DC vs AC Batteries Pros and Cons

[Bob Smith]

Hi,

I wonder if I am the only struggling to understand the pros and cons of having DC or AC batteries as I could not find any thread discussing this.

As I understand it:

1. DC batteries will be fed "directly by the PV array", then the inverter will convert it to AC for use in the Home and/or export to the grid. They cannot be charged directly by the grid as they need a converter from AC to DC
2. AC batteries will be fed by the inverter which would have previously converted the DC from the panels into AC. They can be charged directly by the grid.

So far I see only one disadvantage. If the DC batteries have been charged by the grid, then the current is converted a second time (DC to AC) in order to supply the house with electricity. In the case of AC batteries, whatever the scenario the current is converted only once.

If you have any references where I can educate myself, I'll take Thank you very much for your time reading me.

 

[SPECIAL LOCATION]

I think you are referring to AC coupled -vs- DC coupled systems...

The actual battery itself will be a device that stores DC energy...
i.e. with one +ve & one -ve terminal..
https://en.wikipedia.org/wiki/Electric_battery

quick google brings up this link..
https://www.deegesolar.co.uk/ac_coupling_vs_dc_coupling_solar_battery_storage/

not sure if you have already seen it?
but it may be of some help in explaining the differences?

 

[binky]

Hi,

I wonder if I am the only struggling to understand the pros and cons of having DC or AC batteries as I could not find any thread discussing this.

As I understand it:

1. DC batteries will be fed "directly by the PV array", then the inverter will convert it to AC for use in the Home and/or export to the grid. They cannot be charged directly by the grid as they need a converter from AC to DC
2. AC batteries will be fed by the inverter which would have previously converted the DC from the panels into AC. They can be charged directly by the grid.

So far I see only one disadvantage. If the DC batteries have been charged by the grid, then the current is converted a second time (DC to AC) in order to supply the house with electricity. In the case of AC batteries, whatever the scenario the current is converted only once.

If you have any references where I can educate myself, I'll take Thank you very much for your time reading me.

Not seen DC batteries for year's. Can't charge from the grid last time I saw those.

 

[johnb2713]

Hi,

I wonder if I am the only struggling to understand the pros and cons of having DC or AC batteries as I could not find any thread discussing this.

As I understand it:

1. DC batteries will be fed "directly by the PV array", then the inverter will convert it to AC for use in the Home and/or export to the grid. They cannot be charged directly by the grid as they need a converter from AC to DC
2. AC batteries will be fed by the inverter which would have previously converted the DC from the panels into AC. They can be charged directly by the grid.

So far I see only one disadvantage. If the DC batteries have been charged by the grid, then the current is converted a second time (DC to AC) in order to supply the house with electricity. In the case of AC batteries, whatever the scenario the current is converted only once.

If you have any references where I can educate myself, I'll take Thank you very much for your time reading me.

All batteries are DC and require the stage converting DC back to AC for domestic use which is done by the inverter. Inverters that have solar PV fed into them charge the batteries directly from the DC power provided by the panels via MPPT controllers built into the inverter. Hybrid inverters also have the facility to charge the batteries from grid power, usually during off peak energy times. All of the above comes under the term DC coupled. The advantage of DC coupled is the easier integration and power management of the solar / grid / battery.

AC Coupled is a battery the same as above but with it's own inverter / charger, it will take energy from the AC supply which may come from the grid or a grid tied inverter and charge the batteries up. When demand for power increases it can feed out into the grid / house supply via normal grid tied methods using it's own inverter. The advantage of AC coupled is in physical placement of the battery/inverter it can be located elsewhere to the solar PV inverter or it sits as a standalone energy store on the power system.

 

[Bob Smith]

I think you are referring to AC coupled -vs- DC coupled systems...

The actual battery itself will be a device that stores DC energy...
i.e. with one +ve & one -ve terminal..
https://en.wikipedia.org/wiki/Electric_battery

quick google brings up this link..
https://www.deegesolar.co.uk/ac_coupling_vs_dc_coupling_solar_battery_storage/

not sure if you have already seen it?
but it may be of some help in explaining the differences?

Thanks a lot!! This is a great article (for my level ) Thank you for sharing.

 

[runningpiglet]

One disadvantage of having the battery at the DC side is that when the solar generated output is charging the battery you will be exporting less to the grid. This correlates to the advantage of AC side battery which can be charged with imported electricity as well as own generated solar electricity. One advantage of DC side battery is that it can be possible to charge the battery in the absence of the grid so it can useful for self-sufficiency during prolonged power outages. Don't get too hung up on the conversion efficiencies to and fro AC-DC etc, modern units like inverters, chargers, trackers are pretty efficient. The least efficient part is probably the battery, you never get out as much as you put in (LOL)

 

[Bob Smith]

Hi

Info taken from the link SPECIAL LOCATION gave us
https://www.deegesolar.co.uk/ac_coupling_vs_dc_coupling_solar_battery_storage/

I am confused with this diagram. It would mean that the electricity generated by the panels are going straight to the batteries and the house is powered solely from the energy stored in the batteries. I thought even with a DC couple. The priority is always given to the House's needs, then if there is an excess it charges the battery






Someone in that forum (sorry I can't remember who) said that he can programme his inverter to set the priorities between the house, the batteries, export to the grid, charge the battery from the grid, heat the water...
am I correct to say that the above diagram is over simplified and that there should be a line from the Panels to the Inverter and no line between the panels and the battery.

Therefore a more realistic diagram should look like below?



The inverter will then act as a switch to either convert the DC from the panels to the house and any excess will be diverted to the battery?

 

[Bob Smith]

Another point I see from the link
https://www.deegesolar.co.uk/ac_coupling_vs_dc_coupling_solar_battery_storage/



Why can't AC Current be converted to the DC current and stored in the batteries? Surely some inverters are capable of doing so?

 

[johnb2713]

DC coupled systems can be charged from the grid with a hybrid inverter, your diagram is correct (the lower one).

 

[Bob Smith]

To continue on the diagrams. I have created the one below which is as far as I know one option for the extension of my system
Using DC batteries:




I already have 4kWp system that includes the PowerDiverter (iBoost and the like) that includes the FiT
I want to add more panels but not have to change my existing system to avoid losing my FiT

The additional components will have PV panels, batteries and a new inverter.

I also want my existing panels to contribute in charging the batteries. I do not see why it could not be possible.
I appreciate that the electricity generated by my original panels which will go to the batteries will be converted a fair bit (DC-AC-DC-AC) before it gets used by the house but I cannot see what else I could do if I were to use DC Batteries.



Do you see any problem with such a configuration?

Thanks

Question mainly for @binky since he said he likes challenges

 

[johnb2713]

Watching with interest to see what the guru has to say, youre doing pretty much what I am currently putting together except I'm not adding more panes, just the inverter and batteries.

 

[Bob Smith]

Watching with interest to see what the guru has to say, youre doing pretty much what I am currently putting together except I'm not adding more panes, just the inverter and batteries.

Why would you add an inverter in that case?

btw happy to share with the community all my Up and Down with this project. It started in march, design still not completed... Why so much pain. First I wanted to add batteries only. when I was told that for a complete system there would be no VAT (I thought VAT was 20%. I thought great, but it is 5% only) but this got me thinking. Now I am trying to get accurate quotes. Only one company quoted (well made 3 quotes, even the last one is not optimizing the space I have!! I am also waiting what the DNO will authorize me to do. This is taking ages...).
Other companies have no interest in putting panels on a wall. And now holidays seasons and covid is still impacting the workforce

If this is fitted before Christmas. I'll be lucky!!

 

[Bob Smith]

Watching with interest to see what the guru has to say, youre doing pretty much what I am currently putting together except I'm not adding more panes, just the inverter and batteries.

I see in another thread that you are having AC couple batteries. For now, the diagram is about DC. Once I master the pros and cons. I intend to do the same with AC.

Locked to prevent being resurrected (again)