Charging batteries from 250VDC

[carlb23]

HI,

I have a grid tied solar array with no battery backup. The system will not run when the grid is down and I am looking for a way to charge batteries by taking the panel output before the inverters.  Because it inverters require high voltage and low amps the 5 strings of 10 panels are producing in full sunlight aprox 250VDC @ 7.5amps each.  If i combine these strings in parallel the maximum output would be 250VCD @ 37.5 amps.

Is there any way to utilize this for battery charging.  I don't currently have a battery bank but I am open to any and all suggestions.

Thank You

Carl.

[Nando]

Yes, and you have at least two ways to go.

One is to have a 240 volts battery bank and from there you could make a simple MSW AC inverter ( MSW = Modified Sine Wave) or even a Pure Sine Wave with more Electronics.

The other case is to get a AC PWM PFC regulated power supply with the desired output voltage to charge the battery bank you may have in mind and by the power you have available a minimum of 48 volts bank, though I suggest 120 to 240 volts battery bank, ask if you need more info.

AC = Alternating Current

PWM = Pulse Width Modulated

PFC = Power Factor Correction or Corrected.

Also, you can trick the GRID Inverters to think that they have GRID power available for local power generation if you need to have that.

Nando

[electronbaby]

easiest way is to rewire modules for 120maxVOC. Use mppt charge controlers to buck to 48vdc, change out inverters for battery based grid tie, and install a 48vdc battery. sorry,...its probably cost prohibitive at this point. decisions like that are best made before the system goes in (much more hardware intensive).

I think you would be hard pressed to find  dc-dc converters that could handle that large a voltage window, let alone the combined current of the array aside from splitting it up into isolated 120vdc strings.

anyone else have any other ideas?

Roy Rakobitsch

KB2UHF

[craig110]

If I understand your question correctly, you're trying to find a way of capturing the output of the solar panels when the grid is down and (presumably) feed the power back later through your GTI.  This is a bit different from the normal backup battery setup where the batteries are intended to power the house while the grid is down.

Batteries like to be full.  Any time they are drained down, it effects their longevity. Capturing the power when the grid is down would mean keeping your batteries normally fully drained which is really not good for them.  (If the batteries were kept full, there wouldn't be any capacity left to capture the "grid down" solar power.)  I suspect that the lifespan of the expensive batteries would be low enough in this setup that the cost of replacing the batteries would far outweigh the amount of money earned by capturing the "grid down" solar power in them.

That said, the notion of capturing the power is a good one.  I'd suggest looking into some kind of mechanical storage - such as having the grid-down power running a motor that lifts a heavy weight that can later spin a generator - as these should be able to rest in the zero-energy state without being damaged.

[carlb23]

I expected to keep the batteries full at all times and only use them if the grid went down as a backup energy supply. I was hoping to be able to recharge them from the solar panels during the day if the grid went down.  I did not expect to be able to use my current inverters to produce the required AC.

Carl

[craig110]

Oh, ok.  My apologies for misreading your OP.  Check out how much you might be able to get for selling your current inverter.  (Suggestion: Most vendors have a discussion forum and you might be able to get leads on a potential buyer there.)  If the price difference is small enough, selling it and getting a battery-enabled GTI would likely be the most straightforward route.  If that is too pricy, then you're likely looking at a solution that uses a separate inverter to power just a few of your house's circuits via a cut-over switch and another switch to divert the panels' output to a charge controller feeding the batteries.

[mustang19432001]

This might be a streatch, but what if you were to keep your batteries and inverter, and simply get a charger that runs on AC and keep the batteries charged with it. Then you still have the existing system, you're not frying the batteries, and you're simply adding something to it. Granted it's a little strange, but it SHOULD work. Think of it as your own utility source and your using it to charge the batteries and puting the excess on the grid. If need be you can shut down the power going to the grid and devote it to the batteries.

On the other hand you might be able to hook up the solar panels to a pair of heavy gage wires, so the wire would be carrying the proper voltage. I don't know how your system works so there's a possibility that my idea doesn't.

[the beast]

One quick question - how much power do you need when the grid goes down?  You have close to 10kW from that array, so unless you are using some serious juice for long periods when the grid goes down and can justify a massive investment in batteries you won't be able to store much.

I would work out how much power you are likely to need to store, and get batteries to suit.  Then using a few switches disconnect a few of the panels from each string, so you can charge up your batteries this way.  maybe just use 4 panels in series and charge a 48v battery pack?  The remaining panels can be left unused, or you can use the energy to preheat your hot water or something.