Using solar micro inverters with batteries instead of panels

[kundip]

Thanks for the reply... very much appreciated
My batteries are 24 volt DC and I need 40 Volts DC constant I think. Going off the Enphase M215-60-230-S22 manual.
So the buck converter can ramp up voltage and control the current.
The 40 volts DC will turn off MPPT. Which makes it easier to add a second inverter as I understand.
I am looking to produce 240 Volt AC

 

[Ampster]

You are focusing on the DC voltage which is distracting you from the basic issue of getting the micro inverter to work without a grid connection. Also did you read the comment earlier in this thread about how the electronics in the micro would be fried by a battery?
EDIT: I was mistaken about the comment earlier in this thread. It was another thread but that poster did not have any success.

Using solar micro inverters with batteries instead of panels

The subject says it all. I was wondering whether anyone has tried connecting a solar panel micro inverter to a battery bank instead of a panel. I'm talking here about the grid connect micro inverters that go straight into 240V and have their own anti islanding protection. Obviously you would...

diysolarforum.com

 

[kundip]

Thanks again for your patience.
My inverter is grid connected.
I am looking to emulate a solar panel at night supplying from the DC batteries about 215 Watt 240 Volt AC
Continuously 14 hours a night via the micro-inverter.
Re the micro inverter being fried - the Buck Converter should limit the DC current to below the maximum of 10 Amps.
Added 14/04/23: PLEASE NOTE - I no longer use buck converters nor advocate in this situation. See my later posts.

 

[newbostonconst]

I have ran an IQ7x on 24 volts to almost 70 volts increasing one cell at a time.

So how this micro inverter works running it connected to a battery:
It started making power at 33 volts.
It always pulls right around 6.15 amps.
So at 33volt x 6.15amp = 203 watts
Thus at 50 volts x 6.15amps = ~245 watts
It maxed out at 53 volts x 6.15 amps = 326 watts
It makes 326 watts at all voltages above 53 volts.

I never have seen the wattage drop when warm like the specs say....I ran it at room temp.

 

[kundip]

I have ran an IQ7+ on 24 volts to almost 70 volts increasing one cell at a time.

So how this micro inverter works running it connected to a battery:
It started making power at 33 volts.
It always pulls right around 6.15 amps.
So at 33volt x 6.15amp = 203 watts
Thus at 50 volts x 6.15amps = ~245 watts
It maxed out at 53 volts x 6.15 amps = 326 watts
It makes 326 watts at all voltages above 53 volts.

I never have seen the wattage drop when warm like the specs say....I ran it at room temp.

This is great information. I appreciate your valuable time.
So you are saying that the inverter only takes the amps it needs as per the voltage?
Other posts say that the battery will fry as the battery will push all the amps it can into the inverter.
Also (if you don't mind) - Do you think a Buck Converter is a waste?
Thanks
Added 14/04/23: PLEASE NOTE - I no longer use buck converters nor advocate in this situation. See my later posts.

 

[newbostonconst]

I hook the Iq7x directly to the battery bank. So no Buck Converter is used in my setup....not sure for what reason you would need one? (to raise the voltage?)

It always pulled 6 amps when it was inverting. I run to a high voltage of almost 70 volts and it stayed at 6 amps all the time above 33 volts. 6 amps is 6 amps so the battery can't push more then that.

Good Luck with your project, are you going to try the M215-60-230-S22 in this way....It was easy to test, I used a Victron Smart Shunt to do all my measurements on the DC side.

 

[kundip]

Again...Thanks very much.
Yes I have 2 x 24 Volt Systems. So I need to raise the voltage for one system.
The buck converter I have ordered should do the trick. It can take up to 15 Amps.
It has good range on the input side (10V ~ 60V) and the output side (12V ~ 80V). 95% efficiency.

Yes M215-60-230-S22. But I also have an IQ7.

I have a shunt connected. I use a local shunt made by EV Power. (Western Australia).
Their BMS is very neat https://www.ev-power.com.au/lithium-bms/

I will let know how I get on. The Buck Converters arrive 13 Dec 2021.

 

[meetyg]

I wouldn't do it! (but I think there is a solution...)
In the best case, the MPPT algorithm of the microinverter would never lock, and output would fluctuate.
In the worst case it would simply fry your microinverter and/or ruin your batteries.

There are some grid-tie inverters that are actually designed for this (like the SUN 2000W) , but I don't know of any microinverters that have this feature.

What the MPPT algorithm is looking for is a maximum power point. With batteries, the maximum power point is theoretically infinite.

But, maybe you can use one if these DC-DC boost converters in betweenyour batteries and the microinverter, which has a current limiting function.
For example, lets say you set your DC-DC converter to a maximum of 15 amps and 36v. If the microinverter will try to pull more than 15amps, the DC-DC will lower the voltage accordingly. The MPPT should then understand that it went too far with the current and slowly bring it down.

All this is in theory, I haven't tried it, but I did play with some of these DC-DC boost converters. They are pretty nice and cheap, but a bit if a pain to setup (using pots to set voltage and maximum current).

I would suggest you get one and experiment slowly. I would set the maximum voltage a bit less than your microinverter's maximum voltage, and the amps at almost the maximum the microinverters can handle.
But make sure that this is inline with your batteries abilities and your wiring guage.
If you set it properly, it should protect your batteries a bit, but still I would add proper fusing just in case.

There are different versions of these cheap DC-DC boost converters, but I would get the 1800w one, just because it has a temperature controlled fan, and its beefier.
These guys generate heat at high loads, so make sure to keep them indoors and properly ventilated.

18.34US $ 30% OFF|Dc-dc Constant Voltage Constant Current Boost Converter 1800w 40a 10-60v By Fan For 12-90v Boost Power Module Step Up Adjustable - Voltage Regulators/stabilizers - AliExpress

Smarter Shopping, Better Living! Aliexpress.com

www.aliexpress.com

Good luck... and stay safe.

 

[newbostonconst]

I wouldn't do it! (but I think there is a solution...)
In the best case, the MPPT algorithm of the microinverter would never lock, and output would fluctuate.
In the worst case it would simply fry your microinverter and/or ruin your batteries.

Are you just guessing? or have you actually tried what you are trying to say?

As stated above I have connected an IQ7x to batteries for a couple weeks while testing. Voltage and amperage is rock steady. Or are you trying to pay down using a buck converter?

 

[schmism]

Yes confirming that I'm trying to timeshift. And yes a powerwall or other similiar product would do exactly what I want. I just don't want to spend $15,000 especially when I have a number of the micro inverters already on the roof and hooked into the grid.

The microinverters on the roof dont help you rig up the thing you need connected to the battery pack. So now you need to buy more of them to put next to the battery pack. lets assume a 9000w output thats like 40 of the IQ7. thats like $5200 in inverters. And you have no way to control them to tell them when to come on/shut off (time shift) and you have no way to charge the battery off the grid at non-peak time. (so you need a grid connected charger that you can control based on time of day)

you'd also have to use something like a electrodacus to monitor the battery voltage and trigger a very hefty 300a relay to act as the low voltage cut off. (not that electrodacus will help as it doesn't support 36v systems) so i dont know what you use for that. I suppse in theory you could route the output of the micro inverters though a device that is capable of shifting the frequency out of range and useing the microinverters to shut themselves off. However the devices to do that are also extremely limited. The only thing i know of is a solark.....

So yeh you have to setup, configure, wire all that and maintain it instead of just bolting something like a Solark 12k to the wall, setting the time shift features on it and connecting it to a battery.

some other napkin math to consider. The "pile" of used car batteries to start with.... 9kw feedback *3hrs = 27Kwh*2 because depth of discharge on FLA is 50% = 54kwh of lead. /36v (to work in the allowable range of the IQ7) = 1500ah of battery. at like 200ah each thats like a stack of 21 car bats. The .33C rate discharge would also be considerably higher than people like Rolls recomends for long term battery life.

 

[meetyg]

Are you just guessing? or have you actually tried what you are trying to say?

As stated above I have connected an IQ7+ to batteries for a couple weeks while testing. Voltage and amperage is rock steady. Or are you trying to pay down using a buck converter?

I have tried connecting a battery to a GTI, but it wasn't a microinverter. It was never able to lock. Maybe because the battery was an SLA and Puekert effect made the MPPT algorithm confused.

 

[newbostonconst]

I have tried connecting a battery to a GTI, but it wasn't a microinverter. It was never able to lock. Maybe because the battery was an SLA and Puekert effect made the MPPT algorithm confused.

Or not enough voltage?

 

[kundip]

The microinverters on the roof dont help you rig up the thing you need connected to the battery pack. So now you need to buy more of them to put next to the battery pack. lets assume a 9000w output thats like 40 of the IQ7. thats like $5200 in inverters. And you have no way to control them to tell them when to come on/shut off (time shift) and you have no way to charge the battery off the grid at non-peak time. (so you need a grid connected charger that you can control based on time of day)

you'd also have to use something like a electrodacus to monitor the battery voltage and trigger a very hefty 300a relay to act as the low voltage cut off. (not that electrodacus will help as it doesn't support 36v systems) so i dont know what you use for that. I suppse in theory you could route the output of the micro inverters though a device that is capable of shifting the frequency out of range and useing the microinverters to shut themselves off. However the devices to do that are also extremely limited. The only thing i know of is a solark.....

So yeh you have to setup, configure, wire all that and maintain it instead of just bolting something like a Solark 12k to the wall, setting the time shift features on it and connecting it to a battery.

some other napkin math to consider. The "pile" of used car batteries to start with.... 9kw feedback *3hrs = 27Kwh*2 because depth of discharge on FLA is 50% = 54kwh of lead. /36v (to work in the allowable range of the IQ7) = 1500ah of battery. at like 200ah each thats like a stack of 21 car bats. The .33C rate discharge would also be considerably higher than people like Rolls recomends for long term battery life.

Thanks. I have 13 micro-inverters from Queensland for $648USD all up.
Two of them brand new IQ7's. Courtesy of our silly rebate system here.
You can get a 6KW system installed for USD $2,800.
Which means that people are dumping perfectly good 3KW systems cheap as the maximum you can have per house is 6KW (grid- connected)
Quite often the systems are only a few years old.
I am using two micro-inverters on this project and will be using at night only to cover 200-400 watts AC.
I will operate system manually - until I get system working properly.
Then I will add a cut out solenoid on the positive lead from battery on a timer or some switch of the kind.
The batteries are 24V LiFePo4 with plenty of protection.
I am not looking to power any heavy devices like Toasters, Microwaves Ovens etc. - just the small items including our fridge.

 

[newbostonconst]

Thanks.

Ditto....I am allowed 7kw and am adding 2 IQ7x and panels to take out my base load of 700 watts at night. Adding on to my battery to have 35kwh also. I may also automate a couple more IQ7x's to turn on when charging my car. I have Smartthings with an energy monitor that can be programmed to turn on a relay to enable more IQ7x's when we are pulling more power it is just the reaction time is not the greatest with Smarthings.

Note, if you add a relay to turn on and off the Micros do it on the AC side....The AC relays are way cheaper and work better.

 

[kundip]

Ditto....I am allowed 7kw and am adding 2 IQ7+ and panels to take out my base load of 700 watts at night. Adding on to my battery to have 35kwh also. I may also automate a couple more IQ7+'s to turn on when charging my car. I have Smartthings with an energy monitor that can be programmed to turn on a relay to enable more IQ7's when we are pulling more power it is just the reaction time is not the greatest with Smarthings.

Note, if you add a relay to turn on and off the Micros do it on the AC side....The AC relays are way cheaper and work better.

If I am only using those two micro-inverters at night I will never break our 6KW limit.
Good info on the relay I will put it on the positive side.
I have stuck with 24V batteries as I find there is a much better (& cheaper) selection at that voltage.
I will look at Smarthings - slow reaction times are good in this situation as far as I am concerned.
Currently I just use TP Link Smart Plug timers everywhere.The TP Link app is Kaza.
Kaza also responds to Google & Alexa voice commands for manual Turn on - Turn off.

 

[Jordi]

Dear all,

This weekend I will try feeding a 600W MPPT grid-tied inverter with batteries and a DC-DC buck converter with current regulation (max. 5 Amps).

https://diysolarforum.com/threads/testing-grid-tied-inverter-with-battery.32173/

After the test I will post results. Feel free to comment.

Your conversation is very helpful as I see other people have or are following the same working principle I am planning to apply.

 

[newbostonconst]

Dear all,

This weekend I will try feeding a 600W MPPT grid-tied inverter with batteries and a DC-DC buck converter with current regulation (max. 5 Amps).

https://diysolarforum.com/threads/testing-grid-tied-inverter-with-battery.32173/

After the test I will post results. Feel free to comment.

Your conversation is very helpful as I see other people have or are following the same working principle I am planning to apply.

600 Watts is likely going to require 12 amps and the buck converter only suppling 5 amps might lead you down a wrong path but I don't know. You might want to hook it up directly to the battery for a short time to see what it does. Fused of course.

 

[Jordi]

600 Watts is likely going to require 12 amps and the buck converter only suppling 5 amps might lead you down a wrong path but I don't know. You might want to hook it up directly to the battery for a short time to see what it does. Fused of course.

Does not the load of a grid tie inverter addapt to the supply when limited? Otherwise how can 100W of solar panels work with a 600W grid tie inverter?

 

[newbostonconst]

Does not the load of a grid tie inverter addapt to the supply when limited? Otherwise how can 100W of solar panels work with a 600W grid tie inverter?

Not 100% sure but my response of needing 12 amps was derived from my watching the setup I did work.

PWM stands for pulse width modulated, so the power is fed in pulses....so does that mean each pulse is wanting to be at full amperage?

After you try it let us know.

 

[Jordi]

Not 100% sure but my response of needing 12 amps was derived from my watching the setup I did work.

PWM stands for pulse width modulated, so the power is fed in pulses....so does that mean each pulse is wanting to be at full amperage?

After you try it let us know.

I could not perform the test as explained in the originary post due to suspected defective DC-DC buck converter. (Buying at Aliexpress is tricky...)
I also wonder If an undersized battery could be used for such test even If just for a few seconds. Without current limiting, the test seems risky; not for the inverter but for the battery.

https://diysolarforum.com/threads/testing-grid-tied-inverter-with-battery.32173/

I will try this week the inverter with the solar panels, but this is no new stuff to you. In 1,5 months I will receive another buck inverter, maybe this one is good and I can use it for the test.