Two batteries and one inverter

I have a 4.5kW solar panel array with inverter by Solar Edge.

Also I have a Solax 6kWh battery with BMS and Solax 3.68kW inverter in use.
I wish to add a second battery of 4.5kWh which has its own BMS.

How should this connect to the existing Solax inverter or do I need a third inverter ?
If I add the third inverter how do the three all play together?

I don’t know your components but generally yes, you can do this. As long as both batteries of the same voltage and both fully charged when you first hook them up, it will add to your capacity.


You want to hook them up in PARALLEL. This means to connect both positives to each other and do the same for the negative terminals. Make sure you use proper gauge cables and of equal length. Will both batteries be right next to each pretty other or will they be set apart from each other?

as above, and your two batteries just become one big one!

You can install battery packs in Parallel without any issues.
Solax uses it's own interlink protocol for parallel battery packs, therefore your 1st choice should be another Solax Battery but there are "some" external BMS's which "may" be compatible. I would seriously research alternate brands that can work with Solax, prefferably find folks with experience.

Solax Battery Page:

They have a BMS Parallel Box which is the First Time I have seen such a thing... Hmmm

I did find this ditty which is useful BUT would need confirmation.

The solax hybrid inverter is approved to work with LG lithium batteries and Pylontech lithium batteries.

Click to expand...

IF the Solax is indeed compatible with the PylonTech protocol YOU ARE IN LUCK ! There are several battery packs and BMS' (Battery Management Systems for DIY) that support the Pylontech protocol stack.

Here is one of my simple generic diagrams to show how a paralleled component system can be setup to do what you want.

Tomthumb62 said:

I don’t know your components but generally yes, you can do this. As long as both batteries of the same voltage and both fully charged when you first hook them up, it will add to your capacity.

Click to expand...

The 6kWh battery is a few months old, the 4.5kWh battery is 3 years old and may have a different number of cells so the votage would differ

Tomthumb62 said:

You want to hook them up in PARALLEL. This means to connect both positives to each other and do the same for the negative terminals. Make sure you use proper gauge cables and of equal length. Will both batteries be right next to each pretty other or will they be set apart from each other?

Click to expand...

They will be close together.

If the voltages match then is this connection after the two BMS boxes?

If the voltages are different I suppose this is a no-no.

Tomthumb62 said:

I don’t know your components but generally yes, you can do this. As long as both batteries of the same voltage and both fully charged when you first hook them up, it will add to your capacity.

Click to expand...

Both batteries are Solax "Triple Power". The 6kWh one is a few months old. The 4.5kWh one is some 3 years old.
They may have different number of cells so may be different voltage.

If the voltage is the same and I connect them in parallel is that AFter the two BMS boxes?
If the voltages differ I suppose this is a no-no.

Tomthumb62 said:

You want to hook them up in PARALLEL. This means to connect both positives to each other and do the same for the negative terminals. Make sure you use proper gauge cables and of equal length. Will both batteries be right next to each pretty other or will they be set apart from each other?

Click to expand...

They will be close together.

Steve_S said:

You can install battery packs in Parallel without any issues.
Solax uses it's own interlink protocol for parallel battery packs, therefore your 1st choice should be another Solax Battery

Click to expand...

Both batteries are Solax "Triple Power"£ of different ages, see my reply above.

Steve_S said:

but there are "some" external BMS's which "may" be compatible. I would seriously research alternate brands that can work with Solax, prefferably find folks with experience.

Solax Battery Page:

Triple Power Solar Battery | Solax Power

Get the best of both worlds with Triple Power Solar Battery from Solax Power! Save big on your electricity bills and help protect the environment. Get yours today and enhance your energy independence like never before.

www.solaxpower.com

They have a BMS Parallel Box which is the First Time I have seen such a thing... Hmmm

SolaX Power: Solar Battery System BMS Parallel Box

Uncover the Power of SolaX: Boost Your Solar Potential with our Cutting-Edge Solar Battery System and BMS Parallel Box. Maximize Efficiency & Savings Today!

www.solaxpower.com

Click to expand...

I looked at spec for the Solax parallel box but it requires the age diffenmce to be less than 3 months so no good for me.

Steve_S said:

I did find this ditty which is useful BUT would need confirmation.

IF the Solax is indeed compatible with the PylonTech protocol YOU ARE IN LUCK ! There are several battery packs and BMS' (Battery Management Systems for DIY) that support the Pylontech protocol stack.

Here is one of my simple generic diagrams to show how a paralleled component system can be setup to do what you want.

Click to expand...

PaulT49 said:

I looked at spec for the Solax parallel box but it requires the age diffenmce to be less than 3 months so no good for me.

Click to expand...

When dealing with LFP or other Li-Ion chemistries that Lead Acid BULL Hooey does NOT APPLY. Remnants from the past that refuse to die.
With Lead Acid this is a HARD RULE because of their agimg & stratification. This does NOT happen with Lithium Chemistries. The ONLY catch, is that existing packs are all still fully functional and not degraded (abused and damaged).

In fact, you can have Various sized packs within one bank of paralleled batteries. I in fact run with 2x 174AH, 4x 280AH & 1x 105AH ALL on one Battery Bank (this is my personal residence). + I have several systems running out there from Greenhouses to Saw Mills now running on Solar & Battery with mixed gear....

For Clarity for your discussions. (this is a point that can create a lot of confusion)
A Battery = a Number of Cells with BMS and is an independent entity.
A Bank is 2+ Complete independent battery packs set into a Series/Parallel Bank. There is technically no limit BUT many products do have set limits.
Batteries in Series increase Voltage. Batteries in parallel increase the stored energy.
Always use Native Voltage whenever possible, ie 24V Batteries for 24V and 49V for 48V, series connections with Lithium Based is fraught with foibles & quirks.

These are high voltage batteries. 2.8 kWh model is 102.4V, 5.6 kWh model is 204.8V, each at 30A.


Perhaps additional modules can be added to make the 6.0 into a 9.0 or 12.0
(that seems to be the case or BYD high voltage batteries.)

PaulT49 said:

The 6kWh battery is a few months old, the 4.5kWh battery is 3 years old and may have a different number of cells so the votage would differ

Click to expand...

Not necessarily will the voltages be different, but they could be. Different series + parallel combos will give different voltages and capacities.