Expanding LifePo4 battery bank - I want to mix 48v 202AH and 48v 100AH all in parallel

[AHTrimble]

I have 2 x 202AH 48v LifePo4 batteries currently, wired in parallel, no comms in/out of battery, 1.5 years old, no problems with them. I need/want to expand the battery bank but can no longer acquire existing brand batteries. I was looking at buying 2 x 100AH 48v LifePo4 batteries, wiring them together first to mimic a 48v 200AH battery, then add them in parallel with the existing batteries. But I have read 10 or so threads here on mixing batteries and I now have a question: Should I continue as planned or simply wire both of the new batteries in parallel with the existing batteries and forget about doing the "mimic" thing first?

OK, second question...If I simply add the two new batteries in parallel with the existing two batteries, won't they discharge first since they are only 100AH vs 202AH?

Also, I am open to any other advice, thoughts, and/or opinions

Specs -
BATTERIES
Existing Potential New
Capacity (a): 202AH 100AH
Capacity (kWh): 10.3kWh 5.12kWh
Rated Volt: 51.2v 51.2v
Volt Range: 44.8v - 58.4v 46.0v - 55.5v
Constant Charge Current: 100a 100a
Constant Discharge Current: 100a 100a

* Yes, it appears I would have to lower my bulk/float charge parameters to the lower settings for the "Potential New" but that would mean that the "Existing" would just take longer to charge.

 

[BentleyJ]

The most important part of wiring batteries in parallel is that they are both at the same resting voltage. Same being within about 0.2V of each other.
Also recommend each parallel battery has its own circuit breaker or disconnect so the cables can be connected with the disconnect OFF to prevent sparking because there is going to be some current flow between batteries when the disconnect is switched on.
As far as the capacity mismatch that is not a problem with parallel connected batteries of the same chemistry, LFP in this case. Many of us have parallel batteries or differing capacities and they work just fine. The reason is because when in parallel there is no way for the voltage to get out of balance between battery packs since the current will flow as needed and minor voltage deviations are self correcting.
That said, individual cells inside each battery bank that are wired in series do still need a BMS with cell balancing.

 

[AHTrimble]

The most important part of wiring batteries in parallel is that they are both at the same resting voltage. Same being within about 0.2V of each other.
Also recommend each parallel battery has its own circuit breaker or disconnect so the cables can be connected with the disconnect OFF to prevent sparking because there is going to be some current flow between batteries when the disconnect is switched on.
As far as the capacity mismatch that is not a problem with parallel connected batteries of the same chemistry, LFP in this case. Many of us have parallel batteries or differing capacities and they work just fine. The reason is because when in parallel there is no way for the voltage to get out of balance between battery packs since the current will flow as needed and minor voltage deviations are self correcting.
That said, individual cells inside each battery bank that are wired in series do still need a BMS with cell balancing.

Thank you for the great info! It helps clarify some of what I had read earlier...but the earlier info didn't read as clearly to me as your explanation.
Since each of the four batteries are rated at 51.2v the resting voltages should be the same. Do you agree?
Each of my existing batteries has its own BMS, and the 2 batteries I am looking to purchase also each have their own BMS as well.
Each existing battery will have its own Outback 125a/300vDC circuit breaker on 1/0 wire to the Victron PowerIn (acting as a busbar), less than a 10' round trip circuit.
The new batteries each have a 125a circuit breaker built in and I will also use 1/0 wire on them to the Victron PowerIn as well, less than a 10' round trip circuit.
The Victron PowerIn is attached to a Victron Lynx Shunt with a 325a CNN fuse.
From the Victron Lynx Shunt, via another Victron PowerIn, to the inverters (dual Quattro 5kw/120) I will use a 1/0 wire with 125a MEGA fuse to each inverter.
So, if I am stating it correctly...my set-up should match the configuration as you outlined...and function correctly.
That is a huge worry off my shoulders! Thank you so much

 

[BentleyJ]

Since each of the four batteries are rated at 51.2v the resting voltages should be the same. Do you agree?

The value of 51.2 is an industry term called the "nominal voltage". It is the result of each cell having a nominal voltage of 3.2 multiplied by 16. Its easier to think of nominal as being like an average of the normal working voltage range of an LFP battery or cell.
The resting voltage of fully charged cell after sitting idle for a few hours with the charger off is about 3.35V per cell or 53.6V. Once a cell has been discharged and its resting voltage drops to around 3.0 its basically depleted. Using the nominal voltage value is an easy way to calculate how many Watt hours to expect from a battery of given Amp hour capacity. [nominal Voltage x Amp hours = Watt hours]

 

[AHTrimble]

Great detail...good knowledge.
Since both are 16 cell, and both are charged at 55.5v (even if existing can run up to 58 and new ones can only run up to 55.5), and both have the same nominal voltage, wouldn't they end up with similar resting voltage as you described...or at least close enough for practical application?
The math and technical insight is getting tough! I sure do appreciate your help. Amazing! Thank you

 

[BentleyJ]

Since both are 16 cell, and both are charged at 55.5v (even if existing can run up to 58 and new ones can only run up to 55.5), and both have the same nominal voltage, wouldn't they end up with similar resting voltage as you described...or at least close enough for practical application?

Yes

 

[AHTrimble]

Yes

You have really helped me sleep better last night. Thank you!
Now the work begins!

 

[AHTrimble]

Yes

Well, more headaches and confusion...please help if you can...before my head explodes.
I found that SOK batteries have voltages that are almost identical to the batteries I have now. But, the SOK is a 100Ah battery vs my existing batteries which are 202Ah.
I called SOK to get an availability and shipping quote...and while I was at it I ran past them what I was going to do. They said it would NOT work since the batteries are different capacities (100Ah vs 202Ah), different manufacturers, IR, etc.
So now I am questioning everything...What say You?

And yes...anyone else can feel free to jump in on this!

I have watched Will's "Current Sharing 101" several times now and get the idea I can proceed with my plan. But Will's video doesn't explicitly say you can mix batteries that are 100% different in Ah capacities. What am I missing?

 

[AHTrimble]

OK...UPDATE ! ! ! !

This only applies to LifePo4 batteries.

So I talked with Trophy and they said their engineers stated it was "not recommended" to mix battery capacity. Not quite the cut & dried as the response from SOK.

But, I spent an hour on the phone with a local solar installation company, the owner actually, and he is a licensed electrician...doing commercial, off-grid and grid-tied systems for 20+ years. Here's the bottom line...

• Operating voltages must pretty closely match.
• ABSOLUTELY the same number of cells in each battery.
• Each battery must have its own quality BMS to protect that particular battery.
• The batteries should not be connected to each other with comm cables in an attempt to communicate and coordinate with each other.
• No battery should be connected to the inverter via a comm cable.
• Starting out the batteries should be virtually the same resting voltage before they are all turned on together.
• It is best if the batteries are connected via a busbar system.

He mentioned that doing it this way the batteries are in "dumb mode" operating independently of each other. Also, the batteries will try and "balance" voltages with each other. He explained that is called "micro cycling" which over time will use up some full cycles. However, over the life of the battery it might only amount to less than 50 full cycles.

Now for the good part...he is going to provide a battery to me at his cost so I can see for myself how this works.

I will update folks on this thread after installation and real life application testing.

 

[Quattrohead]

You will be absolutely fine as you seem to be well aware of the safety and wiring aspects of what you are doing.
I have four 200 amp hour batteries plus homemade 230 amp hour and 280 amp hour all connected together and yes they do sometimes charge or discharge each other but at the end of the day they all get on like best friends.
As has already been pointed out but I will mention yet again, just make sure all the batteries are close in voltage and charge level.

 

[AHTrimble]

You will be absolutely fine as you seem to be well aware of the safety and wiring aspects of what you are doing.
I have four 200 amp hour batteries plus homemade 230 amp hour and 280 amp hour all connected together and yes they do sometimes charge or discharge each other but at the end of the day they all get on like best friends.
As has already been pointed out but I will mention yet again, just make sure all the batteries are close in voltage and charge level.

I really hope I have the whole wiring thing down pat...and the fusing...and the circuit breakers. The only thing I am not up on real well are t-class fuses. I've seen Will use them and swear by them. I don't have them in my system as shown below. Any thoughts?

 

[Quattrohead]

I use all circuit breakers in mine, no fuses.
The four Felicity solar batteries are connected together and then to the bus bar via a single 125 amp breaker. Each of my homemade batteries are connected via their own 125 amp breaker to the bus bar.