Adding batteries to existing battery setup

[xcentric]

Have 10kWh of batteries and 4kW of panels, which are topped up by off peak electricity too. May need more battery capacity - if I wanted to add batteries I know it’s preferred to add more of the same, but I may be able to get others much cheaper. I assume I’d set up a separate battery bank with its own BMS to manage it? How would it connect to the inverter - in parallel with the other bank? Or best to set up new completely independent system with own inverter (or even just powered via E7 hence switching from charge to dis garage on E7 cycle)?

 

[timselectric]

If same type and cell count, you can just parallel them together. Each should have its own breaker or fuse.
Run them to bus bars, with the same size and length cables. And connect the inverter to the bus bars.
Make sure that the two batteries are at the same voltage, when you connect them together. To avoid large current transfer.

 

[xcentric]

All are LiFePO4. First 2 communicate via rj45 connections fir bms issues and are inside a sealed metal box so hard to be sure. So let’s assume they may be different (same voltage, obvs.) - what then? Original 2 are part of a modular unit so am not sure I can just parallel the others to the inverter outputs. Model of inverter is ESS 5500 (voltacon supplied and branded in uk, think is generic Chinese)

 

[timselectric]

If both require the same charging parameters. Then it's not a problem.
Assuming that currently you have a positive and negative cable running from the battery to the inverter.
Change it to cables running from each battery to bus bars and from the bus bars to the inverter. With over current protection on all positive conductors.

 

[Steve_S]

Let's address a few things first so communication is clear.
A Battery bank is a set of Batteries set into either series/parallel or combo thereof. A Battery Pack is a complete independent battery and one places multiple packs into a "bank". Batteries in Series increase Voltage, Batteries in Parallel increase Stored Energy (AH). Battery Packs in Parallel Divide & Share both Load & Charge proportionately*.

Unlike Lead Acid battery variants, you can add LFP packs in parallel at any time without negative consequences. With Lead, the weakest battery drags down the rest, this is not teh case with LFP due to teh management systems and the very flat voltage curve. You can even have 100AH, 200AH & 300AH Battery packs within a Bank without issues (albeit some careful math is required during design & implementation). Note that if using different AH packs, the EndAmps/Tailcurrent Calculations have to be adjusted. Also with Different AH Packs the way they Charge & Discharge is proprotionate to their capacity.

Battery Packs that have SmartBMS' that interconnect & communicate with the Inverter does change the picture somewhat. Generally, it is best to stick with Same Brand & Similar Model of Battery/BMS to ensure that it works. There are several BMS' in se which can interconnect & communicate BUT they are not all compatible with each other... There are no default set Communications Standards or Protocols as yet for this particular sector.

Below is a Generic Template for an AIO (All-In-One) based system... which includes Solar Controller/Inverter/Charger in one unit. This shows 3-AIO's in Parallel but it is the same for 1-AIO just eliminate the upper pair of bars.
Each pack uses an Identical Length Same Gauge wire from Pack to Busbar.
The Fuse is in-between and not calculated for length difference. Up to 36V you can use MRBF, after 36V, use of Class-T or MEGA fuses. For a 100AH Battery a 150A Fuse is appropriate.

NOTE - WARNING !
Not ALL Busbars are equal and the cheapo's out there WILL burn your house down ! I Very Strongly urge you to buy only Quality Busbars from Bluesea, Victron, Pike Industries and so on.... AVOID those cheaper "pairs" on Ebay/amazon.

 

[xcentric]

V helpful. As I assumed - the integrated communications of the bms means adding others is at least questionable. Will get more of same.

 

[Bossrox]

I'm guessing you're wondering if a smaller bank combined with a larger bank will drag down the smaller bank faster? The answer would be no. The inverter will draw from the strongest banks 1st & all banks will maintain an equal voltage. The smaller bank can't be drained faster than the largers "cuz the larger banks will back feed the smaller if it falls below the larger.

 

[xcentric]

the problem is more that I don't know what BMS is there, and there are comms between the batteries and the inverter. So adding different batteries to the existing chain of 2 batteries means they get power but without integrating their own internal bus with the existing one, which might cause problems. Or will it? will they all just see voltage available to charge and then deal with it accordingly? The 2 I have have the bus from one connect to the other, and that then connect to the inverter, so one battery bms talks to the other battery, and the other talks to the the inverter. Adding in another battery to that chain has less integration which may cause issues..... hence the q.....

 

[timselectric]

It won't affect the function.
But could skew the data logging.
If some are closed loop and others are open loop communications.
As in, the inverter could show that its using 5kw, and the battery is reporting that its providing 2kw.
Charging will also be limited to only what the communicating BMS's request.
Personally, I would just lose the BMS communications.
But if you want to keep it, add compatible batteries.

 

[xcentric]

can you just lose the bms comms? I guess I should rephrase my q as "what does those comms leads do?" - it can't be battery management itself, is it just for comms so that an app or similar reports more detail?

Idealyl will use same batteries but may have access to supply of cheaper batteries, and/or might want the flexibility. Is there a problem in adding the extra ones then - I don't know how I tell ion it's open or closed loop comms (or even if I really know what that means!)

 

[timselectric]

can you just lose the bms comms? I guess I should rephrase my q as "what does those comms leads do?" - it can't be battery management itself, is it just for comms so that an app or similar reports more detail?

Idealyl will use same batteries but may have access to supply of cheaper batteries, and/or might want the flexibility. Is there a problem in adding the extra ones then - I don't know how I tell ion it's open or closed loop comms (or even if I really know what that means!)

Closed loop communications just means you have a communications cable between the BMS and the charging source. The BMS controls the charging parameters.
Open loop communications just means that you don't.
You setup the charging parameters yourself. And everything is based on battery voltage.

 

[xcentric]

Ok / I read here that some/many closed loop systems are a hassle.

So it’s ok to add different batteries as long as I match voltages at the start? Or is closed loop better overall?

 

[timselectric]

I don't use closed loop.
I'm not interested in the BMS controlling charging.
The BMS takes care of the battery and the charger takes care of the charging. All according to the parameters I choose.

So it’s ok to add different batteries as long as I match voltages at the start?

Yes
Any difference in voltage causes amps to flow from the higher SOC battery to the lower SOC battery.
The larger the difference, the larger the current flow.
Make sure that they are within 0.5v of each other, and you are fine.

 

[xcentric]

Thanks. Where are the parameters set, if the system is expecting closed loop comms? Or is it rtfm?!

 

[timselectric]

Thanks. Where are the parameters set, if the system is expecting closed loop comms? Or is it rtfm?!

It's only expecting closed loop, because you have set it to. I don't know your unit, so I don't know what setting numbers are for what.
But yes, it is in the manual.