on the esperance link, are there any proposed locations that have no grid connection at all?
even a little 5kw connection makes all the difference
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Re: How DC chargers work

Post by jonescg »

All sites have grid power, but the supply is weak. As in the HV transmission lines are also small.
You could totally scale it back but I feel 60 kWh would be a practical minimum. Knowing you wouldn't see too many travellers in a week means you risk a bit of an overspend, but it also works the cells less.
At least the added bonus is the reliability of power for the host site.
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Re: How DC chargers work

Post by brendon_m »

60kWh works for 1 car (or maybe 2) but what happens on Easter and every person and their dog is heading out bush. I'm not saying it's a bad idea to decrease the capacity to save dollaridoos but there will be a lower limit before it becomes unreliable for groups travelling.
But I guess something is better than nothing

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Re: How DC chargers work

Post by antiscab »

I wonder if lead acid would be a reasonable option for this situation.
at 160kwh, deep discharges would be rare (unless it's part of a VPP)
50 x 300Ah Gel batteries would be around $25k
with a 50kw DC fast charge, that would only be a C/3 discharge rate (more like C/3.5 or C/4 if the grid is contributing 5kw and the solar is contributing 5kw as well)
with 13kw of solar maxing out that inverter mppt at 10kw, that's only C/16 recharge rate, most of the time much slower (even charging 5kw from the grid is C/32)

Batteries in this situation I think may die of old age rather than from cycling
Actually, do we have usage data on the DC chargers from the EV highway, to get an idea of usage rates?
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Re: How DC chargers work

Post by jonescg »

We did have data, but it's not being shared any more. Plus, we have a situation where the chargers are being run down to the point of disuse. Recent data wouldn't actually be all that relevant any more. What we do know from Chargefox et al. is that most people do their rapid charging during daylight hours, roughly matching the output of the sun.

The other bonus of LFP over lead is the voltage stability. As the DC bus from lead acid droops, the charger would need to constantly change output.
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Re: How DC chargers work

Post by Rusdy »

35k is way less than I expected for a power upgrade. So, western power only need to upgrade the transformer? I.e. the grid already have the power?

Different case from eastern states my guess, where there is an FCAS market https://thedriven.io/2020/05/07/new-gou ... rging/amp/

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Re: How DC chargers work

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In case you're wondering what FCAS stands for, as I was: Let's talk about FCAS.
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Re: How DC chargers work

Post by jonescg »

Yeah but put it this way - once the new transformer is in, the HV lines feeding the town are more or less at capacity. No FCAS market to speak of in WA, and certainly not out in the bush.
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Re: How DC chargers work

Post by EVdownUnder »

Could there be better solutions than batteries as energy buffers? Maybe not. But I would love to see someone try.
Compressed air (in small scale) is reasonably cheap, extremely reliable and much "greener" than batteries.
I have been thinking about it as house energy storage. Not sure how much instantaneous power can be pulled from it though? Easy for a house, much harder to rapidly charge an EV.
But if possible, close to no loss when waiting for the next car, fibre-glass tanks have a maintenance-free 30 years life expectancy (according to one of the salesman I spoke with) and compressors/generators are getting better every year.
This is one company in the process of using it for mobility:
https://www.mdi.lu/about

Although very expensive, this technology can be scaled up to impressive levels:
https://reneweconomy.com.au/australias- ... val-36150/
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Re: How DC chargers work

Post by antiscab »

Round trip efficiency on compressed air is pretty bad, around 45%, gets worse when trying to go higher pressure to get the volume down. Not terrible for storage between seasons in the right circumstances.

Energy density is terrible too, both in weight and volume. Worse than hydro.

100kwh usable you're looking at 30kL air at 70 bar.

Edit:
compressed air can be closer to 60% if you can store the heat from compression and use it during expansion.
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Re: How DC chargers work

Post by jonescg »

I think the trick is to know your likely demands, and size accordingly. 60 kWh worth of battery is only $30k, which makes the total just shy of $100k for a full setup.
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Re: How DC chargers work

Post by francisco.shi »

The second hand leaf batteries are probably not a bad idea.
At 65% of the original capacity when they are removed under warranty to get 160kwh you would need about 10 leaf packs. I am sure there are more than that floating around in Australia. I know of 6 packs that were in a recycling warehouse in Brisbane.

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