Should we get more home battery capacity?

[Everest]

That sounds like a great idea, but there is no reason why you couldn't also plug in the 48 V DC battery charger to the same circuit and top up the house battery from the EV's V2L output when there is no mains power, so the devices powered from the battery can be kept powered up as well.

I agree, but there could be a caveat in that as Rolfe's installer said it is not possible it could be that Rolfe's system uses HV batteries, rather than standard 15S or 16S "nominal 48V" low voltage system. As far as I can see, Givenergy have both LV and HV systems

 

[Bam Bam]

I think the whole taxation thing is indicative of just how broke the country is, equivalent to searching down the back of the sofa for loose change.

A different perspective is that it isn't a good idea to have 'loopholes.' If you can make money from solar tax free but not from other sources then this will give people an incentive to invest in solar and not in other things.

Hmmmm, doesn't sound so bad actually. Could be a 'good distortion.'

But at any rate, the general idea is to avoid these sorts of distortions.

Governments do not live up to this ideal, creating all sorts of little loopholes that become big loopholes.

I agree, but there could be a caveat in that as Rolfe's installer said it is not possible it could be that Rolfe's system uses HV batteries, rather than standard 15S or 16S "nominal 48V" low voltage system. As far as I can see, Givenergy have both LV and HV systems

I believe the hybrid system that both Rolfe and I have is 48V. The battery seems to go up to 54V.

 

[Everest]

I believe the hybrid system that both Rolfe and I have is 48V. The battery seems to go up to 54V.

Then it should be possible. Just her installer CBA. And @Rolfe.. I didn't mean a cat bite abscess

 

[Bam Bam]

The idea wasn't to connect to the EV DC circuit, but rather, a 48 V DC battery charger being powered by the V2L and the output from the battery charger connected to the battery at the end of that cable with the two ring terminals (at least that is what they look like) by adding ring terminals with the same size bolt hole to the 48 V DC battery charger outlet cables.

T1 Terry

Thanks for clarifying.

So something like this?
LiFePO4 Charger (Fogstar Energy) - 48V 35ADefault Title

My worry would be that a lot of time, money and effort could be spent setting all this up and the Givenergy system will just say 'no, I don't know what that connection is so I will just shut everything down"

There is a Givenergy thread on this topic for the (presumably high voltage) All in One system, though they are waiting for Givenergy to officially allow it rather than discussing these DIY jobs.

Connecting an EV with V2L to an AIO

I agree that V2G is a very interesting technology for many of us, but is frustratingly slow in coming. One thing that should be possible now is to use V2L to drive an individual load in the house, therby reducing the demand on our AIO. I have an oil boiler driving a large underfloor system...

community-beta.givenergy.cloud

 

[Everest]

So something like this?
LiFePO4 Charger (Fogstar Energy) - 48V 35ADefault Title

IMHO, no. For reasons mentioned in my posting below...
Better to have an adjustable supply, as @johnb80 mentioned - see my link in post #24. Or telecoms PSU such as Eltec Flatpack, Emerson or Chargeverter.

Post in thread 'Solar battery and V2L'

May 6, 2025

@Everest Which LifePO4 charger would you recommend

One that is adjustable with regard to output current and voltage and needs to be of a galvancially isolated design. As @wattmatters and @johnb80 have said, the Fogstar one is too much current IMHO. It is also lacking in any detail whatsoever with regard to output voltage.

As I mentioned in my post #8 you will need to consider maximum charge rate that your BMS supports and match (or adjust) the charger to not exceed that current when your inverter is running on full charge rate.

I note there are no fuses shown in...

• Everest

My worry would be that a lot of time, money and effort could be spent setting all this up and the Givenergy system will just say 'no, I don't know what that connection is so I will just shut everything down"

Provided the PSU is voltage adjustable to not exceed battery or vehicles current limit, the existence of the PSU will be invisible to the inverter, thanks to Kirchoffs Current Law

Edit; link to earlier link...

Post in thread 'Connecting a V2L car to a house as a battery.'

Dec 2, 2025

Hi John. This sounds exactly like the setup I'm after. I haven't been able to find anyone that's actually tried this yet, so this is quite exciting. When you say 'feeding onto the wires', how have you done the connection? Paul

Hi Paul,
I connected to my DIY battery directly onto the DC wires (big black and big red) that go from the inverter to the battery. At this point the 48v DC is being fed into the inverter via the BMS (Battery Management System all batteries have). It's very easy to do and you could just purchase an extra battery cable if it's a commercial battery you have or...

• johnb80

• 
• 

 

[T1 Terry]

Thanks for clarifying.

So something like this?
LiFePO4 Charger (Fogstar Energy) - 48V 35ADefault Title

No voltage control and very crude output filtering, generally resulting in AC ripple superimposed over the DC square wave, causes all sorts of confusion to sophisticated electronics, generally resulting in a shut down because it just doesn't like the dirty signals it is seeing.

I was looking through the Victron gear and realised they don't make a 48V battery charger at a price that would be better than a 48V Multiplus II inverter/charger. That is really strange, so an adjustable DC supply would be the better option.

T1 Terry

 

[johnb80]

I am told that this isn't possible with GivEnergy batteries.

'Some bloke in Lincolnshire' told you it can be done with any batteries. The nominal 48v DC coming out of your batteries to the inverter is supplemented by another source of 48vDC from the MG4. The inverter doesn't know or care where the energy comes from, the batteries will simply see a reduced demand from the inverter.

Is there battery cables that come of the battery that go to the inverter, if so, connect the battery charger negative and positive there and set the battery charger to a max output voltage of 54vdc, below the point the BMS will attempt to do a cell balance on the whole battery .....

T1 Terry

The easy way Terry is to increase the voltage output of the power supply until you reach the desired load coming from the car eg 2kW or 3kW etc, do this when the house batteries are at circa 75%. Once set, the car can be connected and as the house batteries discharge the load is transferred to the car.

No voltage control and very crude output filtering, generally resulting in AC ripple superimposed over the DC square wave, causes all sorts of confusion to sophisticated electronics, generally resulting in a shut down because it just doesn't like the dirty signals it is seeing.

T1 Terry

1) It will definitely have voltage control
2) Being a Switched Mode Power Supply will give nigh on zero ripple.
3) The output will be very smooth DC, not a square wave.
4) No shutdown will occur because of this power supply.
5) The energy from this supply will be as good if not better than the charging unit built into the inverter.

 

[T1 Terry]

'Some bloke in Lincolnshire' told you it can be done with any batteries. The nominal 48v DC coming out of your batteries to the inverter is supplemented by another source of 48vDC from the MG4. The inverter doesn't know or care where the energy comes from, the batteries will simply see a reduced demand from the inverter.


The easy way Terry is to increase the voltage output of the power supply until you reach the desired load coming from the car eg 2kW or 3kW etc, do this when the house batteries are at circa 75%. Once set, the car can be connected and as the house batteries discharge the load is transferred to the car.


1) It will definitely have voltage control
2) Being a Switched Mode Power Supply will give nigh on zero ripple.
3) The output will be very smooth DC, not a square wave.
4) No shutdown will occur because of this power supply.
5) The energy from this supply will be as good if not better than the charging unit built into the inverter.

The reason for requiring the adjustable voltage control is to stop the 48vdc charger, that the BMS has no control over, pushing a cell into the cell damage voltage zone. This will cause the BMS to shut down the system, the very thing the system owner is trying to avoid.

Try putting one of those cheap ac to dc chargers on a scope while powering something with no battery in parallel to act as a giant capacitor .....
Just like the Onan generators, the output has so many frequencies ghosted over the 50 hz that anything with a modern electronic control system will simply give up and go into error mode .....

Spent 12 yrs doing off grid battery systems, there are crap inverter/chargers out there, I not disagreeing with that, but a quality inverter/charger that uses a heavy toroidal transformer will outperform any electronic switch mode device as far as delivering clean power .......

T1 Terry

 

[Bam Bam]

It is these kinds of discussions that are going to really put Rolfe and me off tinkering with things that are way beyond our expertise!

It may or may not be possible to do this with Givenergy equipment but I am not the person who is going to prove this.

 

[Rolfe]

I hear what you all say, but I certainly wouldn't be tackling this myself and as my installer isn't keen then it's a big hurdle to overcome. The fact is that power cuts are few and far between around here, and prolonged ones pretty well unknown. I see the EPS more as an insurance policy than anything else. I've been heard to joke that by fitting it I've ensured that we'll never have a power cut again!

There is a scenario where the power goes off at a time when the battery is low (but that would be late evening and I'd just go to bed) and this was in very dull weather in the middle of winter so that next day's solar wasn't enough to recover power in the battery (assuming the power cut continued into the next day), but it would be all the stars aligning in malign positions. That's when I'd need to draw on the VtL directly. But it's not a likely scenario, to put it mildly, and I have that work-round if it does happen. If it ever actually happened I'd probably try again to get something sorted out.

 

[johnb80]

The reason for requiring the adjustable voltage control is to stop the 48vdc charger, that the BMS has no control over, pushing a cell into the cell damage voltage zone. This will cause the BMS to shut down the system, the very thing the system owner is trying to avoid.

Hence why I suggested that the system is started up and with house battery down at 75% the voltage from the power supply is increased until the required output current is achieved eg 40 amps for 2kW. If the load on the inverter is less than 2kW or Solar starts to generate, the battery voltage would start to rise and the current from the car would reduce. In my system, at 80% the current from the power supply drops to zero, at 75% I'm pushing 3 kW into the Inverter / Battery combination. LifeP04 as you know has a very flat charge / discharge curve so it happens very quickly.

Try putting one of those cheap ac to dc chargers on a scope while powering something with no battery in parallel to act as a giant capacitor .....

Some allowance has to be made under the described no load conditions but thats not the real world. With the slightest load applied they work absolutely fine. My system runs on some DELL ex server power supplies and is absolutely fine, no reliability problems at all. A bigger concern is the quality of power from the MG, that does leave a little to be desired.

Just like the Onan generators, the output has so many frequencies ghosted over the 50 hz that anything with a modern electronic control system will simply give up and go into error mode .....

Not in my experience. Generators with the exception of Honda Inverter one have loads of distortion on their outputs. Using a generator to feed the power supply above would IMHO make it very suitable for charging the batteries. THD is pretty much irrelevant when it's being fed into a switched mode power supply, it's only converted to DC at the input.

Spent 12 yrs doing off grid battery systems, there are crap inverter/chargers out there, I not disagreeing with that, but a quality inverter/charger that uses a heavy toroidal transformer will outperform any electronic switch mode device as far as delivering clean power .......

Makes you wonder why all modern tech uses switch mode power supplies if theyre so bad? how does anything manage to work? Even the car charger is switch mode.

 

[johnb80]

I hear what you all say, but I certainly wouldn't be tackling this myself and as my installer isn't keen then it's a big hurdle to overcome. The fact is that power cuts are few and far between around here, and prolonged ones pretty well unknown. I see the EPS more as an insurance policy than anything else. I've been heard to joke that by fitting it I've ensured that we'll never have a power cut again!

There is a scenario where the power goes off at a time when the battery is low (but that would be late evening and I'd just go to bed) and this was in very dull weather in the middle of winter so that next day's solar wasn't enough to recover power in the battery (assuming the power cut continued into the next day), but it would be all the stars aligning in malign positions. That's when I'd need to draw on the VtL directly. But it's not a likely scenario, to put it mildly, and I have that work-round if it does happen. If it ever actually happened I'd probably try again to get something sorted out.

Well at least it gets some discussion going doesn't it? To be absolutely honest, the only time I have used our MG5 and the DC charging of house batteries has been during Christmas Day and Boxing Day. It has worked flawlessly without the slightest concern anywhere. The idea is really simple and carries very little risk. That said it's hardly worth bothering about for the amount it saves and financially probably not worth it. I'm in the fortunate position that I have access to the power supplies from my I.T. business, they cost me nothing and I can do the installation myself being a retired electrician from a previous life in my late teens / early twenties.

 

[johnb80]

It is these kinds of discussions that are going to really put Rolfe and me off tinkering with things that are way beyond our expertise!

And that is absolutely fine but it's not a reason NOT to have the discussion?

It may or may not be possible to do this with Givenergy equipment but I am not the person who is going to prove this.

Absolutely, the real consideration is that the DC voltage that come from the battery, the inverter wont care where it comes from, DC is DC no matter what as long as the voltage is ok then it will be fine.

You are right to be cautious if you're not confident in your own mind installing such a system. Contractors will usually shy away from such ideas, they don't need to install any such items and it's the 'safe' blinkered view option, I really don't blame them.

 

[Rolfe]

Well at least it gets some discussion going doesn't it? To be absolutely honest, the only time I have used our MG5 and the DC charging of house batteries has been during Christmas Day and Boxing Day. It has worked flawlessly without the slightest concern anywhere. The idea is really simple and carries very little risk. That said it's hardly worth bothering about for the amount it saves and financially probably not worth it. I'm in the fortunate position that I have access to the power supplies from my I.T. business, they cost me nothing and I can do the installation myself being a retired electrician from a previous life in my late teens / early twenties.

Yes, it's a good discussion.

I appreciate that it's something simple for you, but put together my own lack of electrical expertise (sob, my father was an electrical engineer, but he gave that up well before I was born) and my installer's reluctance to go down that road, and it's an issue. People also tell me that GivEnergy batteries are a more difficult nut to crack for this purpose.

I really wouldn't bother about it just on account of running out of battery on the occasional dull solstice day with heavy cooking loads. It's only happened to me once, on New Year's Day last year, when total generation was only 320 watt-hours and I was preparing steak pie and roast potatoes and so on for a guest, and the battery ran out about 9.30 in the evening. (And even then, if I hadn't been locked out of my GivEnergy account by a password issue at the time, I could probably have rejigged things to avoid this.) And as you say, the financial gain is very modest. I've got the EPS, and that's likely to cover any likely power cut eventuality, given that the home battery can normally be recharged from the solar generation each day - at least enough to carry me through another day if I'm careful with my usage. In the event of a prolonged power cut in December/January with a succession of sunless days, direct use of the VtL for everything other than the central heating boiler, saving the solar to run the boiler from the home battery, would cover it.

It's more of a notional idea that it would be really really neat to have the capability to do it, than actual necessity.

 

[Bam Bam]

And that is absolutely fine but it's not a reason NOT to have the discussion?

Indeed.

Thanks for getting involved!

Didn't want to @ you but you found it anyway.