Charge 48V battery bank from higher voltage array?

[William P. N. Smith]

I'm looking for a charge controller to charge a 48V battery bank from
a higher voltage array, does anyone know of one that'll take 300 to
600 volt DC input and charge 48V banks?

Because of the ease of wiring for high voltage arrays, and the normal
use of these arrays in a grid-tie application (probably using the SMA
Sunny Boy), and the layout of the building's roof, there will be as
many as 8 high-voltage arrays and up to 5 inverters.

If the grid power goes down for long periods of time I'd like to be
able to charge the batteries that run the critical systems using the
solar panels, but it'd be incredibly inefficient to just connect the
(4 amp) arrays directly to the batteries...

Does anyone make such a beast, or has anyone solved the islanding
problem with grid-tie arrays? There will probably be a generator, but
I can't guarantee that the load will always exceed the array output,
FWIW.

Thanks in advance for any thoughts!

 

[William P. N. Smith]

Ok, so you have this massive grid tie. You are selling most of the
time. But for safety, you want to keep some batteries charged anyway
for "critical systems".

Yeah, I'll let the (probably Outback) inverter keep it's batteries
topped off from the grid. Somewhat less efficient than charging the
batteries directly from the arrays, but it's a lot less complex.

Fair enough. You say you might have eight
arrays. Pick one or two of them, and have a grid-powered NC relay that
will divert to an inexpensive MX-60 for the batteries when the grid
drops.

The problem is that the array voltages vary from 300 to 600 volts DC,
depending on their size and configuration, and I don't know of any 48V
chargers that'll handle that kind of input. Connecting them directly
to the batteries will allow them to charge, but only at about 4 amps,
which isn't very useful...

 

[boB K7IQ]

Yeah, I'll let the (probably Outback) inverter keep it's batteries
topped off from the grid. Somewhat less efficient than charging the
batteries directly from the arrays, but it's a lot less complex.


The problem is that the array voltages vary from 300 to 600 volts DC,
depending on their size and configuration, and I don't know of any 48V
chargers that'll handle that kind of input. Connecting them directly
to the batteries will allow them to charge, but only at about 4 amps,
which isn't very useful...

One way (today) you can charge a 12V to 48V battery, is to use a
batteryless grid tie inverter (Sunny Boy for instance works) and
connect its AC side to the output of an inverter such as the OutBack
VFX3648 or VFX3524. Doesn't have to be a Grid tie battery based
inverter. The Sunny Boy will try to sell to the FX inverter AC output
and drive power from the FX's AC side to its DC (battery) side.
You may have to add some diversion to make sure the battery voltage
doesn't rise out of control.

Anyway, that's one way to get say, 450 VDC to 48 VDC to charge
batteries. AC coupling as SMA calls it.

boB
K7IQ

 

[William P. N. Smith]

One way (today) you can charge a 12V to 48V battery, is to use a
batteryless grid tie inverter (Sunny Boy for instance works) and
connect its AC side to the output of an inverter such as the OutBack
VFX3648 or VFX3524. Doesn't have to be a Grid tie battery based
inverter. The Sunny Boy will try to sell to the FX inverter AC output
and drive power from the FX's AC side to its DC (battery) side.

So the FX inverters will charge their batteries if their _output_
rises higher than they are driving it? Are you certain about this?

 

[RF Dude]

The Grid Tie Inverter.... then an independent AC powered 48V rectifier would
be the most off-the-shelf solution that people have mentioned. And if you
suffer a failure to the PV system, you can still charge your battery from
mains and use your battery based system as a UPS for your critical systems.

Going to AC then back to DC is a double conversion for which an efficiency
penalty is paid. But you have the most flexibility in this approach.

To eliminate the double conversion, you would require a DC-DC converter that
would take your 300VDC and make 54.0VDC (or as recommended by your battery
vendor) to float your battery plant. This way you have one conversion only.
You can look at companies that specialize in these converters
(http://www.absopulse.com/ is one, another is
http://www.analyticsystems.com/converters.htm ), or role your own using a
converter brick from www.vicr.com and there are many more.

Realize that a simple SWITCH mode 48V rectifier will have a bridge rectifier
and filter capacitor in the front end that takes 240VAC and creates 340VDC
for the subsequent stages to chop up and create your desired output voltage.
If you are a hacker and can find a surplus 48V switch mode rectifier from
the telecom world, you could convert it to work with DC input. Remember
that we are talking dangerous and potentially leathal input voltages so if
rolling your own with the Vicor bricks or making modifications to other
equipment, you should be educated in power electronics and know what you are
doing. Otherwise, talk to a DC-DC converter manufacturer as mentioned
above.

Good Luck.

 

[William P. N. Smith]

The Grid Tie Inverter.... then an independent AC powered 48V rectifier would
be the most off-the-shelf solution that people have mentioned.

Yeah, though if the grid goes down for very long, I can't recharge the
batteries on my 48V backup system.

converter brick from www.vicr.com and there are many more.

I just took another look at the Vicr BatMod, and it has 150V and 300V
DC input options, so I'll look at that a bit more carefully.

Thanks!

 

[SpiderG]

Since boB works for Outback, I'd be pretty certain he knows what he said...

SpiderG

 

[boB K7IQ]

Since boB works for Outback, I'd be pretty certain he knows what he said...

SpiderG

Yes, it has been done by me personally, and others too.
Thanks,
boB K7IQ

PS. I'm not at OutBack anymore...

 

[Jim Baber]

Jim Baber replys: I am running about 250 V on all 4 of my Sunny Boy
inverters at this moment.
I Have been thinking about this very problem, and I recommend Mr.
Smith look into the Sunny Island inverter from SMA. (They designed the
popular and successful Sunny Boy inverters)
I personally intend to use one of these inverter / controller units
on a small battery bank to generate the synchronization A/C references
needed to fool my main inverters into thinking the grid is still alive.
The Sunny Island is designed to interface between Solar PV, batteries,
the grid, wind generators and motor generators, it does include some
load switching capabilities. Its principle drawback is that it is only
4200 Watts at 120 V single phase, however, that output voltage can be
fed to one side of a 240 V center tapped auto transformer to develop the
240 V grid reference signal (normally from the grid) for my 4 regular
Sunny Boy inverters that I currently am running. I do not even need to
draw much of the output from the Sunny Island in the daytime, and it
wouldn't draw the battery string down until after dark. This means I
will have as much as 10 kW at 240 VAC available during the day.
Seriously I have really seen it produce that much, but normally it is
between 7-9 kW.

 

[William P. N. Smith]

look into the Sunny Island inverter from SMA.

Ah, perfect, many thanks! Bidirectional inverter sounds like a new
technology, and sounds like what Outback is doing as well, so I'll
have to do some more research!

Thanks!