Charging Battery bank with underpowered wind turbine to be able to run a too big grid tie inverter.

Yves · March 2016

Hi!

I have been learning lots and lots about wind/solar/battery's/arduino/wiring etc. but I have one thing which I can't seem to figure out myself.
This is because I run my system in a unlogical & unbalanced way with the equipment I have gathered (mostly for free).
So I might be the only one trying to do it in this fashion so google is not giving me any help.
I know that it does not make a lot of sense to connect these unmatched components but I'm trying to get the best performance with what I have without having to invest in a system that is never going to be very effective anyway. :-)
(I might in the future invest in a separate solar system but that's another story)

My question:
I run a small wind turbine to charge my battery bank, which is actually too big for the wind turbine, to power my 1000W grid-tie inverter which is also too big for the wind turbine. The Inverter can't run straight from the wind turbine obviously, so I use the battery's as a buffer to store energy.
The way I wired it up I can control the grid tie inverter with an Arduino, so that it turns on when battery's reach a setpoint voltage, and turns off when reaching another setpoint voltage.
The system runs fine but I am uncertain about the setpoints to convert as many available power in to the grid.
Right now I have the 3phase rectifier connected straight to the battery's, so the voltage rises as they get charged.
I could use a DC-DC converter (I have laying around) to convert rectifier voltage to bulk charge voltage/float charge voltage.
And at which setpoints should I turn on the inverter to discharge the battery's to the grid? Slow charging so maybe 100% charge rest voltage, or maybe float voltage?
I plan on using only 20% DOD to keep what's left of the battery's working for as long as possible (converting power to the grid is the main goal)

Thanks for reading and hopefully you have some advice to get me going and improve my system!

Details:
Wind turbine, 24V 3phase AC (rated 400W, but producing more like 50W)
Battery bank, 32 x 12v,24Ah AGM battery. Wired for 24V. (Used battery's) (http://www.yuasabatteries.com/pdfs/NP_24_12_DataSheet.pdf)
Grid tie Inverter, 24V, 1kW output 240v AC (actually outputting between 700-900W to the grid).

bill von novak · March 2016

I run a small wind turbine to charge my battery bank, which is actually too big for the wind turbine, to power my 1000W grid-tie inverter which is also too big for the wind turbine. The Inverter can't run straight from the wind turbine obviously, so I use the battery's as a buffer to store energy.

The way I wired it up I can control the grid tie inverter with an Arduino, so that it turns on when battery's reach a setpoint voltage, and turns off when reaching another setpoint voltage.
The system runs fine but I am uncertain about the setpoints to convert as many available power in to the grid.
Right now I have the 3phase rectifier connected straight to the battery's, so the voltage rises as they get charged.
I could use a DC-DC converter (I have laying around) to convert rectifier voltage to bulk charge voltage/float charge voltage.
And at which setpoints should I turn on the inverter to discharge the battery's to the grid? Slow charging so maybe 100% charge rest voltage, or maybe float voltage?
I plan on using only 20% DOD to keep what's left of the battery's working for as long as possible (converting power to the grid is the main goal)

Thanks for reading and hopefully you have some advice to get me going and improve my system!

Details:
Wind turbine, 24V 3phase AC (rated 400W, but producing more like 50W)
Battery bank, 32 x 12v,24Ah AGM battery. Wired for 24V. (Used battery's) (http://www.yuasabatteries.com/pdfs/NP_24_12_DataSheet.pdf)
Grid tie Inverter, 24V, 1kW output 240v AC (actually outputting between 700-900W to the grid).

>The Inverter can't run straight from the wind turbine obviously, so I use the battery's as a buffer to store energy.
First problem. Your grid tie inverter isn't designed to run that way with a stiff voltage source like a battery. So you risk (at best) poor/erratic operation and (at worst) damage to the converter. (Of course, if it's the sort of unapproved inverter I am thinking of, it's not going to be a great performer to begin with.)

>I could use a DC-DC converter (I have laying around) to convert rectifier voltage to bulk charge voltage/float charge voltage.
A DC/DC converter will result in a poor impedance match between the wind turbine and the batteries. In low winds it can "pull down" the generator and cause it to run far too slowly to be efficient. In high winds this can cause the generator to "run away" and be destroyed. A diversion load is a much better solution there. Diversion loads are popular, and controllers like the old Xantrex C40 can manage a diversion load and maintain the correct bulk/float voltages.

Other problems - if your turbine is insufficient to keep the batteries charged they will be rapidly destroyed. Reduce your battery bank size until it is a good match for the generator (power in amps should be 5-15% of the amp-hour rating of the battery.)

The inverter you are using, if it's what I am thinking of, is not legal for connection to the grid, and you risk actions like fines and shutdowns by using it.

Yves · March 2016

First of all thanks for the quick reply!

The inverter is indeed an unapproved one, which seems to work without any problems and is sufficiently fused, same for the battery bank itself. I have not read anything about a grid-tie inverter not being able to run from a battery bank so that's new to me. I'll read in to that why a battery bank would not be a good source, and a continuously changing source like a wind turbine would be. (I am really curious so if you know a place for information let me know). For now though this is the inverter I have, and I would like to use the system as is, without spending +/- 100 euro's on a new controller. I have a redundant switching system in place which is able to switch on the inverter if the battery's are full, and shorts the wind turbine if the voltage keeps rising after that (that would mean the inverter is not working to reduce charge). I could diy a diversion load to be switched on in case the inverter doesn't function instead? (I understand shorting the turbine between the AC phases is not ideal, and it is only as a safety backup)

For the legal part, I live in Holland and we currently do not have regulation for grid tie inverters. The only way it needs to be certified is if I would want to apply for goverment funds for installing a complete solar/wind system. Of course I do not want to harm anyone and am trying to make the system operate as safe as possible, in this case all the switching relays are in the disconnect state at the moment any power would drop, disconnecting the system from the grid before and after the inverter.

Te wind turbine is able to charge the whole battery bank, with a small current though, and taking more time to do so. The system has run a couple of charge/discharge cycles now but I have not been able to record the exact amount of time it takes to run a cycle. Is it really harmful/dangerous for the battery's to be discharged about 20% DOD, and then slowly charged back up to 100%? The biggest difference being that the charge voltage and current is lower than a normal charge controller would supply. (which makes me wonder at which voltage the battery's are full).

Thanks for the help and hopefully you can tell me a bit more.

mike95490 · March 2016

most low voltage grid tie inverters have a MPPT input circuit, which is expecting to see a Current Source, like a PV panel. A battery is a Voltage Source, and the inverter's MPPT will not be able to regulate power well

Yves · March 2016

Thanks, I completely missed the fact that it was actually capable of MPPT. Will I quickly destroy the unit by using it straight from a battery bank? Or is it mostly really inefficient?

BB. · March 2016

This probably where you need to ask the manufacturer of the specific charge controller. Some (like the smaller MorningStar 15 amp MPPT controller) are designed to work with a battery bank as the input power source.

Solar panels due not have "surge current" capabilities (cell output current is proportional to amount of light falling on a cell--Solar panels are "current sources"). Battery banks, being voltage sources, can output a lot of current if the load demands it--I guess it is possible for an MPPT controller to be designed in such a way that the switching transistors would draw and excessive amount of current and could fail. But, have not heard of it happening to any forum members (that I recall).

-Bill

Yves · April 2016

Just in case anyone might be interested; my system still seems to be running just fine! The inverter is just a cheap Chinese knockoff but seems to be fine with a battery bank as a source.
Still haven't quite figured out the right set-points though. Does anyone have any advise as to which voltage should be the upper charging setpoint? (the point at which the inverter is automatically switched on). I am mostly unsure because I'm really slow-charging the battery bank. This results in a slowly increasing battery voltage in the bank, not a certain voltage supplied by a properly matched charger(for which there is endless information about multiple stage charging). Should I use the float voltage rated for the battery's? or should I use the bulk charge voltage as my setpoint?
Thanks for you reply's!

Yves

BB. · April 2016

More or less, once the battery bank is >90% state of charge, you want the battery to sit at "float" voltage (between 12.7 and 13.8 volts for a 12 volt battery bank). If the battery drops below 12.7 volts (at 75F), the battery is being discharged and will need to go through a full charging cycle (to 14.5-14.8 volts for flooded cell lead acid battery for ~2-6 hours, deep discharge, longer "absorb" time).

-Bill

Yves · April 2016

Hi Bill,

In the battery's documents I found 13.2v to be the correct float voltage. Can I assume the battery to be near 100% when the float voltage is reached? or do I need to keep it at least at float voltage for a certain time to be able to fully charge? I could set the system to let the voltage slowly rise to 14.5-14.8v but this would take at least a full day from 13.2v -> 14.5v to reach this voltage. So it's not possible to just set the right charge voltage because the wind turbine is not able to provide enough "juice" to do this. So it's directly connected with a 3ph bridge rectifier to the battery's.
I do have the possibility to individually disconnect the battery's and one by one charge them from the AC mains->Dc-> DC-DC converter which I can set to a certain voltage & amps. If I need to for battery maintenance (which only costs me electricity from the grid in staid of generating).
Thanks for the answers!

Yves

BB. · April 2016

The problem with AGM/Sealed batteries is you cannot measure the specific gravity of the electrolyte--The "gold standard" to figure out if your battery is fully charged or not.

Otherwise, if you can, disconnect all loads and charging sources and measure the resting battery voltage. In theory, 12.7 volts resting is "charged"--But you may find your AGM batteries will read even higher (lower self discharge).

The charge controller "decides" when to go into float... Typically after holding ~14.4 volts for a couple of hours (if battery is cycled), then falls back to float (13.2 volts does not charge the battery, just keeps it from self discharging). But this is purely a "software and set point" decision--Not anything directly measurable from the battery. This is a common "issue" with sealed batteries--Making guesses on how charged your battery bank really is.

I am a bit confused here--Your battery bank is (if I got it correct) ~384 AH @ 24 volts... If you had a minimum rate of charge of 5% (minimum I would suggest for a seasonal off grid system):

384 AH * 5% rate of charge = 19.2 amps
19.2 amps * 28.8 volts charging = 553 Watts minimum charging rate

If your turbine is outputting ~2 amps @ 25 volts (50 Watts)--That is hardly enough to keep the batteries happily charged, but not much in the way to recharge a battery bank after significant loads. AGMs do have very low self discharge, so without loads, your turbine can probably keep the battery bank charged....

But, if you want any sort of power from your system (loads, feeding energy back into the grid, etc.)--I do not see how you are doing it without some other charging source (solar, grid, or other).

-Bill

Charging Battery bank with underpowered wind turbine to be able to run a too big grid tie inverter.

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