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Does anyone run power backwards through their inverter ?
Caution - this only works with some inverters not all... 

After reading just over 40 pages of a very long forum post (138 pages) in relation to modifications to an inverter part of the discovery was that if an inverter is disconnected from the grid and a grid tie unit is connected to the inverter output the grid tie will back feed through the inverter and charge the batteries. Granted there is no voltage control so your grid tie will over charge you battery if it is not disconnected, howerver I had not realised this works on a lot of inverters (not all).

The unit I have is a Power Star IR3048C and after reading through 40 pages I gave it a try and plugged my grid tie into the output of the inverter and switched it all on. Once the grid tie connected up power started to flow the invetrer was showing a "load" level of 14% as the device is just reading current flow and it is not setup to normally work this way so had no idead the current is flowing back into the batteries.

This now provides a 2-3kW charge route option from my grid tie....

I doubt that UPS units will work, but I'm not sure and don't really want to fry any more UPS units.

I wondered if anyone else here had tried this or have thier powerwall setup to charge from a relatively standard inverter ?
This would probably only work with inverters that have a heavy iron low frequency transformer. The lighter high frequency ones are not likely to work.
Trouble is (I think this is how it works) you're relying on FET diodes at the battery end of the device & they may not cope too well with load sharing or the full currents.
You've basically got a pretty expensive battery charger with no features!
But hey, it's working!
Running off solar, DIY & electronics fan :-)
(09-06-2018, 11:04 PM)Redpacket Wrote: This would probably only work with inverters that have a heavy iron low frequency transformer. The lighter high frequency ones are not likely to work.

Yep, this is what I was thinking too. As long as there's a large transformer, it probably will work. I doubt UPS units will do this tho, because of the way they power on.
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UPS units I have not even considered trying as the output voltage detection / stability could trip the unit offline.

I was hoping to find someone else who had thier powerwall setup in this way :

2kW Solar --> Grid Tie --> Off Grid 230V Network <--> 3kW Inverter <--> 28kWh Battery

The battery is also connected to two separate LV solar charge controllers with 2kW and 3kW soalr attached (1kW of solar is away at the moment...)

I was a bit cautious at first, but, the guy who explained the stages in the inverter, along with the earlier one who started the post (who is an electrical product design engineer...) I started to realise how they worked in terms of voltage management and saturation in the transofrmer when the inpout voltage is low for example (another bit I did not realise). So, gave it a try.

Last few days been charging and using the inverter for various tasks, like a 3ph circular saw with a soft start, 3kW kettle (most important...), 900W toaster, 1600W electric grill, 1100W slow cooker, 300W portable washing machine, 400W sander, cuddly toy, clock radio (older UK readers should get that...). That is with 500W of solar going in (a lot of shading and other issues on 2kW of panels I need to sort...)

The upside (with risks) is that potentially if I had 2kW of solar going into the unit and the unit is rated 3kW I can then run 5kW of loads (not one big 5kW load) as long as the sun it out ! Or, more realistically a 3kW load is then netted off with 2kW of solar and only 1kW is going back through the inverter (losses) and never going above 3kW loading.

Reverting back to HV string inverters from the smaller inverter chargers seems to make a bit of sense as it reduces a lot of heavier cabling, multiple separate string breakers, isolators, etc.. Plus, the HV inverters seem to be more of a commodity and lower in price on a $/£ per kW capability.

Overall I am wondering if it works out more efficient as it bypasses the LV stage when using solar string inverters at HV for whatever load is being matched with the solar. Apparently (measurements in the posts) the charging via the back feed is more efficient than the inbuilt charging unit in the inverter...

I'm still trying to think a bit through (getting 100% comfortable with) as it seems like a solution I had been looking for (solar and battery is at the bottom of the garden).
EVTV is doing this with their new “Selfish Solar” 100kwh tesla battery based PowerSafe100 (the enclosure looks like a gun safe). The inverter manufacturer told them it wouldn’t work, they proved that it does, and now they are in talks to rework the inverter a bit so that this backwards flow is properly handled.
Guessing it is this :

Thanks for that, brilliant read, bit of a long read, skipped a lot of the intro... I like the early experiment notes...

"But if you take a 300 watt inverter working from a 12 volt battery, and use that to trick a grid-tied inverter into operation, experience indicated that it would work, but a few minutes later the 300 watt inverter blew up, and sometimes set the battery on fire. Then the house. Then the neighborhood."
"And that lesson has become part of the tribal lore of solar power. Don’t do it."

"within certain constraints it worked very well because the transformer based H-bridge inverters we were using allowed the excess power to basically reverse “rectify” right into the battery as DC."

" We hit a peak of 280 amperes, 13,500 kw of power IN to our battery through this inverter WHILE the entire shop was running on the Solar Edge output at about 4,500 watts. And we did 230 amperes for 40 minutes until the battery was charged."

Made me laugh : " I’m not very good with the “being told” thing"

"I was disappointed we couldn’t test with the Solar Edge inverters. But magically the clouds parted and we had a PEAK charge rate into a 100kw 48v pack of 242 amperes. The 12k never whimpered. Not a peep."

"All that said, we are an outlaw outfit doing outlaw hardware and software. You’ll never be able to do any of this legally with anything from us. I would say no legitimate solar installer would touch it, but actually we are hearing from a number of them that want to learn about it anyway."

With that addition of another person going through the same thought process and testing and proving it works, I'm convinced this is the route I am going now as it solves a lot of wiring capacity/distance issues for me. Just need to find the right inverter for less than the cost of an arm and a leg...
DCkiwi likes this post
Bit of an update... ok, a diagram is needed here as it is now way off normal setup... experimental and also intended to provoke ideas.

Next question is - can you feed a solar inverter with rectified DC from either the grid or a generator ? The solar inverter I have has a limiter setting and it set to 3kW to allow for loading of many hours if needed and to hopefully limit the current, is this enough ?

The 5kVA inverter will back feed and charge the battery if there is too much power on the "local grid" - the "local grid" is all after the mian incomming supply fuse, i.e. all the house wiring. This works on the 3kVA unit I have and will be adding/building a new 5kVA unit.

This is a system, which I am hoping to implement and only the battery charging from a generator or the system grid when there is not enough sun (always 2 months in winter). Intention is to charge at the home install from the Octopus Agile tariff (half hourly) which may end up with the charger switching on/off several times of the day at any time of the day depending on loading/sun.

Charging is intended to pass through the "local grid" rather than a separate charger directly into the battery bank because of 1. better overall efficiency 2. distance between grid point and battery bank 3. cost of cables and parts

The reasons for this setup is due to the wiring and location of the battery pack, solar panels and efficiency gains. It is also an outline for a the remaining separate setup off-grid over 1 mile from the nearest house. This location one of the inverters is a 1kW grid tie wind unit, which is bypassed when it's very windy and the wind turbine is just connected to the battery to allow for over 1kW gusts from the 700W turbine !

The reason for the separate 2kW inverter with limiter is to provide additional surge handling capacity by backing off the main 5kVA unit after a load is applied rather than going for a 10kVA unit. Bit of a separate experiment...

Please do not comment asking if this is approved, non regulation, etc. because it is not approved nor fits in with the wording of any regulations (probably in any country) at the moment. This is my experimental setup.... and it may take 20yrs to complete the experiment.... Big Grin
(01-25-2019, 11:01 PM)completelycharged Wrote: Bit of an update... ok, a diagram is needed here as it is now way off normal setup... experimental and also intended to provoke ideas.


This is my experimental setup.... and it may take 20yrs to complete the experiment.... Big Grin

Wow, if you crack this one, many people will be thrilled.

I have been approached more than once by people looking to add a battery to their grid-tied PV setup (ie micro-inverters on their panels, and they get an AC feed down from their rooftop).  At first I was ignorant that so many people were tricked/chose this setup ... then I got keen and thought it could be worked around ... then I gave up as it seemed quite difficult to solve.

So yeah ... power to ya ... I will be watching this unfold as Im keen to provide a battery option to any that want me to provide.
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I am an iSDT fanbois.  also a proud owner of two SC608AC's .. totally recommended.
All SMA Sunny Island and Victron Off Grid Inverters work like this. They even control the battery charging by slightly increasing the AC frequency and this makes the Sunny Island Grid Tied Inverters limit their output. They have been maufacturing this system for 20 years now and this is the standard way of installing off grid battery systems with solar panels.
rottenkiwi and completelycharged like this post
I tried running my inverter backwards last week and it's alleviated a lot of my dilemmas. I'm off grid with 5000 watts of solar but my 48v 30kwh batteries are going bad. At night time I have to run my generator. I've got generators ranging all the way up to 12000w. Running the larger generators gobble up way too much gas. Out of my Honda eu3000 and noisy 12000w I prefer the Honda eu2000.

The eu2,000 puts out 124v. My inverter puts out 120v.
When I ran it backwards I was hoping that it wouldn't charge my batteries so the power that I was consuming would basically come from the generator and if I ever started up a huge load that would normally overload the generator the voltage would sag and the power would pretty much just come from the batteries through the inverter.

But I got a big surprise when 1500w started flowing into my batteries. Okay, that's okay I can handle this. It's hard for me to figure out this early weather that power slowly drops as the batteries voltage goes up or the opposite. I've never done a controlled test this week to find out.

the voltage output of my inverter is adjustable but it requires tethering it to my laptop to make the change. I don't have my laptop right now but I'm getting it back in a few days to experiment. As of now I have managed a primitive way to reduce the voltage difference between the two which lowers the charging power. if I tether the two together with a very long light duty extension cord I can get my charging down to 150w. now I have a whole mess of extension cords out there so that I can dial in the wattage I want. Lol.
it's a balance though because on one hand I want to be able to max out the generator but that will over charge my batteries pretty quickly. The charge is shared with the load so if I have the correct cord for a 1000w charge and I'm consuming 1000 watts in my house then nothing will go to the batteries.

My AGM batteries suck and they get hot far below the recommended absorb voltage. When they start getting hot they start absorbing even more power. So with this unregulated charge I can't tell you what would happen if it got up to the proper absorb voltage but I can tell you that after holding at a lower voltage for an hour I have to shorten the cord.

this is been a dream come true when it comes to dinner time after dark and people are starting the microwave, well pump, and other heavy surge loads that used to overload the generator.

Before I try this mad scientist approach I was using a Schneider SW 4000w inverter & had set the incoming amps on the AC in breaker of the generator so that it would not pull too many amps from and overload the generator. if I began drawing more power than the generator could put out the inverter was supposed to pull power out of the batteries and help my generator out. But it was slow to respond and did not work for huge start-up surges like the propane clothes dryer and the well pump. Anyway for reasons unrelated I am now using an advanced energy 5000w inverter. It's 150 lb 20 year old piece of magic that has an solar mppt charge controller and grid-tie all built-in. I am 100% off grid though. It has a breaker for the generator and a breaker for the grid but I've never been able to get it to charge my batteries from incoming AC until I decided to run it backwards.

Before I also tried hooking a standalone battery charger to the generator but it was very inefficient with all the conversion and I burned 3 gallon vs 2 in 16 hours overnight with a 400 watt average load. Using the standalone charger enabled me to start the heavy surge loads but would not work for when I needed more than 2000w because then the generator would overload. This is because my standalone charger is capable of 3000 Watts. I even took the cover off the charger and turn the trim pots all the way down. it's an old iota that has trim pot for the voltage output and a 2nd trim pot for its maximum amperage output.

The only time I had a problem was when I turned on an 1800 watt convection oven. Every time the heater element comes on I hear a heavy load on the generator and inverter and the generator turns on it's red overload light and it stops putting out power. I even tried the long cord so the generator was only passing 150 watts and it still happened. The sound of its heavy load was more of a high-pitched frequency sound instead of the deep hum so I think for some reason it was getting out of sync. It does fine with everything else I've tried so this really confuses me.

My next project will be turning my 05 prius daily driver into a backup generator when I'm home instead of the Honda.

Anyway I hope that my experimenting has enlightened you to try burning up your own equipment for our entertainment.
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