Parallel 2 48V lifepo4 batteries advice

macdad22

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Apr 11, 2021
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First of all I would like to thank Will Prowse for the forum and all the great YouTube videos as I've learned so much from him and others over the past few months.

About a month I turned up a Growatt 12k inverter with a 48V 4s4s configured EVE 280AH battery bank. Each 4S battery has a Daly 250A 4s BMS w/ bluetooth and its so interesting to watch all the voltage and amp draws. The system has been working great and under typical scenarios it provides between 3 and 4 hours of AC power to my critical loads panel if I where to lose utility grid power. Almost every day I've been tested a power out scenario and letting a large portion of my house run off of batteries in a test scenario of power out.

My entire plan was to grow the system over time a system that has a little over 40kwh of battery backup as currently there ~ 14kwh. The plan is to add 2 additional 48V battery banks in parallel over the remainder of the year. I'm almost ready to add the 2nd bank which will raise the system to ~28kwh in total.

I've now completed building a new16s battery with CATL 271ah cells I got on Alibaba, however one of the things I'm most concerned with is the energy transfer from one battery to the other when they are first connected. I'm going to do my best to have them at the same ( no load ) voltage prior to connecting them. The 2 battery banks will be connecting via busbars with 1/0 sized cables on. They will have identical cable lengths and bluesea 300A fuses from the busbars to them and on the other includes Anderson PP185 connectors to each battery bank so that it is easy to disconnect/connect a battery bank.

What has been other people's experiences when first connecting 48V batteries in parallel? I've done this quit a bit at lower voltages and have always been very careful in terms of planning and safety.

I think a lower voltage like exactly 48V for each battery might be best with each battery and then let the Growatt charge them back up vs have them top balanced ~56-58V and connecting them together?

I think the 1/0 cables due to short length can handle the current inrush between the batteries in almost any scenario ie.. 1C is still less than what the cable and fuses are rated at but would rather not risk blowing a fuse or worse.

Let me know if I'm overthinking this.

Cheers,

Chris
 

Steve_S

Offgrid Cabineer, N.E. Ontario, Canada
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Ummm, colour me confused.
48V Pack requires 16 LFP Cells @ 3.2V, with a 16S BMS.
12V required 4 LFP cells in a 4S Config with a 4S BMS.
It is NOT recommended to put 12V packs in Series, 1 12V pack cuts off and the other BMS take the Voltage Hit and goes POOFDA !

Having Battery Packs in Parallel are no problem until you start to get past 4 packs and things become a tad more complex.
Having a minimal difference between packs is Best ! 271AH & 280AH is fine, but 150AH difference is NOT, that creates other issues. The closer the better.

NEVER EVER MIX CELLS ! If building a EVE 280AH Pack use only the EVE 280AH cells. Do not intermix brands & models even if the same AH Rating, they will NOT behave the same, they will be unbalanced.

IMPORTANT TIP !
Never put an uncharged battery into a battery bank. Always Top Balance the cells, then build your pack and charge it to full before putting it into a bank. The Bank should also be in a state of Full Charge as well. Once installed the packs will balance themselves out and on next two or three charge/discharge cycles they should come into a uniform operation. Do NOT forget to BIND THE CELLS to control Expansion/Contraction which is normal for LFP.

Normally when a Bank is setup, a Shut Off switch is placed between the battery banks DC Bus & the Inverter/SCC equipment to allow you to shutoff battery powering the gear to maintain and change battery systems/components. I use a BlueSea 9001E which allows me to use 2 Battery Banks ( I run 1090AH of LFP & 428AH of FLA) but they have several in their family of switches.
LINK: https://www.bluesea.com/products/category/11/41/Manual_Battery_Switches/e-Series

Below is a Chart from Southwire for Royal Excelene Welding/Battery cables.
1/0 is rated (with this fine copper wire cable, coarse wire has lower ratings) can handle a MAX of 260A without derration.
12,000W @ 48V = 250A (uncorrected) 287A corrected for 15% efficiency loss.
GW systems are Low Frequency and can handle 3X Surge or 36,000W momentarily. That translates to 750A Surge handling.
EVE & Lishen cells can handle up to 1C Rate for Discharge and 0.5C-Rate for charging MAX. But parallel packs will Divide Load & Charge fairly evenly which reduces that "hard pull" & push to the cells.

Fine Copper Wire IS BEST but as you know copper is not getting cheaper by any means, quite the opposite. At MINIMUM I would not consider anything less than 2/0 for your system. Use exactly the same length of wire for ALL of the Battery Packs to the common DC BusBars. Keep your (+)&(-) wires together (zippy ties or whatever) to reduce RFI/EMI from the DC Wires. Use ONLY Tinned Copper LugEnds like the Tinned SelTerm lugs (heavier copper & thicker tinning) LINK: https://selterm.com/collections/tinned-copper-battery-cable-lugs/2_0_AWG
To properly do this, the same gauge of wire is used from Batt Terminals to Busbar to Inverter/SCC.

Excelene-Wire-Info.JPG

Hope it helps, Good Luck
See the links in my signature for more information that will help you.
Steve
 

macdad22

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Apr 11, 2021
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Steve-

Thanks for the thoughtful and quick reply. While my current 4s4s setup works just fine as a standalone battery bank with the growatt as the inverter shutdowns when a BMS takes down one of the 12V batteries I can now see the challenge with the 4s4s setup working in parallel with a 16S battery or frankly any other battery bank as that risk would be there. Losing a

Everything else you mentioned is how I setup up the system, including the 2/0 wires from the inverter to switch and then to the bus bars.
 

mrzed001

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If the 2 pack is nearly the same V then there will be no inrush. So keep the difference lover than 1,5V
That is only 0,1V per cell. On such a low Vdiff minimal amount will flow.
You can test it. Connect a 3,3V cell into a lab transformer and set it to 3,4V. How many A is flowing ?
This is the main problem with LFP cells connected parallel. They balance each other very slowly (low Vdiff means low A flows).

So the best solution is top balance new cells and 100% charge old ones ... let them rest a bit (to reach battery relax V), then connect parallel the 2 pack and ready.
 

Steve_S

Offgrid Cabineer, N.E. Ontario, Canada
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4s4s is ? 4 in series 4 in series ?? Never heard of that, dunno what it is.
I'm assuming that is 4x 12V (4S packs) in series. to make 48V.
I am sorry to put it bluntly, but THAT is WRONG ! That is an open invitation for Murphy's Laws to be applied to you in the worst & most expensive way possible.

Really. The proper way to to make a 4S for 12V, 8S for 24V and 16S for 48V.
If you want to parallel cells then use ONLY Matched, Batched & Binned cells that are identical through their operating range and THEY costs more bucks.

I HAVE seen what happens with 12V Lithium in series when one pack fails, it was MNC and made a Nice Lovely HOT Fire ! Thanks to the craptastic BMS which was also for the wrong chemistry.... The dude was trying to save a few pesos and blew up a few grand in the process of being smarter than everyone else. BTW, I got shit on for it because HE did not listen to what he was told to do, he then decided to take it out on his wife, a Major Mistake that put him in the hospital (She's also ex SSF LOL) bad move. Now he is "single".

PLEASE look at this, simplify life: https://diysolarforum.com/resources/luyuan-tech-basic-lifepo4-guide.151/
 

mrzed001

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Hungary - EU
4s4s is ? 4 in series 4 in series ?? Never heard of that, dunno what it is.
I'm assuming that is 4x 12V (4S packs) in series. to make 48V.
I am sorry to put it bluntly, but THAT is WRONG ! That is an open invitation for Murphy's Laws to be applied to you in the worst & most expensive way possible.

Really. The proper way to to make a 4S for 12V, 8S for 24V and 16S for 48V.
If you want to parallel cells then use ONLY Matched, Batched & Binned cells that are identical through their operating range and THEY costs more bucks.

I HAVE seen what happens with 12V Lithium in series when one pack fails, it was MNC and made a Nice Lovely HOT Fire ! Thanks to the craptastic BMS which was also for the wrong chemistry.... The dude was trying to save a few pesos and blew up a few grand in the process of being smarter than everyone else. BTW, I got shit on for it because HE did not listen to what he was told to do, he then decided to take it out on his wife, a Major Mistake that put him in the hospital (She's also ex SSF LOL) bad move. Now he is "single".

PLEASE look at this, simplify life: https://diysolarforum.com/resources/luyuan-tech-basic-lifepo4-guide.151/

I totally agree that the proper way is 16s.
But if you use a BMS for the 4s 48V pack (4x12V) then it is almost as safe as the 16s (48V) version. (like with 4pcs 12V bigbattery LFP battery)
The reason that NMC pack exploded is ... that it is NMC :D Simple.
That does that sometimes. With wrong BMS even more and without the proper 4s 48V top BMS even more.
 

Steve_S

Offgrid Cabineer, N.E. Ontario, Canada
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12V + 12V + 12V + 12V = 48.
But 12v+12V+CUTOFF+CUTOFF you have 24V live 48V being demanded = NOT Happening.
Or your charging away and pack 3 & 4 hit full cutoff and the 12V BMS all of a sudden gets 36V and then 48V.... oops SHTF !
There are piles of Burnt BMS' out there from the Lessons Learned dept that OTHERS had to learn the hard way and Pay the Price too.
We share lessons learned to prevent others from doing it and suffering the consequences of it.

As for BigBatt - No Comment, there is plenty out there.
 

mrzed001

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12V + 12V + 12V + 12V = 48.
But 12v+12V+CUTOFF+CUTOFF you have 24V live 48V being demanded = NOT Happening.
Or your charging away and pack 3 & 4 hit full cutoff and the 12V BMS all of a sudden gets 36V and then 48V.... oops SHTF !
There are piles of Burnt BMS' out there from the Lessons Learned dept that OTHERS had to learn the hard way and Pay the Price too.
We share lessons learned to prevent others from doing it and suffering the consequences of it.

As for BigBatt - No Comment, there is plenty out there.

False.
12V + 12V + 0V + 12V = 0V
They are in series.
Anyone is cut then the current flow is cut.
No circuit ... OL (open loop)
You can put a switch between them. That will also cut the flow.
Some even put a fuse on top of every cell.
 

macdad22

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Apr 11, 2021
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I think I'm back to believing this will work well, presuming have both both bank at similar voltage before connecting.. thanks everyone
 
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