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DC Charging of the Model S from a DC Home PV Solar Installation

TonyWright

New Member
I have a 9 KW (40 panel) Grid tied PV Home Solar Installation that produces more power than I need for my home. We planned this on purpose to have excess power to charge a Tesla Model S. Of course I can set up a 240V AC charging station, but then I am converting DC from the panels to AC, and then, on board the Tesla, back again to DC, suffering significant power loss through two inverter conversions. It would seem pretty straightforward for the DC current from the panels to be gated through a solid state current controller to charge the Prius batteries directly without inverters. Does anyone know if Tesla or a third party will be manufacturing such a system. Similar systems may be already available for people with off-grid solar home installations that power lead acid battery storage banks.

Especially in Europe or Japan with very high gasoline prices, I would think this approach really improves the economics of EV ownership. I would think solar city and Tesla would get together to write a lease agreement for tesla + Solar PV installation as a package!
 

VolkerP

EU Model S P-37
Jul 6, 2011
2,464
27
Germany
Hi Tony,

charging from PV was discussed before here
or in the TMC forum here and here.

The baseline is, there is no economic point in this approach. If you got a lot of cash to burn, you might set up an off-grid / island system.
Feeding DC to your batteries means, you either run out of sun before your car is full or you must dispose surplus energy. You need VERY big battery storage banks to balance production and consumption capacity for an electric car.
 
Last edited:

Robert.Boston

Model S VIN P01536
Oct 7, 2011
7,844
42
Portland, Maine, USA
Tony, you also need to look very closely at the value of Massachusetts "Solar Renewable Energy Credits." These are stupid-valuable (~40 cents/kWh), so you may be a whole lot better off financially selling your solar power to the grid and buying generic power to charge. This is just a bookkeeping transaction; your contribution to reducing pollution is unchanged regardless.
 

TonyWright

New Member
Hi Volker,

I already have a grid tied system so I can easily take the recommendations of the previous posts, But I see many one can buy a DC "charge controller" that is 93-98% efficient for charging home battery banks. Instead I suggest Tesla and PV companies supply a similar charge controller for shunting DC power direct to the Tesla when the sun is strong, thus avoiding two inverters at 0.85 x 0.85 = 72% efficiency. And to reply to Robert, I do have the SERC credit system in place. It is based on net metering so I diminish my credit if I charge from the grid.
 

Lloyd

Well-Known Member
Supporting Member
Jan 12, 2011
6,335
2,202
San Luis Obispo, CA
tony,

If you are getting say 15 cents per KHW when your PV panels are producing and you are charging at night at say a cost of 6 cents per KWH. You have a pretty big effeciency in cost to overcome. I believe if you run all the numbers, charging from the PV array directly is not the way to go.
 

VolkerP

EU Model S P-37
Jul 6, 2011
2,464
27
Germany
There seems to be no economic point in charging directly from PC. You can justify that investment only with other topics, e.g.
- prove that anybody can do it (same like driving an EV)
- get autonomous energy supply in case of grid outage (there were some roadster owners immobilized this year during Irene)
- take overall energy efficiency to the possible maximum

any other ideas?
 

TEG

Teslafanatic
Aug 20, 2006
21,914
9,065
There seems to be no economic point in charging directly from PC. You can justify that investment only with other topics, e.g.
- prove that anybody can do it (same like driving an EV)
- get autonomous energy supply in case of grid outage (there were some roadster owners immobilized this year during Irene)
- take overall energy efficiency to the possible maximum

any other ideas?

I think you covered it. This question comes up from time to time as it just seems "elegant" to take DC directly from solar panels and charge the battery pack directly. Unfortunately the irregular voltage output, and irregular current output from Solar panels means that you still likely need a bunch of electronics in between to treat the batteries right and you are limited to charging during daylight hours only.
 

W.Petefish

Active Member
Apr 29, 2011
1,060
11
The only economical way to do this is with a grid-tied system using micro inverters.

But if you want to do something like an off-grid system, it'll cost at least $30k. (figuring $16k for the batteries alone) Not to mention that the size of the battery array that would be needed presents serious space issues. (think truck container just for the batteries) If you had an implausibly large array you could do it, BUT Tesla has repeatedly said that they are not going to sell the DC fast charger. (I tried to get one installed, and was even willing to pay for the install and they still turned me down.)
 

widodh

Model S 85 and 100D
Jan 23, 2011
6,855
2,808
Venlo, NL
This thread is old but still relevant. Now that there is a CHAdeMO adapter for Model S, wouldn't it be possible to build a mini-CHAdeMO charger at home?

As far as I know you can charge with as little as 1A using DC. So if your Solar array is generating 1kW you can put that directly into your Model S. Super slow, I know, but a lot more efficient then going from DC -> AC -> DC.

The DC protocols are also much more advanced then the J1772 AC signaling.

Would be kind of cool if you could charge a Model S (or any CHAdeMO vehicle) using DC directly from a solar array.
 

FlasherZ

Sig Model S + Sig Model X + Model 3 Resv
Jun 21, 2012
7,028
1,025
I'm not familiar with the DC protocols, but am wondering if they would permit a rather sudden decrease in power availability (e.g., clouds) without generating an error on the car?
 

widodh

Model S 85 and 100D
Jan 23, 2011
6,855
2,808
Venlo, NL
I'm not familiar with the DC protocols, but am wondering if they would permit a rather sudden decrease in power availability (e.g., clouds) without generating an error on the car?

I don't know either, I would love to read the specs somewhere. But as the internal charger of the car is by-passed I think that the outside charger is in full control.
 

AWDtsla

Active Member
Mar 3, 2013
4,274
4,242
NE
I'm not familiar with the DC protocols, but am wondering if they would permit a rather sudden decrease in power availability (e.g., clouds) without generating an error on the car?

You would still need DC-DC conversion. Supercharger/onboard/chademo are not just AC-DC converters, they have control over the DC level. You can't just take solar panels and plug them directly into a battery. You could try, but that system would be extraordinary inefficient as it would throw away any energy produced above or below the required voltage. DC-AC-DC conversion isn't so bad, just multiply the two efficiency numbers together. There's not much to gain.
 

Chris TX

Active Member
Sep 30, 2013
1,531
188
Dallas, TX
You would still need DC-DC conversion. Supercharger/onboard/chademo are not just AC-DC converters, they have control over the DC level. You can't just take solar panels and plug them directly into a battery. You could try, but that system would be extraordinary inefficient as it would throw away any energy produced above or below the required voltage. DC-AC-DC conversion isn't so bad, just multiply the two efficiency numbers together. There's not much to gain.

Solar panels-powerwalls-CHAdeMO would do it. If there's a way to dial down the DC amps advertised to the car, it would allow for continuous charging. The PowerWall would just be to absorb any fluctuations.

Electric Motor Werks, Inc. - QuickCharge-25000 HV - a 25kW PFC charger for HIGHER voltage batteries
 

wk057

Vendor & Senior Tinkerer
Feb 23, 2014
5,722
12,200
Hickory, NC, USA
I charge my two Model S with my off-grid solar system. When there is sufficient sun the cars do charge directly from the PV power without using battery storage. PV->Charge Controllers->Inverters->Model S AC to DC Charger->Battery. Long road. By my estimates I end up with about 77-82% of the incoming PV power at the panels as power in the Model S battery this way.

I think a direct DC-DC converter cutting out the DC->AC inverters and onboard AC->DC chargers would be a little more efficient... but would be pretty complex and probably not worth the effort for maybe a ~10% efficiency gain. In my case I would need something like a 500 amp DC input from my main DC bus for my system to convert to 20kW worth of DC power for the Model S. Those components aren't going to be cheap either.

Long story short, unless you designed the system with this in mind from the beginning, and were willing to just accept that it isn't cost effective, there really isn't going to be a good cost benefit to doing something like this. It would likely be cheaper in the long run to just add 10% more PV to account for the efficiency losses.

Now I would like to get something like a 40-50kW DC charger for myself... but probably not worth the effort either. lol.
 
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Chris TX

Active Member
Sep 30, 2013
1,531
188
Dallas, TX
...Long story short, unless you designed the system with this in mind from the beginning, and were willing to just accept that it isn't cost effective, there really isn't going to be a good cost benefit to doing something like this. It would likely be cheaper in the long run to just add 10% more PV to account for the efficiency losses...

Couldn't you just tap into the PV-Battery DC high voltage lines and connect it to a CHAdeMO charger? It would pull from both PV and battery.
 

wk057

Vendor & Senior Tinkerer
Feb 23, 2014
5,722
12,200
Hickory, NC, USA
Couldn't you just tap into the PV-Battery DC high voltage lines and connect it to a CHAdeMO charger? It would pull from both PV and battery.

Doesn't quite work like that, at least not for my setup or any other configuration I can think of. The mid-voltage PV (~170V for me) is connected to charge controllers which regulate the voltage that goes to the batteries (~38-50V, depending on SoC) at high efficiency (93+% usually). That isn't really bidirectional either. Nothing can feed from battery to PV.

Edit: For my setup the best place to tap would be the main DC bus (~44V nominal) with a high efficiency DC-DC step-up converter of some sort to reach Tesla pack voltages.
 
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green1

Active Member
Mar 25, 2014
4,548
1,494
Calgary, Alberta, Canada
Doesn't quite work like that, at least not for my setup or any other configuration I can think of. The mid-voltage PV (~170V for me) is connected to charge controllers which regulate the voltage that goes to the batteries (~38-50V, depending on SoC) at high efficiency (93+% usually). That isn't really bidirectional either. Nothing can feed from battery to PV.
That said, if the batteries were configured at a higher voltage, it should be trivial. (though few setups do that)
 

Bangor Bob

Member
Jan 5, 2015
670
487
Bangor, ME
Couldn't you just tap into the PV-Battery DC high voltage lines and connect it to a CHAdeMO charger? It would pull from both PV and battery.

You'd need to build a Chademo that accepts DC input, they're mostly expecting to receive three-phase AC, aren't they? At least, the one ABB unit I just checked the specs on does.

[edit] And variable DC input, at that. Ick. You would need a very flexible (and efficient) DC/DC converter. [/edit]
 

wk057

Vendor & Senior Tinkerer
Feb 23, 2014
5,722
12,200
Hickory, NC, USA
That said, if the batteries were configured at a higher voltage, it should be trivial. (though few setups do that)

Yeah, higher voltage pack would be ideal, but a lot more safety related mechanisms needed in that case vs low voltage. Tesla cheats with the PowerWall by using a built in DC-DC. The issue is the availability of off-grid inverters that can utilize a HVDC battery bank.

You would still need to regulate the voltage getting into the car, though, regardless.
 
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