Bob is right in that it is often in the inverter that the voltage gets
boosted first, then chopped into a sine wave.
In fact, I have an inverter laying around that I want to do this, not
for solar charging (I will explain why in a minute)
but for running it off my Nissan Leaf pack, which consists of 2 banks of
180V series cells with a disconnect in between
(which poses its own challenge, as you do not want to defeat the
disconnect with your connected inverter.

Since the full pack of a Leaf is typically between 350 - 395V the
emergency solar charging that I eventually going to plan for
is to properly and legally install a grid-interactive inverter with the
solar panels wired to provide a MPP of just under 400V
(the inverter is pretty efficient at that voltage, so that is a plus)
and let the Open Voltage go as high as needed, but under the
600V that the inverter is rated for (note that this is legally required
to be calculated at freezing temperatures, so it is always
a lot higher than the Voc spec of the solar panels at 25 deg C!)

Now in an emergency, when the grid is down and I *need* to charge my EV
as backup bank for my home or for travel, I can
open the DC disconnect switch next to the inverter (it does not have a
built-in switch), safely jerry rig the panels to the plug that
I have connected to my EV battery bank (for this purpose) and turn on
the DC disconnect switch to DC-charge my battery bank
while keeping an eye on the charging voltage.

Why am I charging my EV battery instead of directly using solar panel
power in my home?
Simple - solar panels are current limited devices. This means that there
output voltage varies strongly with the load.
So, if you plug in your panels directly into your home inverter and
there is little load, the panels will deliver their Voc which will
typically be much too high for the inverter, it can be almost 50% higher
than the max power point voltage.
Then, when the inverter tries to draw a large start-up current because
the fridge or furnace blower motor comes on, the 
solar panel will deliver its max current limit and the voltage on the
inverter will crater, causing the inverter to turn off due to
undervoltage and you never get that load started, even if it just takes
a peak of one second long. So, you need a buffer and a constant load
to capture the energy from the panels and effectively use it when
needed. That is why I want to connect the panels to my
(EV) battery bank (essentially a mobile PowerWall) and run the inverter
from the battery bank.

The inverter that I have laying around is actually a UPS without its
120VDC battery bank. I used it on my 120V Ford Ranger conversion
but removed it before selling the truck.
I noticed that (like all inverters in USA) it converts the voltage up to
almost 200V to chop a 120V AC waveform, but unlike other inverters
where the cap with the 200VDC and the chopper gets switched back and
forth between Phase and Neutral outputs to create the + and the - half
waves
from the same floating 200VDC, this inverter seems to have 2 caps and
choppers, each taking care of one half wave on the Phase output (which
is the usual H-bridge),
so the neutral is always connected to the center connection between the
two caps.
That opens possibilities to feed the inverter from the 2x 180V banks of
a Leaf pack, but first I have to make sure that what I traced in this
inverter
is actually the way it works.

The other option is the use of one of the two SURT inverters that I
have, one takes a single 192V battery bank, the other (the SURT-8000)
actually has two battery banks in series, which is pretty much what the
Leaf pack also has, so it seems a natural fit to use. They need an
output transformer due to the weird choice in how they use their input
AC, causing a weird offset AC voltage on their outputs and that is one
of the reasons that you need an output transformer to isolate and
convert it back to 2x 120VAC output. I happen to have 3 of those
transformers, if anyone is interested in a SURT and/or the 5kVA output
transformer (rack mount)
then let me know. I plan on creating a vertical rack: mounting two
L-shape brackets to the garage wall so the 19" rack mount provided by
the two brackets is not
vertical as usual, but horizontal, so I can hang the UPS and the
transformer in between the two brackets, hanging down along the wall for
minimal intrusion into
my garage space. Then I can run the 2x 120VAC output into a subpanel to
feed critical circuits in my home.

Enough plans to keep me busy for a while...
Cor.

-----Original Message-----
From: EV [mailto:ev-boun...@lists.evdl.org] On Behalf Of Jay Summet via
EV
Sent: Friday, October 06, 2017 10:40 AM
To: ev@lists.evdl.org
Cc: Jay Summet
Subject: Re: [EVDL] Solar Emergency Charging

The device you are describing is called a "micro-inverter". Typically
they are designed with "anti-islanding" proteciton, so they require a
system (the grid) already making 240v sine waves before they add 240 VAC
back to the grid. You can purchase microinverters without this
protection, but it may be easier to "simulate" your own grid with a
small battery and a true sine wave inverter.


I think your easiest solution is to have a "traditional" solar system
set up with it's own (small?) battery bank and inverter, and just use
each component in the "normal" way. (e.g. Solar cells feeding a charger
that charges the battery bank, and an inverter that produces 120 (or
240?) VAC to use to charge your car.

Perhaps you could find/use a specialty inverter/charger combo box
specifically made for the solar industry, but depending upon size of
your system, it may be cheaper to use a separate 12v inverter and one
(or more) 12 volt solar charge controllers.

A single 12 volt battery may be enough if you are primarily going to be
charging your car while the sun is shining and have more solar PV panel
wattage than the car charger draws. The battery will give you a little
buffer to provide for brief clouds, etc

If you had a slightly larger battery bank on your solar system, you
could also use it to run a fridge overnight, etc...

Jay


On 10/06/2017 09:39 AM, Robert Bruninga via EV wrote:
> I want to develop a way to charge an EV from a Solar array when the 
> grid is down cheaply.
> 
> My thiking is that typical modified sine 12v/120VAC inverters do their

> inversion first from 12VDC to high voltage DC first, and then they 
> chop it to make it modified sine.
> 
> If that is the case, it should be possible to inject high votage DC 
> from the solar panels at that same point and produce the same 120VAC.

> This then can drive any standard 120v EVSE.
> 
> We can tap into solar panels in increments of 30 VDC and should find 
> one close.  The problem will be the starting process to bring the VOC 
> down to the Vmax that matches the HV in the inverter..  That can be 
> done with a trivial big resistive load a few caps and then transfer
relay maybe.
> 
> Thoughts?
> Bob
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