@vince31 

will one 15kw battery be enough? if you really were to go down the diy route, why not assemble your own battery, using one of the many kits available? you will need to look at getting an inverter that can charge whatever battery bank you decide upon within the time you have AND can also provide the max output you might need. We have a 5kw Sunsynk but have an 8kw shower, so we cant provide enough power for the shower to run without 'leaning' on grid power at the same time.

its fine for everything else, so i can accept the odd bit of grid support, but at some time in the future i may install a larger inverter. 

the important thing is to plan and ADD future proofing which is what i havent always done ..... and it will probably cost me more in the long run !

@misterb thanks for your valuable advice, I appreciate it.  I already have an 8.2kWh battery with 3kW AC connected inverter charged form solar and cheap rate tariff. Adding another 15 kwh battery and 5kw inverter will give me a total 23kWh storage and 8kW inverter supply (peaking up to 12kW for short bursts).  I don't have any electric showers etc as my heat pump takes care of all the hot water and feeds the showers with mains water pressure, so I think I'm pretty much covered for 95% of all scenarios.  The heat pump can take up to 4kW when its running full power in the winter, but soon settles down to between 1 and 2kW after a while. I'm more than happy to build my own 16s battery from commercial prismatic cells at about £100 each but then there's the BMS, cabinet, busbars, cabling, fusing and contactors to take care of, so a commercially available package containing all of this in one box for £2499 seems like a bargain. The only extra thing I think I might need in the future is some extra Solar panels (say a second 4 kW system), but importing at 7.5p will make the payback time for extra panels be 10 yrs plus.

You can buy a Seplos kit for £500 less than the Fogstar ready made which comes with everything you need! The only bits you would need extra are the same bits you would need with the Fogstar ready assembled! 

I like the Fogstar ready assembled, plus it has a heater, so am certainly not going to put you off buying it, I was thinking that if you were going to buy two batteries, it's a £1000 saving .... Even more if you used grade B cells, though I would stick with grade A EVE cells. 

My main point though was to add capacity in the inverter for future expansion, which you have already considered. Best wishes.

PS do you leave the heat pump on 24/7 in the winter? 

Posted by: @misterb

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PS do you leave the heat pump on 24/7 in the winter? 

Yes, its on 21C 24/7 from end Sept until start of May. Obviously it only runs when there is a need to heat the water in the tank that feeds the Rads.  Had it since 2015 and works out £200 pa cheaper than the mains gas CH boiler it replaced. The key is insulation, insulation, insulation.

SITREP #5

No improvement on the system.

Everyday, it is charging 1% less.

Today was 84% then stopped charging. User timer set to 100%SOC.

I'm convinced it is the Sunsynk that is the issue. What setting it is that is learning/reducing the charging, I cannot affect it. It just won't output the voltage to charge the battery.

I've contacted both Sunsynk and Fogstar.

Sunsynk - we need a Sunsynk/Deye specific BMS protocol (Pylontech is not acceptable) which the Fogstar unit does not have.

Fogstar - the inverter is in charge of charging, there is nothing that can be set on battery to improve this.

Fogstar say it isn't them. Sunsynk say it isn't them

Bottom line, something has to change - and I'll likely change the inverter.

Really disappointed, will lose money, no doubt.

Sunsynk not the best in my experience. Poor software, lacking in features….

very poor overload capability.

Posted by: @ggw

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No improvement on the system.

I'm sorry to hear this is the case.

Posted by: @ggw

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Fogstar - the inverter is in charge of charging, there is nothing that can be set on battery to improve this.

This is not the case as the battery can send signals to the inverter to stop charging (e.g. outside temperature limits), and the Pace BMS looks to have charge/discharge MOSFETs it uses to protect the battery.

Posted by: @ggw

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Sunsynk - we need a Sunsynk/Deye specific BMS protocol (Pylontech is not acceptable) which the Fogstar unit does not have.

Sunsynk does list Pylontech batteries as being compatible with the inverter, so it could be that the messages are being populated by the Fogstar battery BMS (Pace) in a funky way.

It would be helpful to understand the signals between the inverter and BMS ... don't suppose you're willing to fork out for a CAN sniffer, are you?

@chickenbig 

Sunsynk support claims that Pylontech is not supported.

Tho the BMS screen on the inverter is properly populated with the correct data. So Comms is happening.

Can you recommend a CAN sniffer and I'll consider.

Stopped charging @79% today. Very frustrating 

Posted by: @ggw

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Sunsynk support claims that Pylontech is not supported.

Yes their manual lists specific battery types as being supported, not BMSes.

Posted by: @ggw

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So Comms is happening.

Indeed when you enabled the BMS_Err_Stop it confirmed this was very much the case. At the very least an intelligible message was being sent periodically.

Posted by: @ggw

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Can you recommend a CAN sniffer

I'm afraid that my knowledge is rather "high level"; sniffing the CAN bus is not something I've had to do. A standard CAN bus adapter ought to do the job. Are there any experts on the forum? I suspect @transparent will have significant expertise in this area.

Sorry I don't have knowledge of CAN protocols/standards.
All my in-house stuff is based on Modbus over RS485...

... but we have had some success with a Modbus sniffer to report the module ID.

Has anyone here had a look at Sunspec Organisation who are defining a set of standard communications protocols?

Its success will obviously depend on how many devices implement the protocols.
Some manufacturers appear to have only implemented the Sunspec system on 1 or 2 products in their range, just to get their name in the database.

@ggw When the sunsynk is used in lithium % mode, it also still makes use of the basic voltage figures from the lead acid mode - these must be set correctly(I think I used 16x3.5v).  I suggest charging as much as possible, then preventing any power out so that you can investigate.  
When your inverter stops charging, I suggest checking the overall battery voltage that sunsynk find, and that the Seplos says - are they the same?.  Compare this to the Seplos and sunsynk maximums that are set.

Also the 16 of individual Seplos cell voltages - are they balanced?  

@robl 

Ill take a photo of the screen tonight & also in AGMV mode

- From memory, it is set to 56.7V

- Unless i factory reset/run it in AGMV mode - i cannot charge to 100%.

sunsynk find, and that the Seplos says - are they the same?

The BMS screen on the Sunsynk - it is the same.

On all other sunsynk screens, the V reads 0.3V higher than the battery screen states.

Also the 16 of individual Seplos cell voltages - are they balanced?  

- Yes, I have checked and ran the A/B balancer for couple of days - looks good.