Battery PV -> Battery Bank -> HOUSE/GRID Question.

[Dave Edwards]

Hi.
Keeping it ***VERY*** simple, can anyone explain (using as few three letter acronyms as possible!) the following:

I currently have 4kW Photo-voltaic connected into my house grid supply before the house meter, so I use whatever power is generated and when PV generation is greater than my house usage, the red light comes on solid on the meter & I export to the grid.
I'm completely happy with that concept and how it works and what's happening. I don't need any help or explanation of that concept..

However, I'm thinking of getting a battery bank, charging that with the PV and using the stored power to power the house.

What I want to know is in a setup like this, how it is connected to the house and the grid.
I'm imagining the PV would not have any connection to the house supply and would purely charge the battery until the battery was full and then somehow once full, it would then dump into the grid again. But that would mean some sort of controller to bypass the battery charging controller and 'push at the grid' again, just as it does now.

I'm also imagining that the battery pack, no matter how full would never export any of it's energy to the grid, even when it's attempting to feed the house yet the house is using very little power.
How does this work???
Answers in "electrical stuff for dummies" type speak please!

 

[Deleted member 26818]

Standard PV systems use an inverter, which will automatically shut down in the event of lose of power to the grid.
This is to prevent your system from attempting to power your neighbours and to protect anyone working on the grid to fix the fault.
The simplest method, would be to connect your batteries between your panels and the inverter.
That way the inverter would still shut down in the event of a grid power loss, and you would still be able to export to the grid.
If you wanted either your panels or batteries to power your house in the event of a loss of power to the grid, you would need an auto changeover switch and either a new inverter, or your existing inverter to be re-programmed.

The batteries would be considered as a new power source, and would need to be registered and passed by your electricity supplier.
I don’t know whether the batteries would affect your FIT payments.

 

[Dave Edwards]

Ahhh.... so there isn't "A correct way" I know many people are buying Tesla power wall battery packs and installing them to use their PV and power their home.
I just want to know how it's done....
I know there are acres of red tape around what the DNO will or won't allow. I'm not (at the moment) interested in that, as I know people are doing it.
I just want to know what is connected to what and how it works.
If as you say the batteries go inbetween the PV and the inverter, then there has to be some sort of charge controller between the PV and the batteries.
I'd like to find out what the basic scematic of components is, and how & why it hangs together.

 

[Deleted member 26818]

It may be that the Tesla batteries, just store the energy to use at night?
From what I understand, the system where the batteries are connected between the panels and the inverter, work similar to a car or motorbike charging system.

 

[plugsandsparks]

One i am installing has a inverter / charger combo. Batteries connect to the inverter/charger as do the solar panels.

 

[Deleted member 26818]

One i am installing has a inverter / charger combo. Batteries connect to the inverter/charger as do the solar panels.

Does it treat the batteries as a separate string?

 

[Dave Edwards]

Hmmm...
OK, so this is probably a really silly thing to say, but what stops the batteries exporting/emptying into the grid?

 

[Gary K]

Personally,I wouldn't bother!
ATM, the cost of buying and replacing the batteries is hardly covered by the amount that you would be saving. This will change in the future as battery tech improves.
Not sure about the idea of charging the batteries before the power gets to the inverter as then you will not get paid the FIT for that energy. That's assuming the charging system could handle the voltage.

 

[Dave Edwards]

I know others are interested in all sorts of other aspects of this....
I'm not interested in the FITS or the cost, or payback.
I know it is done, I just want to know how it works.
Maybe this is the wrong place to ask...

 

[Deleted member 26818]

Hmmm...
OK, so this is probably a really silly thing to say, but what stops the batteries exporting/emptying into the grid?

Nothing.
Same as with a car, the alternator tops up the battery and runs the electrical components in the car.
When voltage drops from the alternator, the battery takes up the slack.
When the alternator picks back up, it tops up the battery again.

 

[Dave Edwards]

But the car has only one power input, the alternator.
With this there is the grid providing power and the PV also providing power. I know I'm probably being thick, but When there are no batteries PV to inverter to grid seems really straightforward, but when the batteries are in the equation, I'm just not sure how it is working and how the stored battery power doesn't get exported to the grid.
I know it powers stuff in the house, but how and what decides to use battery power rather than grid power?
So I turn on a heater in the house, it uses power from the batteries (if there is any) rather than power from the grid. Is this because the voltage (pressure) is slightly higher in the batteries? And if the voltage is slightly higher in the batteries, then (Like I said) why don't the batteries just export or discharge onto the grid.

 

[Deleted member 26818]

The batteries and the panels will try to equalise.
You have a 48V battery, the panels pump out out 48V, everything stays at 48V.
If the panels output drops to 38V, the batteries will drop to 38V.
If the panels rise to 50V, the batteries will also try to rise to 50V (this is where a voltage regulator would kick in).
The only time the batteries would completely discharge, is if there is no voltage from the panels.
i.e. at night.
The inverter would need to have a cut out that prevented it from working below a certain voltage.

It is possible, that the inverter could be used to charge the batteries from the grid.

 

[plugsandsparks]

You need the correct inverter/charger. They are programmable. The inverter programming stops batteries discharging to the grid by dropping the output voltage. It is also clever enough to allow cheap over night electricity to top up the battery charging. Basically it is how it is set up. Look at Victron inverters.
MultiPlus - Victron Energy - https://www.victronenergy.com/inverters-chargers/multiplus-12v-24v-48v-800va-3kva

 

[Lucien Nunes]

To make the system work as you want, requires a specific type of PV inverter. The domestic power distribution has to be disconnected from the grid and connected to a separate distribution board fed via the inverter. That way, the inverter's management system can monitor and control power flows to/from the grid separately from those serving your house. It contains, or interfaces to, a battery controller that maintains the battery charge state and allows intelligent priority control of which source to use to power your loads, according to availability of PV, battery status and grid status.

There is possibly also a system where the inverter distinguishes between domestic load and grid load by sensing the current in the meter tails with a transformer, computing the domestic load as the difference between what it is delivering and the load on the tails. I am not sure whether this actually exists or I invented it in a recent discussion!

I am not up on the individual models but you might be looking at something like this, although the primary application of this system is to provide battery backup during grid loss:
https://solar.schneider-electric.com/solution/residential-grid-tie-with-backup-power/