EV low current charge from solar

[-sandro-]

Hello,
I was thinking of creating a simple 12V 250W solar panel with an inverter to 120V or 230V AC to charge cars like the Volkswagen e-up at a very slow rate, maybe like add a 1kWh of charge in a sunny day. I'm talking without any battery in between, just straight from solar.

However this EV like many others won't charge at currents below 5A so if the math is correct considering the input from the solar and that the battery in the car is 400V+ that maximum current the built-in charger will be able to output is less than 1A so it won't work at all.

The questions are:
1) why do EVs manufactures care if you charge at a such incredible low current/wattage? Is there some kind of technical limitation I don't see?
2) is there a way to overcome this while keeping the panel small and the system overall cheap?

I know I could install a bigger solar panel system on the roof directly but I'm talking about a hobby here, I'm not trying to create a charging station at home. I already know I'll probably never "break even" even with just a 100W panel.

 

[-sandro-]

Have I asked something so idiotic as a beginner?

 

[gnubie]

No, just that at that sort of low power there isn't really anything that readily springs to mind. Strictly speaking you can do it but the issue is going to be getting the car to work with the variable output from the PV side. Your car will typically expect that it can just pull some number of amps based on the supply voltage / sense lines from the 'charger' (its not really a charger) and perhaps even your own manually entered power limit via the car's console.

Your solar power generation will go up and down based on sun exposure. A cloud goes past, output dips etc. When that happens the car will still want to draw what ever amps it is trying to but the solar array won't be able to hold it up so the solar inverter will turn off or misbehave, then the car will stop charging, maybe declare an error or all sorts of potential things happen.

Could an EV's onboard charger be designed to do it? Sure, but you then have to consider the practicality of such low power. 1kWh in terms of moving a car isn't all that much chop so the benefit of a 250 watt panel all things considered, ie even being able to get direct sunlight on it in the first place, it's just not worth the trouble. The few EVs that have built in solar panels really have them there for the swank factor more than any practical application.

 

[-sandro-]

Hi, someone told that 100W is not enough to power the charging system to actually charge the battery so that would be one of the reasons among others why this is not allowed.. Anyway it would have been cool to get "free" charge from solar directly at your own house just to experiment with cheap diy stuff. Especially for people like me that use the car just once a week for 10km.

 

[mikeb]

i am no expert but just a beginner like you but i do think i can help you a little. in my view your charging voltage system needs to be higher voltage
than your battery. to me i would think low current wouldnt matter if you had too much current that might be more of a problem becouse of heating the batteries. i believe your system of solar to inverter then to charger then to car could work.
might work much better with solar to 12v battery then to inverter then to charger then to car to smooth out highs and lows

 

[Ampster]

Hi, someone told that 100W is not enough to power the charging system to actually charge the battery so that would be one of the reasons among others why this is not allowed..

Yes, that is my understanding as well because the charger has some overhead. That is why 240 volt charging is more efficient than 120 volt charging.

 

[-sandro-]

i am no expert but just a beginner like you but i do think i can help you a little. in my view your charging voltage system needs to be higher voltage
than your battery. to me i would think low current wouldnt matter if you had too much current that might be more of a problem becouse of heating the batteries. i believe your system of solar to inverter then to charger then to car could work.
might work much better with solar to 12v battery then to inverter then to charger then to car to smooth out highs and lows

Yeah a battery in between will solve many problems. However as I said usually these EVs don't accept currents below 5A (from a 240V system) aka below 1.2KW. So even if you can get a higher voltage than the battery (>400V) with a converter the current would be too low.

 

[Ampster]

is there a way to overcome this while keeping the panel small and the system overall cheap?

If you are on the grid have you looked at EV rates from your power provider?
Back to your original question, smaller panels are more expensive. You can buy used 250-300 Watt panels for $50 and used grid tie microinverters for less than $100. That would only work if you have a grid connection.

 

[-sandro-]

What do you mean by "ev rates"? Where I'm based (Italy) I don't think we have those, you just pay based on the kWh you use.
How do grid tie inverters work?

 

[Ampster]

What do you mean by "ev rates"?

In California, USA we get a special rate if we have an EV. That means we can charge overnight at $0.15 per kWh versus the usual rates of $0.30 per kWh.

Where I'm based (Italy) I don't think we have those, you just pay based on the kWh you use.
How do grid tie inverters work?

The short answer to how grid tie inverters work is they send any excess energy to the grid instead of to batteries. GT inverters are less expensive than most battery inverters and you save the cost of batteries. To do that requires a Net Energy Metering agreement with the power company. I don't even know if that is a possibility in Italy.

 

[-sandro-]

I see, yes that's possible.
I thought it worked the other way, basically it gets the power from the sun and the remaining from the grid.

 

[Ampster]

It works both ways. With a good net metering (NEM) agreement you can store excess energy produced in the morning and use it in the evening with little cost. That is the concept of the grid as a battery and only works if NEM agreement is available. That is not universal in the USA since each state has different regulations.

 

[nosys70]

the problem is charging an EV require a special connector that doe not only provide power, but also negotiate charging parameters trough several signals.
read about https://en.wikipedia.org/wiki/Combined_Charging_System
so it possible the protocol list a bunch of parameters like charging power available and in that list there are only a few possibilities (like 5, 10, 50, 100, 200A) . So if your EV charger is finnicky , it will not trigger charge without the handshake allowing charge.
there is a openCCS station you can buy and probably fiddle with to allow any power to go trough.

OpenEVSE - Electric Vehicle Charging Solutions

OpenEVSE provides electric vehicle charging solutions worldwide based on SAE J1772 and IEC 61851 standards. Our experts provide components, software and services to accelerate the development of new charging technologies.

www.openevse.com

It is also possible that the cooling sytem while battery is charging ask for some power, and without the minimum guaranteed to run the colling system, the car will not initiate a charge.
If you don't know why they prevent that, the best would to try to follow the rules instead blowing the home.
but you can fiddle with that even with a cheap arduino

Arduino EV J1772 Charging Station

Arduino EV J1772 Charging Station: Based on the OpenEVSE project OpenEVSE OpenEVSE Store Arduino Electric Vehicle Charging Station "Electric Vehicle Supply Equipment" (EVSE) implementing the J1772 protocol. J1772 is used in the current generation of Electric Vehicles and Plug ins suc…

www.instructables.com

 

[cwstnsko]

I dont know of a way to do it without a battery, but with a battery, you could build a cart that housed the battery, the solar charge controller, an inverter, and a low power (probably L1) EVSE) I'd suggest that you could park the cart connected tothe solar panel during the day to charge the battery, then after the sun went down, you could roll the car to the car and use it to charge the car for about an hour before the battery would have expended it's capacity.
Based on what I have looked at for my project, I think I could build the setup for $500-1000, depending on the battery and whether you already own a low-power EVSE. At the rate I pay for power, it would take about 20 years for the idea to break even, and that would be if all the components would last that long.