Adding second 9.5 kwH battery

I'm considering adding a 2nd battery to cover the winter months that are looming.

Presently we have a Gen3 Hybrid, a 9.5kwh battery and 14 420w panels on our three-bed detached. This all works really well, and the battery serves us through the evening

My question is how does a 2nd battery draw work, does it wait for the 1st battery to deplete before kicking in for example?

The £3,500 outlay is a lot and this added about £115 to the payment of the system, am I using that extra cost in the Winter by not having a 2nd battery? Time will tell, it's certainly overkill for us this time of year, although today was hopeless for solar generation, and may battery is already at 45% at 19:30 (I like to dump so battery back into the grid from 21:00 - 23:00 for extra SEG.

Finally from a warranty perspective I guess EON will need to come back and complete the install? (They were the original installers)

Answering your questions in turn:

The second battery is daisy chained into the first battery and your inverter will see it as one giant battery. In practice both batteries will charge and discharge together but the first battery in the chain will probably charge and discharge a bit faster than than the second, and so the SoC's of the two batteries will drift apart, typically 10-20%.
It is important to be aware that when one battery reaches 100% or 4% then (full or empty) then all batteries will stop charging/discharging. Therefore you won't get access to the full 9.5 capacity, you'll probably get up to 20% less. This is annoying but lots of people have confirmed that that's what happens to them.

Adding a second battery won't change the throughput of your inverter so you won't be able to charge or discharge the battery any faster, just you'll have more capacity. So if you charge overnight you'll be able to charge for longer but you'll have to do the maths to work out whether you can completely fill the battery in your cheap rate period.

Whether this makes financial sense or not, hard to say. You have said that your current battery is sufficient in summer so the only benefit of additional storage in summer is that you can charge the batteries more and then if you have a peak-rate export tariff like Octopus Flux, you can discharge more in the peak and still give yourself enough to run the house through the evening.

Winter it comes down to your overnight charge period and whether you can get enough charging in that the overnight vs day rate saving makes it worthwhile.

I have two inverters and one battery on each so with two inverters I get double the charge and discharge rate. I've looked several times at getting a 3rd battery but concluded every time that financially its just not worth it. Depending on the scenarios I took and assuming rates remained flat, the payback for a 3rd battery was between 6-22 years. of course economics are not everything but its important.

And your last Q about who installs it. Any GivEnergy supplier can install an additional battery for you but they have to take ownership of the entire system if they do so. There's no concept of different bits being supported by different companies

geoffreycoan greatly appreciate the detailed response , thank you

Midlands_Solar I'm considering adding a 2nd battery to cover the winter months that are looming.

Do you have a heat-pump or some other form of electric heating, or is it primarily just the reduction of solar PV that you need to make up for in the winter?

ISTR you consume a lot of power heating water? I wonder if you can do something like a bigger water tank, so that you can heat more water overnight, rather than having to top it up during the day ?

DD

No heat pump however our house is electric supply only so no gas or LPC, just the lack of solar + water heating + conventional house heating was my thought process here

In the summer months I'd just dump the surplus battery to the grid for additional SEG, so something towards the cost of the battery there at least (40p per Kwh for EON currently)

Midlands_Solar Have you priced things like storage heaters ? Rather than storing overnight electricity chemically in a battery, effectively store it as heat instead. Possibly cheaper than a battery, though of course only useful in winter, whereas a bigger battery might be useful all year round. (Though relative benefit in summer may be limited.)

(And of course insulation is a good way to reduce amount of heating required.)

Midlands_Solar In the summer months I'd just dump the surplus battery to the grid for additional SEG, so something towards the cost of the battery there at least (40p per Kwh for EON currently)

Yes you are getting a great SEG rate with Eon. Very jealous. (As an aside I saw on another video that Eon are now advertising their Eon Drive v3 tariff as being for EV AND Solar home owners, so making the use of the tariff for solar owners official and not an “its ok”)

Anyway, r/e battery one thing to look at is the inverter charge rate and the overnight cheap charging duration that you can fill the battery. You get 7 hours at 6.9p on Eon Drive so with a 3.6kWh charge rate you’re using about half of that with your current battery, so should be able to fill another 9.5 in the same period.

Profit on charging and discharging the extra capacity would be about 9.5 x (40-6.9-2) = ~ £3 a day so could cover the installation costs in ~ 3 years
(2p subtracted from the profit to allow for ~ 20% inverter and battery conversion losses)

That’s the best payback I’ve ever seen on a battery but it does assume you only use the extra battery for SEG discharging and Eon keep the export price this high.

Midlands_Solar Depends what you are looking to achieve! I have 13 panels facing east and south with a gen1 5kw inverter and a 9.5kw battery. I also installed a 6kw A2A heat pump, which because the house is pretty much open plan serves all our heating in winter. Personally, when I had my install done in Nov 22,, which I was told was oversized for my property size and usage, I was hoping to part cover my energy costs. We downsized from a 5 bed place in Wales to a 2 bed bungalow in Essex, and had some spare cash earning no interest at that time, so I thought I'd get a better return by putting it in solar. If I could get at least a 5% return, it had to be better than 0.5% in a savings account.
Well in March 23, Octopus brought out Flux. That was a game changer for me as my 'oversized ' system started earning real money and by December 23, I had exported enough in the summer to completely cover my electric imports, all gas and all standing charges with a £125 extra profit on top.
On that basis I was getting a return of around 17%, way better than the 5% I was hoping for, and although it will be less this year as fuel prices have come down, it's still been a great investment and I now still hope to be energy neutral financially.
I too have thought about adding another battery, but whichever way I work the figures out, for me it just wouldn't work financially. What I would do, if I had space on my roof, would be to put on more solar panels. That would give me greater export in summer and would mean I would import less in winter. That would be far better value for money!