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Discharging battery to grid whilst charging EV

 
Energy Storage
Last Post by AndrewJ 7 months ago
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 AndrewJ
(@andrewj)
Reputable Member Member
Joined: 11 months ago
Posts: 132
Topic starter 14/05/2025 9:53 am  

What would be the outcome in this situation?

I'm on Octopus Intelligent Go so smart charging can set a schedule at any time period without my control.  I have 16kW of home battery storage (SigGen) which powers my house during the day and is recharged during the cheap 23:30 - 05:30 period.  I want to be able to discharge the battery at 23:30 to the grid to be paid 15p/kW and then charge it back up again at 7p.  

The only strategy I have at the moment is to configure the SigGen batteries to discharge 23:30 to 01:00 and then charge 01:00 to 05:30.  If I plug the car in, I reconfigure the SigGen batteries to only charge between 23:30 and 05:30.  Then, in the morning, change the configuration back to my discharge/charge settings.  Problem is it isn't possible to edit settings, I have to, essentially, delete the configuration then set it back up again.

It's not that difficult but it is a faff and is subject to me remembering!  Given I charge the car maybe once or twice a week, what would happen if I leave the configuration of the battery alone and the scenario arises that the smart charge schedule covers 23:30 to 01:00 and thus pulls from the grid (7kW) whilst the battery is trying to discharge to the grid (3.6kW)?  I don't want to break anything.

If it's not a problem then I'll just forego the 15p payment on the evenings where the car is charging.  I'm hoping to get Home Automation up and running which I think would sort this but I'm finding it a bit confusing to say the least so it will take me some time.



   
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Majordennisbloodnok
 Majordennisbloodnok
(@majordennisbloodnok)
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Posts: 1214
14/05/2025 12:12 pm  

I'm going to qualify what I say right at the start by saying I'm no expert so if anyone contradicts me later they may well be right.

That said, I don't see how you could break anything but things might not be quite what you expect. Eventually, you only have one cable from the grid to your house and your import or export is simply a measurement of the net flow of electricity in one direction or the other. Therefore, if you charge your car at 7kW at the same time you're telling your battery to discharge at 3.6kW then the car will charge from your home battery and will pull extra from the grid to get up to the full 7kW. That's not ideal since you're not going to be paid for the energy being discharged from the battery and you're also wearing your home battery unnecessarily.

You should also bear in mind the likely lifetime of your battery vs its initial cost so you can get an idea of how much a charge/discharge cycle costs. You may well find that the 8p difference between export price and cheap import price isn't as much after all. I've been through it and found playing the "import now to export later" game gives only relatively low payback.

All that said, you're quite right that a bit of home automation is the way to go to get much better control (along with keeping an open mind on tariffs too, of course). Assuming you're talking specifically about Home Assistant, I'm sure there's plenty we could do to help reduce the confusion so shout if you want help.


105 m2 bungalow in South East England
Mitsubishi Ecodan 8.5 kW air source heat pump
18 x 360W solar panels
1 x 6 kW GroWatt battery and SPH5000 inverter
1 x Myenergi Zappi
1 x VW ID3
Raised beds for home-grown veg and chickens for eggs

"Semper in excretia; sumus solum profundum variat"


   
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 AndrewJ
(@andrewj)
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Joined: 11 months ago
Posts: 132
Topic starter 14/05/2025 3:32 pm  

@majordennisbloodnok I should probably clarify: I wasn't expecting that it would charge at 7kW and simultaneously export 3.6 for payment just not interfere with each other (or break something!!)  For the evenings where I do charge the car, and assuming it is ok to discharge at the same time, I'm expecting my export payment to be zero.  But I offset that small loss against having to change the configuration twice every time I plug the car in.  I'd ask my installer but he's currently on paternity leave and I don't want to bother him.

As for Home Assistant (I keep getting that "A" in the acronym mixed up), if I need some help I'll be sure to ask.  It will be related to hooking in to the Octopus API and re-configuring the SigGen batteries for a smart schedule.  Anyway that's a different thread!!



   
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Majordennisbloodnok
 Majordennisbloodnok
(@majordennisbloodnok)
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Joined: 4 years ago
Posts: 1214
15/05/2025 8:10 am  

Understood, @andrewj.

It might be simpler if you remember that the EV charger isn't a special device from your inverter's point of view; it's just another load the same as a phone charger, a laptop or an oven. If your fridge is still running whilst you're discharging your home battery then the only thing that'll change when you try charging your EV is the size of the load.

Does that help?


105 m2 bungalow in South East England
Mitsubishi Ecodan 8.5 kW air source heat pump
18 x 360W solar panels
1 x 6 kW GroWatt battery and SPH5000 inverter
1 x Myenergi Zappi
1 x VW ID3
Raised beds for home-grown veg and chickens for eggs

"Semper in excretia; sumus solum profundum variat"


   
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 AndrewJ
(@andrewj)
Reputable Member Member
Joined: 11 months ago
Posts: 132
Topic starter 15/05/2025 2:08 pm  

@majordennisbloodnok Well, I tried it and nothing burned down 😀  Actually, I forgot to think about the 8kW inverter so what happened was that the battery entered its forced discharge and 7kW ended up in the car and 1kW ended up in the grid so all works fine and I don't need to be messing with configs.  Although, of course, I won't be getting paid the £1.05 for that 7kW so it would seem unlikely I'll be a millionaire this time next year!



   
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