Deciding whether to buy a second growatt battery for moving to Octopus Flux

[Dave79]

Hi. We have 10 x 405W panels, a Growatt SPH3600 inverter and Growatt 5 kW battery (AXE 5.0L-C1 5kW). We are currently on the Flexible Octopus (Import) and Outgoing Octopus (Export) tariffs, but are thinking of switching to Flux to make use of the cheap Flux rate at 2-5am.

So I recently changed the "Battery First" settings to force charge the battery between 2 and 5 am (cheap flux rate period @ 16.13p/kWh) as a test to see how long it took the charge the battery from 11% to 100% and then how long the battery lasts during the day while we are at work. The reason is to see if there would be enough kW left in the battery to get us through the peak Flux period (4-7pm) as we do not want to have to import during this period (37.64p/kWh instead of standard 26.88p/kWh) - that would negate the point of moving to flux. As part of this I was also wondering if a second 5 kW battery would be a worthwhile investment (our battery is already 6 months old so need to make a decision as cannot mix and match new with old according to Growatt manual).

The AXE 5.0L-C1 has a nominal voltage of 51.2V, max charging curent of 60A and max charge/discharge of 3 kW. So on this basis I expected the battery roughly to be charged from 11% (0.55 kW)) to 100% (5 kW) in approx 1.5 hrs (ie 5-0.55/3). But it actually took the full 3 hrs to charge the battery. Obviously I forgot to factor in the load drawn by the house, which for that period was about 0.2-0.47 kW, but even factoring that in it seems to have taken a lot longer to charge the battery than expected. Am I thinking about this wrong?

The discharge numbers (to the house only) were as follows:

2am - 11%
5am - 99%
6am - 94%
7am - 80.6%
8am - 75%
9am - 67%
10am - 61.2%
11am - 54.8%

On this particular day we had very little solar energy produced. As it was November we had only around 7 hrs of daylight, The day load stayed pretty constant between 0.2-0.5 kW. So left wondering if we moved to Flux there would have been no energy left in the battery by the time we got to the 4-7pm Peak Flux period. So then we would be Importing at a high rate just as we return home from work. Not what we want. In this case then a second battery might be worthwile to get us through that period even if we have to charge at the Standard Day rate (same rate as we pay on the Flexible tariff so no big deal to do this). Then of course we also need to factor in the cost of the baterries and their life-span!

This is really hurting my head trying to work out what is the best thing to do. Can anyone recommend the best settings so that we dont Import from the grid during the Peak Flux period? Should I just tell the system to keep the battery charged for much longer than the 3 hr Flux Period so that there is enough battery left for the Peak period? Any thoughts gratefully received.
Thanks, Dave

 

[SeaGal]

...But it actually took the full 3 hrs to charge the battery. Obviously I forgot to factor in the load drawn by the house, which for that period was about 0.2-0.47 kW, but even factoring that in it seems to have taken a lot longer to charge the battery than expected. Am I thinking about this wrong?

The charge rate won't be linear all the way to 100%. Typically the battery management system will reduce charge rate when above 95%, if not 90% and then will taper down the charge rate as the SOC approaches 100%. This is to a) help cell longevity and b) allow the balancer time to do its business.

... So then we would be Importing at a high rate just as we return home from work. Not what we want...
... Can anyone recommend the best settings so that we dont Import from the grid during the Peak Flux period?
... Any thoughts gratefully received.

It sounds as though you don't have enough battery. We have 14.3kWh and it just gets us through solar-less winter days from an economy 7 charge up.

If you don't want the expense of more battery, then presumably you can just set the inverter to not use the battery for a few hours in the main part of the day - i.e. when you are out?

 

[Dave79]

The charge rate won't be linear all the way to 100%. Typically the battery management system will reduce charge rate when above 95%, if not 90% and then will taper down the charge rate as the SOC approaches 100%. This is to a) help cell longevity and b) allow the balancer time to do its business.


It sounds as though you don't have enough battery. We have 14.3kWh and it just gets us through solar-less winter days from an economy 7 charge up.

If you don't want the expense of more battery, then presumably you can just set the inverter to not use the battery for a few hours in the main part of the day - i.e. when you are out?

Thanks for your reply. Yes I think you are right. I can buy another battery for £1400, but not sure if I can add it easily myself. It looks realtively straight forward to add it as it is modular and there are instructions in the manual. So would be willing to do this. Just not sure if changes have to be made to the settings and wouldnt know what to change and the app warns against changing anything. But the installers are wanting another £6k to add a second battery, which I think is ridiculously excessive.

Re changing settings on inverter to not use battery for a few hours in the day so there is enough left for the 4-7pm period, I agree also. But this is what I am not sure how to do. Am I right in thinking that if I extend the Battery First from 2-5am to 2am- 4pm, then the battery would stay topped up to 100% through solar first then grid second? Then at 4pm I could set the battery to discharge. If so do I set discharge rate lower than 100%? When you set it to discharge does it just send it all to the grid or will it go to load first, grid second? Sorry for questions, but I find the manual to pretty useless in explaining how it all works.

 

[SeaGal]

But the installers are wanting another £6k to add a second battery, which I think is ridiculously excessive.

Nice mark up for them!

Might be worth posting a new thread on here about adding a battery and put the model details in the thread title - then hopefully someone else with same/similar setup can help step you through what's needed? Just a thought.

 

[Dave79]

Thanks for your help. Yes I will do that. Have a good Christmas!