Adding Additional Battery to existing system

We currently have a 9.5Kw GE battery coupled to our 5Kw G2 inverter (4.8Kw panels) with an average house consumption is around 6-8Kw per day. On occasion our battery level has gone quite low as there has been very little solar input (current minimum battery level set at 20%. I m worried that the deep discharge will shorten the life of the battery unit and that we can get VAT free batteries from February in the UK, I was wondering if it would be a good idea to add an additional 9.5Kw battery to A, Stop the current battery capacity dropping so low and allow me to Import over night at the cheap rate (Octopus Flux) and then export at peak times?

Ill give you my take on this for what its worth

I would save your money and when degradation happens outside of warranty buy a replacement battery then which would likely be cheaper and better, as technology marches forward.

Its a little like buying a van in addition to your car for the odd trip to the skip (I know Im overly simplifying it)

Ortwin
I have 9.5kWh batter and charge 00:30-07:30 on Eco7 at circa 15p. I then have to import from around 18:00 at day rate 30p.
I have 27 panels, but not much sun at present.
Based on my Dec bill my day rate energy was £80 ish and my off peak £40 ish.
If I added another 9.5kWh battery the day rate bill would halve to circa £40/month, but this situation really only occurs for 4 months per year as by March my panels are pretty much enough and by April/May I am generating more than I am using.
Doing the math it really does not compute to add another battery unless I get to a better tariff, but after 4 years my smart meters are still dumb - don't go there!!
Only makes real sense if we are getting lots of power cuts, but my EPS has only ever been needed once.

DreadorUK makes a good point about about attempting to preserve the battery, especially when it should be 10-12 years before you get noticeable degradation and the warranty is for unlimited cycles.

On Octopus Flux you only have a 3 hour charging window, you'll be lucky to put more than 9kWh into the batteries. So in the winter with low solar generation you won't get much benefit, in the summer I would argue that it is better to not import at all ie charge up on 'free solar' and then export at peak times.
So the question is whether spending around £4000 for an extra battery you don't need to run your house is worth it or not? Currently on Flux you might get an extra £1 a day for 6-8 months of the year, say £200 which gives a 20 year payback.

Having said that you'll have seen discussions about using Agile and how you can aggressively manage your system to maximise the export profits - an additional battery would help significantly with that and there is a risk that it will be more expensive.

Myself I'm considering having an AIO and a Gen 3 5kWh inverter (to replace my Gen1 3.6kWh inverter). I don't expect to the change to pay for itself but I will have the benefit of surviving power cuts (worst one was 3 days a couple of years ago) and more storage capacity than I use a day that can be charged at the cheap rate.

Ortwin There's probably already buffers at the top and bottom end of the battery, so your 20% reserve is already on top of a safety reserve.

Can you actually charge more than 9kW in a 3-hour overnight window ? I think the total charging is limited to 3.6kW (or so) across all batteries ?

If you're worried about avoiding degradation, are extra cycles to discharge for export consistent ?

Other factor down the line (once warranty has expired) is I'll be very surprised if someone hasn't either worked back the GE ones or worked out how to fit someone else's packs to them, so I'm sure they'll be plenty of options.

Ortwin I Have Gen 2 5kw inverter and 3 batteries 2x9.5 and 8.2 I am on Octopus Cosy as we also have a heat pump. So you could change to octopus Cosy and charge twice a day which could be upto 21.6kw ? we have 8kw of panels 20No. We sometimes charge from 04:00-07:00 and then pause for between 13:00-16:00. If you are thinking in the future of getting a heat pump then yes I would say you need more batteries. At the moment the maximum you can discharge is 4kW per hour and this will not increase with adding batteries. Before the heat pump we were on 8kw-10kw per day now we are on 45kw per day at the moment.

Its a complex price-benefit trade off of adding extra batteries. I've considered it several times and once you work through constraints of inverter throughput, overnight charging window, etc, I think the best payback you can get is around 5 years, and that's with fully charging overnight on cheap rate in winter and consuming it during the day and fully discharging the battery with charged solar in the Flux peak period in summer.

And outside of needing and being able to achieve both of those activities the paybacks head towards 20 years as others have said.

Certainly don't buy an extra battery to "save the load" on your existing one. They have an unlimited cycle warranty and as also said there is extra capacity in the battery so your 100% Depth of Discharge is really not 100%, there's spare capacity in reserve.

Having looked at extra batteries I have ended up wondering (after the event) whether there ever was a sensible payback on the original batteries. But not going there!

I'm on agile and currently only with a 5.2 battery as my 9.5 is dead. And with an ASHP so high winter demand. The 5.2 gets me through the period 16:30-18:30 or thereabouts, so not even all of the peak period, but overall its still cheaper than being on the normal import rate.
So summary is you don't need a lot of battery capacity to benefit you financially, more kWh doesn't save you lots more £