L2 Charging from DC source?

[miscrms]

Ok, this probably sounds a bit wacky, but anyone have any thoughts on the feasibility? It's perhaps not totally crazy, as most modern switching power supplies convert AC to DC, then generate AC at a much higher frequency, then convert that AC back to DC at the desired output voltage. Basically an AC to DC converter and DC to DC converter back to back. Consequently, many such power supplies (and some chargers) can be fed directly with DC voltage, which basically just passes straight through the rectifier and work just fine. Often even with improved efficiency due to dropping the AC to DC conversion losses. Many can even accept a pretty wide range of DC voltages as they are designed to be 120V /240V compatible without requiring manual switch like old school supplies.

I believe the Leaf OBC operates on the same principle, and might be perfectly happy if fed DC in the right voltage range. The complete unknown for me is what parts of the charger / EVSE initiation process might get hung up if the EVSE was just fed DC?

Any thoughts much appreciated! This is mostly intellectual curiosity at the moment, but could have some interesting possibilities. For example if I were to take a Leaf out to a track day and needed a way to recharge between runs, it's occurred to me that my Prius can easily put out 3kW of 220V DC....

 

[miscrms]

From a brief refresh on the EVSE operation from OpenEVSE, it seems like the EVSE would probably be fine running off DC so long as the 12V power supply used in the particular EVSE is DC capable. For example the one spec'd by OpenEVSE operates on 85 to 264 VAC or 120 to 370 VDC. So it would seem that in this case the EVSE would be able to generate the pilot signal, close the relays, etc as usual when operated on DC.

Next issue might be the voltage sense circuit in the OBC, and whether it senses only AC, or could work on DC....

Rob

 

[GerryAZ]

Rob,

One problem I see is the contactor in the EVSE. It takes much larger contacts and a much wider gap to break DC current. There is a good chance the contacts would fail to interrupt the DC current if the contactor dropped out under load. This would destroy the contactor and maybe the whole EVSE.

Have you looked at the input side of the onboard charger? If it just uses rectifiers to convert to DC, it may work on DC. If it uses SCRs to control the input current, then it needs the zero crossings of AC to turn them off. As you noted, the other issue is voltage sensing. There have been a couple of cases reported of failing to charge due to the input voltage being interpreted much higher than actual.

Gerry

 

[miscrms]

Good stuff Gerry, thanks. Good observation on the contactors. Sounds like a custom EVSE with appropriately sized ones would be smart if one were to consider this.

Hmm, this report (starting on page 7) seems to have details on the charger internals, but its a bit confusing. The pictures are clearly of a 2011/12 type charger, and yet its described as a 2013 and tested up to 6kW. So a little hard to say if the schematic relates to the 2012 charger or not. If so, it seems pretty DC compatible from what I can tell?

Overview slides (page 7): http://energy.gov/sites/prod/files/2014/07/f17/ape006_burress_2014_p.pdf
Expanded Description (page 13): http://energy.gov/sites/prod/files/2015/04/f21/FY14_EDT_Annual_Report.pdf

Rob