Connecting new additional batteries to Solis Inverter with exisitng batteries

soram · 12-03-2026 07:52PM #1

So I have my Solis with 2 Soluna batteries working fine a couple of years now. As per another thread I wanted to increase capacity so I purchased some NKON 5kWh units to supplement the 10kWh already there. Now apart from the mechanicals how do I connect them up? The NKONs are showing around 27% SOC all withing 0.03V of each other at around 52.59V. I am guessing I can connect these together with no issue but I have no way of charging them up except with the Solis itself. I am also guessing connecting them directly to the other 2 batteries with unknown or 100% SOC is not a great idea!

Does the following sound plausible?

Force charge the existing batteries to 100% SOC and disconnect them from the Solis
Connect the new batteries to the Solis and again force charge them to 100%
Connect all batteries together and switch on!

But what do I need to set the Solis to when switching the batteries? The NKONs have Seplos BMSs and connecting cables. Do I need to even bother with the connection to the Inverter? Or will the Solis then not 'see' the Solunas when I disconnect them and so not charge the NKONs? Or is there a better way????

My final state was to leave the Solis communicating with the Solunas and just connect the new NKONs in parallel with no comms. Does this sound OK too?

Many thanks for any tips!

on_the_roots · 13-03-2026 03:24PM

No problem at all simply connecting the NKON batteries in parallel with the existing ones, no matter what the SOC is in any of them. When the inverter comes to charge them, it will provide charge to whatever battery pack is still under 100% SOC and their BMS's will manage to stop charge input when it gets to 100%.

You are unlikely to enable comms across these different BMS so you can leave the inverter communicating with any of the 2 types. The only difference is that the inverter will only know the SOC of the packs connected to it, but the total available energy will be the whole set.

soram · 13-03-2026 07:16PM

Thanks for the tip. I’m still a bit wary of connecting the low soc to the high soc batteries though. Will one not try to charge the other at possibly high current?

AntonP · 14-03-2026 09:03AM

Not a bad advise to have all the batteries at roughly the same charge when connecting them, if they have same voltage and capacity.

unknown if they will work together well. Ideally you want them all of the same type so they can all talk to each other.

graememk · 14-03-2026 09:35AM

If they are the same chemistry, and same cell count they will work together perfectly.

Ideally have them close to the same voltage, and then fully charge them.

Communication doesn't matter.

soram · 14-03-2026 12:06PM

it might be easiest to run the connected ones down and then just connect I’m thinking. I’m away until next week so it will be then.

I need a negative busbar. I have a 5 way megafuse thing for the positive but nothing yet to get all the negatives together!

soram · 18-03-2026 07:31PM

Got the batteries connected in today. I did a force discharge of the existing ones to 40%. The Solis cloud reported the BMS voltage to be 52.6V which was exactly that for the new NKONs - confirmed with a multimeter. I connected them all together and there was next to near no current flow between them.

Connected to the Inverter then and they are discharging away normally. I'll see how the charge goes overnight and how they equalise over the coming week or so but looking good for now.

graememk · 19-03-2026 12:09AM

https://www.boards.ie/discussion/comment/124322492#Comment_124322492

Try to do a complete full charge at the start, that should settle everything out

soram · 19-03-2026 10:25AM

It's been on Force Charge all night until 8am and now the Solar is keeping it at 100%!

I might force discharge it this evening to cyle it and see what it is doing.

soram · 19-03-2026 08:32PM

https://www.boards.ie/discussion/comment/124323416#Comment_124323416

So it seems to be working (ish). Well all the batteries reported being 100% SOC this morning (On the individual displays on the NKONs). Now they are discharging but the Solis reported SOC seems to reflect the size of the 10kWh existing batteries only i.e. it is reporting 2kWh discharge and around 80% SOC.

So is the Solis displaying the SOC reported by the existing battery BMS? If so, will this ever be correct if that BMS is calculating based on 10kWh and not the 25kWh I now have? I'm confused as to how this might pan out! Will I have to unplug the existing battery CAN to the inverter and just tell it it has x lithium batteries and set voltage limits to get it half decent or will it someway magically figure it out which I am a bit skeptical of!

soram · 19-03-2026 11:28PM

Right! So it is not all rosy! Yes, the battery performs but the stats are all off from the Solis if leaving as is! The Solis obviously just takes the BMS values and not its actual production. The attached screenshot shows I am exporting over 3kW but the battery is only producing 1.3kW. So this is the production of my existing battery alone and is coming from its BMS I guess and does not include the piggyback batteries……

soram · 22-03-2026 11:22PM

So after a few days everything looks to be working perfectly. The batteries appear to be in sync. I've dumped them to the grid most evenings and recharged them fully overnight with no issues. I have also just managed to get the NKONs into Home Assistant using the HACS BMS integration and a bluetooth dongle.

The NKONs also come with a USB/RS485 cable. I had thought I could just connect this to the HA server and asked NKON to help here. They were actually to my surprise very 'snotty' about this. 'Johan' told me that NKON also sold Victron and that basically HA was 'bricking' Victron devices. I left it there….

The last issue I guess is that the Solis Cloud is still only seeing the BMS output values from the existing batteries as in the above post. Solis reports BMS Voltage and Voltage as different but not BMS Current and Current. I have raised a ticket with Solis but don't expect anything. So my Solis Cloud data will always be wrong unless I tell the inverter my pack has no communication.

mjatkey · 23-03-2026 10:49PM

https://www.boards.ie/discussion/comment/124333602#Comment_124333602

The Solis can only report the soc, volts, current etc of what ever battery is connected to it, for me I connect the WECO's to the Solis(1) and then use the Seplos BMS integration to monitor the Seplos (2), discharge script just takes the highest soc value to calculate discharge current.

I did run it using User defined battery setting on the Solis but then there is no visibility of the WECO's except on their APP.

I've not tried the Lead acid battery setting on the Solis, but again I'd loose visibility of the WECO's.

soram · 24-03-2026 10:10AM

https://www.boards.ie/discussion/comment/124335882#Comment_124335882

That's sort of where I am at too though hadn't thought of putting the NKONs as a second HA battery and just summed the power values for my main screen!

I also came to the conclusion of leaving the Solunas on the Solis as there is no app for those so I can see them through Solis. The NKONs have Seplos BMSs and I have connected HA to these using a BLE Dongle and the BMS BLE Integration.

On the Solis Portal the battery section has fields for BMS values and other values i.e. BMS Voltage and just Voltage and the same for current and power. The two voltage fields are different and I had hoped the Solis would report BMS Current from the Solunas and also just 'Current' as a DC value at the inverter that might then include the addon batteries. While I was correct for the Voltage, Solis confirmed yesterday to me that 'BMS Current' = 'Current' so that's a no go. Anyway, the Solis integration also does not report both fields so that would also need to change.

It's messing a bit with my calculations for my display and I've a bit more tweaking to do!

mjatkey · 24-03-2026 07:26PM

https://www.boards.ie/discussion/comment/124336434#Comment_124336434

I just use the Solis for the WECO values and the BMS integration for Seplos values, its good enough for my use, basically you charge batteries (cheap rate) , use (PV and Batt) and flog back any surplus between 10pm and 2am.

I did look at using a shunt but you'd need two and not sure how you account for the energy supplied by the separate rectifiers.

I may look at this again at some point, but for now I'm happy with the way it works.

WECO voltage always looks higher on the graph, but if I check it on the APP its pretty much exactly what the Seplos is (which is what you would expect).

soram · 04-04-2026 10:44AM

So a small update here. The batteries appear to be performing fine but with one small issue I am not 100% sure how to resolve or if it is even necessary.

When I connected them together they were both indicating 100% SOC. I have been discharging them to about 30% every evening and charging them to 'full' overnight. The NKONs reach 100% reported but the original Solunas only ever reach 96% before charging stops. Even during the day with full excess solar so theoretically charging all day they may get to 97% but that is the maximum. They seem stable at that figure though and they are not continually getting lower and lower.

I presume it is a fractional voltage difference seen at the inverter somehow. The Solunas are controlling the inverter over CAN but I am guessing they 'tell' the inverter at what voltage to stop charging but somehow the NKONs are fractionally higher (maybe marginally less voltage drop through the BMS) and so it shuts donw early. But I guess I would also expect the batteries as they are connected together to balance out but balanced appears to be 96% to 100%!

As said I'm am happy enough that they are performing well enough so I guess I'll have to live with this! My 5 80V megafuses arrived from Victron this week so I'll be changing out the 32V I had temporarily in place and maybe I'll leave the NKONs disconnected for an hour and see what happens the Solunas. I'm guessing they'll reach 100% but thern settle back to 96%.

Connecting new additional batteries to Solis Inverter with exisitng batteries

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