solar car charging

Schneider XW-Pro hybrid inverter-charger:

https://www.solar-electric.com/schneider-electric-xw-pro-6848-21-inverter.html

The issue with "no battery/no storage" on a solar power system is that our "utility" voltage is assumed to be a steady 120/240 VAC and "unlimited' current (amps) and energy (amp*hours/watt*hours) available. There is no method "today" of looking the AC power coming in and figuring out how much "extra" power is available (i.e., charging care at 100% of system rated power at noon, and only taking 20% of available solar system power at 9am and 3pm when sun is lower in the sky (let alone clouds and such).

Today, you either have 120/240 VAC at 3,000 Watts to charge your car, or you have 0 VAC and 0 Watts (i.e., car takes "3,000 Watts" charging, and solar can only supply 2,999 Watts from array--Only choice is to kill the 120/240 VAC power--Vs "brown out" such as supplying 180 VAC at 2,000 Watts).

Without any local solar energy storage--You are left with the "Bang Bang" control of charging (bang bang is term used in engineering control theory where 100% of energy is used to 'position" something--Such as a gun turret on a ship, 100% from zero to maximum slew rate, then 100% reverse energy to bring from max to zero slew rate--As in positioning and anti-aircraft gun at maximum possible tracking speed). Or full throttle leaving a stop sign to speed limit then maximum braking at the next stop sign. Hard on the equipment and not always the most fuel efficient.

With "today's" tech... There are AC inverters out there that will run without batteries--And you could install 3x more solar panels to harvest from 9am to 3pm and get more "time on charge" for the car battery (i.e., 9,000 Watt array producing at 9am 3,000 Watts)--But you are wasting array energy at noon (9,000 Watt array supplying a 3,000 Watt load).

There is the frequency shifting that is now common in off grid and hybrid inverters.... AC Source supplies 60 Hz -- Means AC Source can supply requested power (i.e., AC source can supply 100% of requested power). And 61 or 59 Hz, the AC source cannot supply that level (or a reduced to zero Watts) of power.

Off grid inverters use this to "modulate" AC sources (such as GT type solar inverters to limit their output current if hybrid inverter cannot accept full AC current for charging battery bank)--And (I think) that is being implemented in some areas/countries by utilities to "modulate" Home GT Solar power Systems to prevent "too much" solar power overwhelming the grid (i.e,. soft feedback to GT inverters at homes to reduce output by XX%, vs the "bang bang" approach where utility grid loses frequency control and at 60.9 Hz "too much solar" and at 61.1 Hz, all GT Solar home inverter systems STOP supplying power to grid--And the utility loses 100% of the GT solar which can be 10% or more of the "total grid load"--I.e., Bang Bang control loop).

To do Solar panels -> car (no batteries)--I think car chargers (or wall mounted charger interfaces) need to change. The simplest would be DC from panels to car, with MPPT controllers in the car charging circuit. Then the car/charger itself would figure out how much the panels can supply at any time during the day.

Or, something could be done with frequency feedback too.

I understand there to be some car/AC Charger interface/feedback that modern EVs have  (such as 1,200 Watts charging on 120 VAC, 4,800 Watts charging on 240 VAC, and the "super charger" that can even have a 300+ VDC direct charging path (intended for 480 VAC 3 phase utility powered charging stations).

It would be interesting to understand more about the Direct DC charging interface to a car (such as Tesla) and see if a ~300 VDC Vmp solar array could power the DC charging port. I believe that even the DC charging port has a digital communications link with the "super charger"--So it would be more complex than just plugging in 380 VDC from a solar array to DC fast charge car port.

10+ years ago, Tesla owners were asking about DC port solar charging:

https://forums.tesla.com/discussion/15586/charging-from-solar

No useful answer then...

2019--Still no DC solar options(?):

https://thedriven.io/2019/04/15/is-there-a-dc-charger-i-can-use-with-a-dc-solar-panel-system/

No such plans--Use AC charging instead (only a few percent lost in DC To AC conversion--So use GT Solar and Utility Power):

https://thedriven.io/2018/09/04/do-i-need-to-convert-my-home-battery-to-charge-my-ev/

Basically, manual (automatic?) switching of AC power draw of the Car's charger to match the AC power available (i.e, 16 amps, 32 amps, etc.)...

https://thedriven.io/2018/09/06/i-live-off-grid-how-do-i-charge-my-ev/

-Bill

From the "standard" battery life charts (for lead acid/agm/etc.) type batteries... They seem to have a constant AH life (if otherwise well maintained).

For example a couple of "models": 25% discharge = 2,000 Cycles and a 50% discharge = 1,000 cycles of life. Using made up numbers that are roughly accurate:

• Say 400 AH @ 48 volt AGM batteries
• Suggest 1/10 to 1/8 discharge rate maximum (for best life)
• 400 AH * 48 volts * 0.25 discharge * 2,000 cycle life = 9,600,000 WH = 9,600 kWH of cycle life
• 400 AH * 48 volts * 0.50 discharge * 1,000 cycle life = 9,600,000 WH = 9,600 kWH of cycle life
• Discharge 25% and 10 hour rate = 2.5 hours of AC Car Charging
  
• 400 AH * 48 volts * 0.85 AC inverter eff * 1/10 hour discharge = 1,632 Watt moderate discharge rate
• 400 AH * 48 volts * 0.85 AC inverter eff * 0.25 discharge = 4,080 Watt*Hour "gentle discharge"
• Discharge 50% and 8 hour rate = 4 hours of AC car charging
• 400 AH * 48 volts * 0.85 AC inverter eff * 1/8 hour discharge = 2,040 Watt heavier discharge rate
• 400 AH * 48 volts * 0.85 AC inverter eff * 1/8 hour discharge rate * 0.50 discharge = 8,160 Watt*Hour "harsher discharge"

The above I would consider moderate to heavy discharging. And anyone that ever asked a battery mfg what is a "discharge cycle"--They say a 5% discharge is a "cycle" and 25%, 50%, and 80% are all a single discharge cycle too.

If you are charging your car during the day--Then you have your solar panels supplementing the battery power, or even "charging" both the car and the solar battery bank at the same time. If your discharging is overnight--The solar battery bank is doing all of the work.

The above are just numbers picked out of the air... You have an existing system and what can you expect from it--Or you have your planned loads (6 hours * XXXX Watts per week) of the EV--And should design the solar system to support that load (and the rest of the home/etc.)?

Lots of variables here (including brand/model/expensive or cheap AGMs, etc... And would you like 4 years from an 400 AH AGM bank or 8 years from an 800 AH battery bank (seems to be a good first approximation--A 2x larger AH battery bank will last 2x longer vs 1x AGM battery bank). The overall costs are the same (8 batteries for 4 year vs 16 batteries for 8 year life).

HOWEVER, you have 2x the up front costs. You should have a larger solar array/solar charger/AC to DC backup charger/etc. to maintain a suggested 10% rate of charge for off grid power system.

Also--What if something does go wrong... Have to leave the house in a hurry (going to doctor, spouse stranded on road, etc.) and forget to turn of EV Charger after X hours (or automatic timer fails) and take bank DEAD (need new batteries)... The 800 AH bank has 2x more $$$ at risk vs the 400 AH battery bank.

No "right answer here"--Your risk tolerance, $$$ available, etc. all play into the decision.

Helpful?

-Bill

More or less, if your bank is at float voltage or higher, the batteries are not being "affected" (life/loading).

Single phase 120/240 VAC inverters pull a 120 Hz sine (like) current waveform (peak current to near zero current as the AC voltage sine wave crosses zero volts). So there is a "ripple voltage" on the Battery Bus.

In theory, if the battery bus is at near 12.8 volts (12 volt bank), the ripple current can cause "micro cycling" of the battery bank... Basically discharging at peak current (120 Hz) and charging at near zero current (i.e., discharging to 12.4 volts and charging at 13.2 volts--Assuming a 0.8 volt ripple voltage--Just for example here--Your ripple voltage will differ).

If you only run the "heavy" AC loads @ 13.6 volts, the battery is always in the "float or charging "realm" and not micro cycling--Micro-Cycling is assumed to be harder on battery life (how much--I have no idea).

Just as a start... I would suggest a solar array that is, at least, 2x larger than your AC charging load to keep the bank at float voltage when charging the EV.

If you want to play around a bit more with a spreadsheet, the PV Watts program does allow you to input your array sizing/tilt/location, etc. and see the average solar harvest wattage hour by hour (another solar model! Yippee):

https://pvwatts.nrel.gov/pvwatts.php

Each day is a "real day of sunlight" based on the average/typical day (month/day) for >20 years worth of days for that date (if this makes sense)--So you can see what to expect (array performance in Watts for each hour of the day)--And play with the numbers to see what makes sense for you to allow "float harvesting" and avoid early micro cycling wear and tear on your battery bank.

-Bill