DC solar directly to EV (no AC!)

[Ampster]

But it is something that we all want to do because DC to AC to DC is just redundant and you lose as much as 40% efficiency.

I lose at most ten percent from my panels and possibly another ten percent from the inverter through the onboard charger using AC charging. When my system is not charging my EVs it is generating credits which I can use later to charge my EVs when the sun is not shining. Overall it is a much more efficient system than dedicating all that equipment to DC charging which might only occur a few hours a day. I have never had a problem adding two hundred miles or more to my EVs overnight. If I want fast charging I can go to a Supercharger.

 

[summit]

been keeping an eye on this AIO with high DC-battery voltage, but not sure how to marry with the EV's BMS, more high battery voltage hybrid inverter seemingly coming

 

[curiouscarbon]

I lose at most ten percent from my panels and possibly another ten percent from the inverter through the onboard charger using AC charging. When my system is not charging my EVs it is generating credits which I can use later to charge my EVs when the sun is not shining. Overall it is a much more efficient system than dedicating all that equipment to DC charging which might only occur a few hours a day. I have never had a problem adding two hundred miles or more to my EVs overnight. If I want fast charging I can go to a Supercharger.

May I ask what charge rate you use for home AC EV charging?

In the past, 240V / 12A was what I used, but the current place only feels comfortable up to 120V / 8A.

I miss being able to do a few thousand watts overnight :')

 

[Ampster]

May I ask what charge rate you use for home AC EV charging?

I can do as much as 48 Amps. One of my EVs can only do 32'Amps. Often during the day when charging from excess solar it varies as low as 8 Amps and up to 25 Amps.

 

[curiouscarbon]

Awesome. Thanks for that info. May I ask, what do you use to throttle charging rate? Does that use the PWM pilot signal?

Sorry about the somewhat OT discussion. Comparing DC direct charging speed to AC charging speed is of interest to me. I have some arduino/microcontroller programming experience and will check out OpenEVSE. I want to charge EV battery from solar, mainly off grid format from DC battery, so throttling the output will be essential to preventing low voltage safety disconnects.

 

[curiouscarbon]

The appeal of direct DC charging is quite apparent to me, due to "less hardware" and higher potential efficiency.

For now, I am still designing with the assumption of DC->AC->DC for EV charging from a DC battery, despite the intrinsic inefficiency there. Safety and feasibility are the primary factors for me, besides ability to charge with grid down. This is mainly because my DC battery voltage is going to be small relative to the EV battery (12-48VDC vs 400VDC), and there's no currently supported method of connecting to the HV battery, and I'm not interested in the risk level associated with the air plasma arcs that a ~70kWh battery at 400V can pull when a mistake is made.

 

[eXodus]

Which specific whitepapers? If I randomly picked a J1772 documentation or whitepaper, I doubt it would say much about CCS and V2X because that is highly tangential.

J1772_201710: SAE Electric Vehicle and Plug in Hybrid Electric Vehicle Conductive Charge Coupler - SAE International

This SAE Standard covers the general physical, electrical, functional and performance requirements to facilitate conductive charging of EV/PHEV vehicles in North America. This document defines a common EV/PHEV and supply equipment vehicle conductive charging method including operational requirements

www.sae.org

J2847/2_201504: Communication Between Plug-In Vehicles and Off-Board DC Chargers - SAE International

This SAE Recommended Practice SAE J2847-2 establishes requirements and specifications for communication between Plug-in Electric Vehicle (PEV) and the DC Off-board charger. Where relevant, this document notes, but does not formally specify, interactions between the vehicle and vehicle operator. This

www.sae.org

the SAE standard documentation. The standard defines the the communication between the vehicle and and any off-board equipment.

As engineer - always go back to the enabling document. There is tons of useful information in the SAE standards.

sorry for the confusion - maybe "whitepaper" was the wrong term.

 

[zanydroid]

As engineer - always go back to the enabling document. There is tons of useful information in the SAE standards.

Thanks. I don't work in this area of engineering. It's very difficult to know where the documentation is in a different field.

 

[zanydroid]

Is there a free way to read SAE stuff, like it is to read UL and NEC? (Through a really atrocious web portal)

 

[eXodus]

May I ask, what do you use to throttle charging rate? Does that use the PWM pilot signal?

the PWM signal can be used to the tell the OBC to reduce the charge rate. It's a simple 1 khz signal which gets modulated.

 

[yabert]

Any insight into this would be appreciated.

You need this: https://enteligent.com/dc-coupled-dc-evses
But sadly it seem to be vaporware.
There is reasons than solar DC EV charger don't exist, it's quite complex.
Imagine, most DC charger (CCS fast charge) don't work properly all the time with all the EV's and they cost 30-50 k$.
So imagine what it could be challenging to sell a reliable 3-4k$ product while it can brick / destroy / put in fire an EV.

You can certainly connect a proper MPPT directly to the battery of an EV, but be prepare to blow fuse, brick and clear fault to ''repair'' the EV and have the potential of a fire...
My suggestion, for the moment stick with the AC charger and live with the 20% effeciency lost imply with it.

 

[zanydroid]

Imagine, most DC charger (CCS fast charge) don't work properly all the time with all the EV's and they cost 30-50 k$.

The price point is supposed to come down to <$2.5K this year for residential installs. Not that I would call it a fast charge, but it is bidirectional DC.

And a lot of that $30K is probably from the power levels.

 

[eXodus]

Is there a free way to read SAE stuff, like it is to read UL and NEC? (Through a really atrocious web portal)

Are you affiliated with a University or still have old student account? If there is a engineering programs at the school the sometimes have subscriptions to SAE.

Otherwise, it's a digital document on the internet. Someone will have backup listed somewhere.

 

[summit]

the PWM signal can be used to the tell the OBC to reduce the charge rate. It's a simple 1 khz signal which gets modulated.

can the charge rate be dynamically modulated in real-time to vary the charge rate ? am thinking of using my EV as a over generation charge dump, sure beats just heating water

 

[zanydroid]

can the charge rate be dynamically modulated in real-time to vary the charge rate ? am thinking of using my EV as a over generation charge dump, sure beats just heating water

Cars are required to respond to control pilot change. I think J1772 has a response time standard. I don't know what that time is.

 

[eXodus]

can the charge rate be dynamically modulated in real-time to vary the charge rate ?

you can change the rate of the PWM in real time but the OBC has a delay depending on the car. When I look at the power graphs of my EV - it takes about 30 seconds to ramp from 0 to 7.2KW. It disconnects instant - but ramp up is slow.

 

[zanydroid]

can the charge rate be dynamically modulated in real-time to vary the charge rate ? am thinking of using my EV as a over generation charge dump, sure beats just heating water

Emporia EVSE + Emporia VUE is supposed to be able to spin up the charge rate when there is extra solar. It'll probably not work for off-grid. Some parts of Emporia feature set is for on-grid normies.

Also I doubt it's as flexible as a custom solution with your own PLC.

 

[400bird]

You, you can change charge current live on J1772.
Here's a random day last month. I removed home loads and other things from the graph. The home base load is something like 500-700 watts, hence the difference between my solar production and EVSE/car charging.

This is using an OpenEVSE. Works great.

Yellow is PV
Orange is EVSE
The car left at about 1-2 PM for school pick up.



While DC PV to DC car battery charging sounds ideal, the extra expense and dedicated solar make it feel like a waste. As others mention, once the car unplugged, my PV was still out to good use (charging the home battery or going to the grid for net metering)

 

[curiouscarbon]

You, you can change charge current live on J1772.
Here's a random day last month. I removed home loads and other things from the graph. The home base load is something like 500-700 watts, hence the difference between my solar production and EVSE/car charging.

This is using an OpenEVSE. Works great.

Yellow is PV
Orange is EVSE
The car left at about 1-2 PM for school pick up.

View attachment 138967

While DC PV to DC car battery charging sounds ideal, the extra expense and dedicated solar make it feel like a waste. As others mention, once the car unplugged, my PV was still out to good use (charging the home battery or going to the grid for net metering)

Wow, thanks for this very descriptive example! Much appreciated.

Seems like OpenEVSE will be able to do what I need: programmatically modulate charge rate drawing from stationary DC battery inverting to AC EVSE.

 

[400bird]

Wow, thanks for this very descriptive example! Much appreciated.

Seems like OpenEVSE will be able to do what I need: programmatically modulate charge rate drawing from stationary DC battery inverting to AC EVSE.

You do need something else to communicate available charge wattage to the EVSE. I'm using a Raspberry Pi, but you could go with anything that can send the correct MQTT commands. Open Energy Monitor is made by the same group as OpenEVSE, they're designed to work together.