Need help with off grid EV charging system design

[furkidmom]

HI, I could use some help designing an off grid EV charging station for our Ford Lightning which is scheduled for production. Where I keep getting stuck is on the battery bank sizing. The standard formulas come up with a requirement of over 700 amp hrs of battery. In his video for his Tesla charging Will is only using two 100Amp 48V server rack batteries. I'm retired and we will be able to charge during the day. We will also have a charging station hooked up to the grid, so don't need an 100% solution. Not worried about the rare occasion when we don't have sun for days or if we have to put a lot of miles on a few days in a row. Just looking the 90% of the time solution. Can someone help me understand this battery requirement calculation? Also, we were going to use the same batteries he had, the EG4's.. and the new UL listed MPP 6048 inverter. But, the reviews say that the inverter doesn't work with the EG4's and a number of other issues, so we may have to pivot to a different inverter. We also need to keep the price of the system reasonable, or we are wasting money and might as well just pay PGE to charge it. Would appreciate any advice. -cj- (PS, we are in California on 2.5 acres, and have a site with full sun for the panels, ground mount)...

KWh Demand​
Miles Driven/Day	50​
Range (1.8 mi/kwh)	​ 1.8​
Battery used KW	27.78​
Solar System Size​
Adj for system losses	29.17​
Inverter efficiency	95%​
Amt of Elec needed to be supplied by panels to inverter and battery	30.70​
Ave. Irradiation hrs/Day	6​
Solar Panel KW needed	5.1169591​
solar panels needed	12.792398​
Battery Bank Size​
Energy needed kwh	27.777778​
Depth of Discharge	80%​
Battery storage needed KWH	34.722222​
Battery Voltage	48​
Amp hrs (1000xKwh/voltage)	723.37963​

 

[MichaelK]

In a daytime charging situation, I'd not be paying much attention to battery size at all. Realistically what you want to be doing is maximizing charging during the daylight hours, and don't try charging at night at all. What you would want is a battery bank large enough to keep the system stable throughout the day, maybe in the 250Ah to 500Ah range.

You should be focusing more attention on getting the watts you need during daylight hours. There are three "tiers" of charging that EVs have standardized on. They are.....

Tier 1: 12A at 120VAC: 1440W
Tier 2: 20-50A at 240VAC: 4800W-12000W
Tier 3: 480VDC Commerical only There are no companies whatsoever supplying DIY components at 480V

I understand that the Tesla can be programed to do as little as 10A at 240V. I have a very large off-grid system, and I can just manage 2400W for about 6 hours per day. I'd say that with ~ 6000W of panels, placed on rotating mounts, you could get 2400W continuously from about 8AM till 4PM. You might be able to get 4800W between the hours of 10AM and 2PM. That would get you close to your goal of 30kWh per day.

I'd suggest you go with a higher quality split-phase inverter. Take a look at the Outback Radian 8048, and the Schneider XWPro 6848 as two quality inverters that can handle this task.

Ultimately, if the goal here is to save money, then you've already lost at the starting gate. Off-grid solar is simply the most expensive way to generate electricity, and if you already have the grid connected, you are just kidding yourselves.

 

[furkidmom]

Since this is the beginners forum, I guess I can ask why off grid is the most expensive way to produce electricity - and whether that is universally true, particularly here in California with our TOU and baseline pricing structure? We built a DIY solar setup for our pond. The pump has to run 24/7/365. Our true up bill went from around 2K to $200. This is in part because the little pond solar system it keeps us below our baseline allocations. It would have cost over 15K to expand our rooftop solar, instead we were able to do this DIY system for the pond ourselves.. I think it cost lest than 5K, and after the tax credit was even less.

 

[BentleyJ]

I guess I can ask why off grid is the most expensive way to produce electricity - and whether that is universally true,

CA leads the nation in expensive electricity. With all the rate increases and TOU replacing the older Tier rate system, I would say it is no longer universally true that off grid solar is much more expensive.

 

[Ampster]

Since this is the beginners forum, I guess I can ask why off grid is the most expensive way to produce electricity

I am in California with high rates, two EVs and a 42kWh battery pack and 9kW of solar. I will try to answer your question. I looked at the cost of my solar system at $2.50 per Watt, assumed a 20 year life and a total production in kWhs over that life and arrived at a cost of solar production of $0.08 per kWh.
My lowest rate in California is $0.23 per kWh and peaks at $0.53 per kWh. Therefore any time I can charge my EVs from solar it is a lot less expensive.
Adding batteries and a hybrid inverter cost $10.000 and I assumed a life of ten years. That is a daily cost of $2.74 per day and If I use 14 kWh per day of that pack that is a cost of $0.20 per kWh of stored energy. Since the source of that energy is the $0.08 per kWh solar, my total cost per kWh of using battery storage is $0.28 per kWh. Since I can use grid power at less than that rate it makes sense to use the grid to charge my EVs from the grid late at night when grid power is less expensive, if I did not get enough from solar during the day. That was when I did the math and invested in a battery storage system.

 

[Ampster]

You should be focusing more attention on getting the watts you need during daylight hours. There are three "tiers" of charging that EVs have standardized on. They are.....

I agree, as I mentioned in my comment above. Don't confuse those tiers with your rate plan. The most practical is the second tier otherwise known as Level 2 charging at 240 volts. I can set the charging rates on my EVs and often during the day only charge at 20 Amps, which is about 4800 Watts. I get between 3-4 miles per kWh on my EVs so during a good five hour solar day I can get almost 100 miles of charging. I only have a 50 Amp circuit so the fastest I can charge is 9.6 kWs which is more than enough to get 200 miles of range overnight. A 40 Amp Level 2 charging station is the most you can put on a 50 Amp circuit. Before you buy a charging station look at the one that comes bundled with an Emporia Energy monitor. It has the ability to change its charging rate based on your solar production. That might be useful, if you are just convenience charging because it would make sure your kWhs used to charge your EV will not exceed to solar production of your solar panels.

 

[furkidmom]

Thanks for the input...I have some more calcs to run now...

 

[Ampster]

...I have some more calcs to run now

A good place to get good solar insolation estimates is PV Watts. It will give you monthly estimates which as you may already have discovered can vary significantly from June to December.

 

[400bird]

So, you already have grid tied PV?
I'd suggest adding more PV to your current and getting a charger that can monitor grid current and charge based on what you are selling to the grid.

Grid tied PV is less expensive than off grid because you don't need a battery.

I have an openEVSE, that combined with an Open Energy Monitor would allow you to charge the car with just excess production. That's a very DIY set up and I love it. I believe that Emporia is coming out with a similar and more consumer level hardware to do the same.

 

[Ampster]

Off-grid solar is simply the most expensive way to generate electricity, and if you already have the grid connected, you are just kidding yourselves.

To be clear, solar generation can be very inexpensive. The thing that is expensive is storing that energy for use during when the sun doesn't shine. In California high TOU rates have made storage optimal to reduce energy bills.

 

[Ampster]

I have an openEVSE, that combined with an Open Energy Monitor would allow you to charge the car with just excess production. That's a very DIY set up and I love it. I believe that Emporia is coming out with a similar and more consumer level hardware to do the same.

I considered the OpenEVSE a while ago, but since then I have installed an Emporia Energy monitor and may get their EVSE which is available now for less than $500. The new product in the works is the bidirectional charger that might allow someone to use their EV battery as storage for the home. (V2H)

 

[GregTR]

I can only share that Emporia Vue with their EV charger works awesome to charge off of excess solar in a grid tied situation.

If you're starting to dump electricity onto the grid it automatically ramps up and adjusts the EV charger to match the load to your generation.

Highly recommend it, beat EV charger for $500

 

[Doggydogworld]

Since this is the beginners forum, I guess I can ask why off grid is the most expensive way to produce electricity - and whether that is universally true, particularly here in California with our TOU and baseline pricing structure?

Off-grid is expensive due to battery amortization. It will help economics a lot if these LiFePO4s truly last 5k cycles as advertised.

Ideally you'd avoid all batteries, inverters, etc. Some MPPTs can take 450V+ from panels and directly charge a 48V DC battery. You just need one that takes 450V from panels and directly charges your 400V DC Lightning pack via the CCS1 port. At only 10-15A it'd be cheap to build in volume. But there's not really a volume market.

 

[Ampster]

You just need one that takes 450V from panels and directly charges your 400V DC Lightning pack via the CCS1 port.

A much simpler solution for the OP will be to use his existing GT system and a J1772 Level 2 EVSE. DC Fast charging from solar is expensive and not optimal.

 

[Doggydogworld]

A much simpler solution for the OP will be to use his existing GT system and a J1772 Level 2 EVSE. DC Fast charging from solar is expensive and not optimal.

I agree in principle he should just expand his grid-tie system, but I figure he has reason not to. Maybe it's grandfathered into a super-sweet net metering scheme from days gone by, and modification would kick him out of that program.

We're talking about 5.1 kW, here, not fast charging.

It just doesn't make sense to me to first convert his (400-500VDC?) string voltage down to 12/24/48V to feed an unnecessary stationary battery, pull all that energy back out of the stationary battery, invert it to 240VAC, feed that 240VAC into the truck where the onboard charger converts it back to 400V DC. Not only do you suffer efficiency hits at every stage, you have to pay extra for all that equipment that causes those inefficiencies.

If high voltage MPPT controllers with 5-10 kW CCS1 output were available would you recommend the Rube Goldberg scheme I outlined above? Of course not. You'd say "just buy a simple, cheap, efficient XYZ box". Unfortunately nobody sells an XYZ box, so we're stuck wtih Mr. Goldberg.

 

[Ampster]

My inverters are about 95 percent efficient in converting the energy from the panels to 240 volts. Only a small part of my system is a DC string because most of my system is microsd. I do not know what the efficiency is on the onboard charger, but from some sources it seems there are also losses relating to the the thermal management system which is also the case with DC charging. So, until cost effective DC charging is available I am not going to worry about it. The cost of a kWh from my solar panels is about $0.08 er kWh. I do agree that going through the extra steps to charge a stationary battery at 50 volts does add a layer of losses and contributes to a shortened life for the stationary battery.

 

[Doggydogworld]

Yeah, the DC-AC-DC route bothers me, but it's the extra batteries that really get my goat. He's got a 98 kWh (or 130 kWh) battery sitting right there. Why spend thousands more to add a handful of extra kWh? And turn more of his panel watts into heat in the process. It's nuts.

I suspect the extra battery might be necessary with AC charging, though. The onboard AC charger is in control once the initial negotiation establishes voltage and current levels. I doubt it would respond well to a current that keeps changing as clouds pass over the panels or whatever. With DC charging the box outside the car has more control.

 

[Ampster]

The onboard AC charger is in control once the initial negotiation establishes voltage and current levels. I doubt it would respond well to a current that keeps changing as clouds pass over the panels or whatever. With DC charging the box outside the car has more control.

I charge my EVs from solar during the day. The grid is the backup if there are clouds but I often change current several times depending on solar output and time of day.
There is an EVSE made by Emporia Energy that can communicate with their Energy monitor to only use available solar energy. I have not heard reports and I suspect it changes charging rate often.

 

[GregTR]

I charge my EVs from solar during the day. The grid is the backup if there are clouds but I often change current several times depending on solar output and time of day.
There is an EVSE made by Emporia Energy that can communicate with their Energy monitor to only use available solar energy. I have not heard reports and I suspect it changes charging rate often.

I happened to have one of those Emporia chargers and it works exactly as advertised. It will change charging current as solar and/or household demand changes.

This is what charging my EV looks like with solar. Only caveat is that since I have a battery backup and it responds to demand faster than the Emporia when my A/C kicked in at one of those times the battery started being used and since I still haven't imported from the grid Emporia had no way to know that the power is coming from a battery and not from the sun. This is a limitation of my particular inverter setup; Emporia can't differentiate between solar and battery power.



And this is what the EV charging looked like for the same time, you can see it ramping up with PV production increasing:

 

[400bird]

No need for a house battery to charge from PV. You loose efficiency and there's significant cost involved.

I'll agree with others, I'd love to charge 380 VDC from solar direct to the 380 VDC car. But that equipment doesn't exist and may limit that array to car charging only. And my assumption is great expense. A $500 EVSE and some monitoring hardware and it works.

With my set up or the Emporia set up like GregTR, the PV still gets used when not charging the car.