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Any way to safely test if an AC-DC converter can accept DC input?

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FyKnight:
Hi, I'm new, please correct me gently if I do something wrong. I did look to see if this question has been asked before but didn't find anything. Also I'm thrilled to find this forum!

I have a 7 kW AC-DC converter that I would like to feed DC directly to instead of AC. Yes, it's the OnBoard Charger in my EV. I'll need to charge it at an off-grid location, where I'll build a dedicated system just for this purpose. It seems silly to have a big inverter there if it can be avoided.

It could well be possible, right? If I keep to half the max current, then even though only one pair of diodes in the rectifier will be used, they shouldn't waste more power than under AC at max current. By the time we get to a capacitor it'll be no different to AC. The big question mark for me though is what will the PFC boost converter algorithm do?

So before I go any further with this thinking I'd like to test it. Is there any way I can do so without a large risk of damage to the OBC? All I can think of is to use the minimum voltage the OBC accepts (85 Vrms AC, so I guess 120 V DC?), at the lowest current (6 A, i.e. getting the EVSE to tell it to only draw 6 A) and try it for just a few seconds.

I'd also be really keen to hear if someone has done this before or even used a modified sine wave inverter to charge their EV (since that is nothing like a sine wave at all)

Thanks!

mikeselectricstuff:
Hard to see how it would damage it, shouldn't be too hard to use a low current DC supply to test off-load.
Might be useful to take a look at the circuitry to see if there any obvious potential issues.
 

NiHaoMike:
Perhaps just use the DC charge mode if there is one? It's often called "fast charging" but it most likely will also work at somewhat slower rates. https://openinverter.org/forum/viewforum.php?f=17

FyKnight:
Thanks very much for your replies!


--- Quote from: mikeselectricstuff on October 25, 2021, 02:32:22 pm ---Hard to see how it would damage it, shouldn't be too hard to use a low current DC supply to test off-load.
Might be useful to take a look at the circuitry to see if there any obvious potential issues.

--- End quote ---
OK interesting thank you! Will using a low current DC supply get much data? The OBC minimum configurable draw is 6 A, so it would just overload a lesser supply very quickly wouldn't it?

For the circuitry, would a photo on its own be any use or would I need to go and reverse engineer the actual circuit? I can probably manage to do that but if it isn't a massive risk perhaps I'll just try it and see.


--- Quote from: NiHaoMike on October 26, 2021, 12:34:13 am ---Perhaps just use the DC charge mode if there is one? It's often called "fast charging" but it most likely will also work at somewhat slower rates. https://openinverter.org/forum/viewforum.php?f=17

--- End quote ---
There is indeed a CCS fast charge mode in my EV, and that was my initial plan. I couldn't find any EVSEs under AU$50K that support DC charging on the CCS pins — they are all designed for fast charging. So I have to build one myself.

The first problem with this mode is that unlike AC charging which is controlled by a simple 1 kHz PWM signal, the DC charging uses a nightmare combination starting with powerline communications protocols over the same PWM control line and getting worse from there: (ROBO) OFDM, Ethernet, "Sounding packets", IP, TCP, TLS, HTTP and the cherry on top: XML! It's disgusting, and you need a beefy microcontroller to run it, but there are modules available and I am trying to order one (fingers crossed *).

The bigger hurdle with this approach though is that the battery is directly connected to the DC pins in the socket, and it's up to the EVSE to generate the voltage required to charge it, so that requires a DC-DC converter of similar complexity to the inverter that I'm trying to eliminate. It's still more efficient overall so worth doing, and I have a couple of these on order too.

So yeah, if I can reuse the hardware that I'm already carrying around, that would be cheapest, easiest and likely most efficient solution. I wish I'd realised this before spending so long going down the HVDC and CCS rabbit holes. Also, thanks very much for the openinverter link! Heaps of great info on that forum!

*: Probably easier than doing CCS is to just put a T junction on the HV lines from the battery and drive the contactors directly... if only the connectors didn't cost $200+

mikeselectricstuff:

--- Quote from: NiHaoMike on October 26, 2021, 12:34:13 am ---Perhaps just use the DC charge mode if there is one? It's often called "fast charging" but it most likely will also work at somewhat slower rates. https://openinverter.org/forum/viewforum.php?f=17

--- End quote ---
The problem with that is that you need to do the current regulation externally - the CCS port is just a pair of contactors away from  the battery terminals

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